FM Receiver For 137 - 141MHz - WorldRadioHistory.Com · 2019. 11. 11. · A Publication for the Radio Amateur Worldwide Especially Covering VHF, UHF and Microwaves Volume No.34

A Publication for theRadio Amateur Worldwide

Especially Covering VHF,UHF and Microwaves

Volume No.34 . Autumn . 2002-Q3 . £5.00

FM Receiver For 137 - 141MHzMiroslav Gola, OK2UGS

Contents

K M Publications, 63 Ringwood Road Luton, Beds, LU2 7BG, UKTelephone / Fax +44 (0)1582 581051, email : [email protected]

web : http://www.vhfcomm.co.uk

Miroslav Gola FM Receiver For 137 - 141MHz 130 - 150OK2UGS

Wido Schäk The Transmission Of Electro- 151 - 159Magnetic Waves in RectangularWaveguides

Sigurd Werner Amplifier For 47GHz Using 160 - 164DL9MFV Chip Technology

Bernd Kaa Precision Directional Coupler 165 - 174DG4RBF For Matching Measurements

Wolfgang Schneider CW Modulator Using Pin Diodes 176 - 180DJ8ES

Winfried Bakalski BALUNs For Microwave 181 - 187DL5MGY and co-authors Applications Part 1

Gunthard Kraus Internet Treasure Trove 188 - 190DG8GB

This issue has lots of constructional articles, I hope that this will please those of youwho have contacted me to ask for more such articles.Miroslav Gola is another new author to VHF Communications, the 137 - 141MHzreceiver project is ideal for the new constructor.If you have an interesting article that is suitable for VHF Communications, pleasecontact me.73s - Andy

VHF COMMUNICATIONS 3/2002

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1.Introduction

Receiving information from FM mete-orological satellites has become an inter-esting hobby for thousands of radioenthusiasts all over the world. Those ofyou who have already tried internetsearches using such keywords as NOAA,METEOSAT, 137.5MHz, WEFAX , Me-teor etc. , will undoubtedly confirm thatthey have found hundreds of links topages of receiver manufacturers, re-sel-lers, professional users and particularlyham enthusiasts.You will find among others, a link to thehomepage of Radek Václavík OK2XDX,which is devoted to these issues [1]. Hisarticle on a downconverter for Meteosatreception was published also in the VHFCommunications issue 4/1999 [28,29].It is worth noting that on 1st April 2000we commemorated the 40th anniversaryof the first transmission of images fromthe satellite TIROS 1. The pictures wereof rather low quality, nevertheless, theystarted an era of space research of theEarth’s surface. The resolving power oftodays images is currently of the order 1pixel = 1 m. You can find more detailedinformation on the internet pages of theNOAA agency http://www.earth.na-sa.gov/history/tiros/tiros.html.

Quite and few hams tried successfully inthe seventies to construct receivers.These obviously did not have the techni-cal specification that can be achievedwith modern components. Images werenot generated using high quality decod-ing programs for personal computers,simply because they did not exist at thattime. The images were “decoded” usingof technology of the seventies; plottingon oscilloscope with medium afterglowand then photographing using an instantPolaroid camera [9].

2.Looking at earth from space

Satellites NOAA (USA - National Ocea-nographic and Atmospheric Administra-tion) and METEOR, OKEAN, RESURS(Russia) are the focus in this article.They are flying on polar orbits aroundthe Earth at the distance of approxi-mately 800 - 1200 kilometres passingover the same place at approximatelysame time every day [23]. Satellites passthe North or South pole on each orbit,that is why their orbits are called polar. Itis possible to determine their trajectoryprecisely using “Keplerian elements”,which describe the current orbit of thegiven satellite. Calculation of the exacttime of a satellites orbit, from the mo-

Miroslav Gola, OK2UGS

FM Receiver For 137 - 141MHz(A double conversionsuperhetrodyne with pll)


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ment when it appears at the horizon tillthe moment when it disappears behindthe horizon, can be made nowadaysusing many programs for personal com-puters. I most frequently use a simpleWindows program called SatWin [10,24]. A version of SatWin was alsowritten for MS-DOS and can be run onolder personal computers of the DX486type. Both these programs can be down-loaded free of charge together with up-to-date Keplerian elements at the follow-ing address: http://www.emgola.cz/. Youwill also find other information about theactivities of satellites plus the signals thatyou can receive and decode using thereceiver described in the following arti-cle. Pictures are transmitted continuouslyfrom polar satellites without beginning orending. When the satellite appears overthe horizon, the edge of the pictures isslightly cramped, gradually resolution ofdetails in the picture improves. At theend of orbit the signal gets weaker andthe picture begins to disappear in noiseas the satellite slips behind the skyline.Inclination is the angle made by the planeof satellites orbit and equatorial plane. Asatellite that passes over both poles (onso called polar orbit) has the inclinationof 90°. The inclination of Americansatellites NOAA 10-16 is 98°, their pe-riod is approximately 102 minutes andheight of satellite is approximately 820 -850 kilometres.Signals from the satellites are in WEFAXformat (Weather Faximile). This is anold, but still useful, system for transmis-sion of black and white visual informa-tion using a standard audio channelwhere a change of amplitude of the2400Hz sub carrier represents the levelof the video signal brightness. Maximummodulation (black) is not zero, but ap-proximately 5%, white is then approxi-mately 87%. This audio signal is fre-quency modulated on the main carrier,e.g. 137.50MHz for the satellite NOAA15. After demodulation by the FM re-ceiver we therefore obtain an amplitudemodulated tone of 2400Hz. This signal is

sent to the input of standard sound cardin a PC and processed by a softwaredecoder such as JVComm32 which canbe downloaded from http://www.jvcom-m.de/. JVComm32 even handles badquality demodulated signals due to theefficient digital filters. The result of thisprocessing is shown in Fig 12 as picturedisplayed on a computer monitor.Transmission of images from NOAAsatellites are composed of lines lasting0.5 second, which correspond with datafrom sensors. They provide one pictureof the Earth surface containing data fromtwo channels. Channel A transmits pic-ture in the visible spectrum (VIS) andchannel B transmits picture in the infra-red spectrum (IR). Each line containstime multiplexed data from both channelsand is composed of separation tonesinterlaced with picture modulation. Datafrom channel A is preceded by and shortimpulse of 1040Hz and similarly datafrom channel B are preceded by andshort impulse of 832Hz. Each line alsocontains a calibration sequence. Thanksto this the decoding program can displayonly the chosen type of picture. You willfind more detailed information at http://www.noaa.gov/. You will find up-to-dateinformation about Russian satellites ME-TEOR, OKEAN, RESURS at http://sput-nik.infospace.ru/. These satellites havehigher orbit than that of NOAA satellites(1200 km). For example inclination ofsatellites METEOR is 82° and their pe-riod is 115 min. The system of picturetransmission from METEOR satellites iscompatible, however slightly different,from that of NOAA satellites. Modula-tion is similar, but pictures contain onlyone photo with higher resolution. Edgesof lines contain sets of phasing lines(alternately black and white), the linesmark end of picture and greyscale. Pic-tures in the infrared spectrum do notcontain the greyscale. The pictures arealso inverted as in comparison withNOAA pictures. Photos from NOAAsatellites show warmer places by darkershade and colder places are brighter. The


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pictures from METEOR use inverse scalewarm seas are white and cold cloudformations are black.It is also possible to decode visual infor-mation from the receiver any time. To dothis it is necessary to save the receivedmodulated signal as a WAV sound fileon a high quality recorder (we had thebest results with SONY Minidisk). If youtake holidays in distant countries, it isrecommended that you use a portable andeasily mounted Quadrifillar Helix an-tenna, see [19], take the receiver de-scribed below and a Minidisk. Duringyour trip you can record exotic picturesfrom any of the meteorological satellites.When you return you can decode thesaved WAV sound files in the samemanner as during direct reception.

3.Description of the receiverRX-137-141MHz

The receiver RX-137-141MHz has beendesigned for high quality reception ofsignals form polar meteo-satellitesNOAA, METEOR, OKEAN and others.It is compatible with the converter from1691MHz to 137.50MHz which is suit-able for reception from geo-stationarysatellite METEOSAT 7 [14, 23].Looking at the Table 1, you will find thatsatellites in polar orbits transmit signalsin the range of 137.30-137.85MHz,therefore a very narrow frequency rangeis sufficient. We have chosen, for practi-cal reasons, a lower frequency of137.00MHz and an upper frequency of141MHz. No meteorological satellitest r ansmi t a t f r equenc ies above137.85MHz but the frequency of

NOAA 11 137.62MHz Not operatingNOAA 12 137.50MHzNOAA 13 137.62MHz Not operatingNOAA 14 137.62MHzNOAA 15 137.50MHzNOAA 16 137.62MHz Not operatingNOAA 17 137.62MHzNOAA beacons 136.77 and 137.77MHzMETEOR 2-21 137.40MHzMETEOR 3-5 137.30MHzOKEAN-O 137.40MHzRESURS O 1.1 137.85MHz

Table 1 : Not all the satellites given are always active. Some of them are stillflying on polar orbits, but their transmitters have been switched off. Someothers do not transmit due to a failure, e.g. the modern satellite NOAA 16 onlytransmits in the mode HRPT at the frequency 1.698 GHz due to a defect. This isthe fate of all artificial satellites, when they fail they can only be repaired usingvery costly methods. Not all the satellites are as important as the Hubble spacetelescope, which was repaired by the space shuttle that we watched withexcitement and admiration.See http://noaasis.noaa.gov/NOAASIS/ml/status.html


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141MHz will make it possible to use theconverter for the METEOSAT 7 satellite.This makes it possible to process infor-mation from both channels, the firstchannel (1691MHz) converts to137.50MHz and the second channel(1694.50MHz) converts to 141.00 MHz.The circuit diagram of the receiver isshown in Fig 1. It was originally devel-oped for the nearby ham frequency rangeof 144-146MHz [3]. The circuit of thereceiver is designed for wideband FM(bandwidth 30kHz). The low-frequencyWEEFAX signal is sent from the outputto the PC sound card. The frequencysynthesiser PLL and LCD display arecontrolled by an ATMEL micro-com-puter.The receiver is a double conversionsuperhetrodyne. Design of the receiverhas been significantly simplified by usingan MC 3362P (IC1) integrated circuitmade by Motorola [5], which comprisesall main elements of modern FM re-ceiver. All that is required to connect tothe MC3362P is an input band-passfilter, a resonant circuit for the first mixeroscillator, 2 ceramic filters for 10.7MHzand 455kHz, a quartz crystal oscillatorfor the second mixer, a resonant circuitfor the demodulator and few other pas-sive components. We will thus obtain anexcellent receiver with a rather simplecircuit and with supply voltage of 2-5V[12].

3.1 Input circuits of the receiverThe signal from the antenna (or from the

converter) go to a capacitance dividerC2-C3 (input impedance adjustment).The divider together with L1 forms thefirst tuned circuit, the “hot end” of whichis connected to T1 a dual gate MOS-FET, preferably “low-noise” type BF982.T1 ensures sufficient amplification of theinput signal. Resistor R3 suppresses thetendency of input amplifier to oscillate,but it does reduce overall amplification.The signal from resistor R3 is furtherfiltered by a band-pass filter L2-C5,L3-C8, L4-C11+C12 with the bandwidthof approximately 4MHz. Critical cou-pling between the band-pass filter cir-cuits is determined by serial connectionof SMD capacitors C6 + C7 and C9 +C10. The signal passes through the ca-pacitance divider C11+ C12 to the inputof the first mixer in IC1 with mixingsignal from oscillator (L5, C33).

3.2 PLL OscillatorThe stability of the oscillator for the firstmixer is achieved using a PLL withreference frequency of 4MHz. IC4 is aPhilips SAA1057 single chip synthesiserdesigned for tuning of VHF FM radioreceivers with medium frequency band-widths [4, 6]. It was produced in 1983,but surprisingly enough it is still avail-able on the market and at very goodprice. In the circuit shown in Fig 1 thesynthesiser can be tuned from 110MHzto 150MHz with steps of 10kHz using amaximum tuning voltage of 4.5V. Thetuning voltage (max. 5.5V) is taken fromthe power supply to the pin 7 of IC4.R14, C25 and C26 are the passive com-

Frequency range: 137 - 141MHz, smoothly in steps of 10kHzFunction SCAN: 137.00 137.30 137.40-137.50 137.62 137.85-141.00MHz,Intermediate frequencies:10.7MHz and 455 kHzInput sensitivity: 0.4µV (rms-typ.) for 12dB SINADOutput signal: 2400Hz amplitude modulated (black 5% and white 87%)Display: LCD single line, 16 displayed charactersCurrent consumption: 70mA, (with converter LNC1700 250-500mA)

Table 2 : Specification of the receiver.


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ponents of the phase detector, C27 filtersthe internal stabilised voltage. The stabil-ity of the PLL is determined by the filterconnected to pins 5 and 6 of IC4, R15,R16, C28, C31, C56, C57 determine thetime constant of active low-pass filter. Itis important here to pay special attentionto recommended values of components.The tuning voltage from the PLL isconnected to the pin 23 of IC1 to aninternal varicap diode. The output fromthe first oscillator of the circuit in IC1(oscillator buffer) is connected throughthe coupling capacitor C35 to pin 8(FFM), the input pre-divider of the syn-thesiser IC4. In the majority of applica-tions of the SAA1057 the referencefrequency is determined by a 4MHzinternal oscillator controlled by externalquartz crystal connected to pin 17 (X). Inour circuit we have chosen an economicoption and used a common quartz crystalfor the reference frequency of both PLLand ATMEL micro-computer [7]. Thequartz crystal X1 is part of the oscillatorin IC3 and the reference frequency forIC4 is connected through the capacitorC24 and resistor R11.For the first mixer we have chosenfrequency one intermediate frequency(10.7MHz) lower than the signal fre-quency. The synthesiser therefore gener-ates frequencies from 126.3MHz to130.3MHz for a reception frequencyrange from 137.0MHz to 141MHz. Thesynthesiser frequency can be finely tunedwith use of trimming capacitor C21. Thecontrol word for setting the dividing ratioof the divider is accepted by the synthe-siser IC4 through the inputs CLB,DLEN, DATA from the micro-processorIC3 via a three wire data bus, C-BUS,which is also connected to the connectorPC-BUS for other uses.

3.3 Intermediate frequencyThe first mixer oscillator is 10.7MHzlower than the input signal. The differ-ence component (fIN-fOSC) is the inter-mediate frequency signal of 10.7MHzbeing amplified by the amplifier in IC1

and fed to the ceramic filter F1, this is acommon type muRata 10.7MHz/180kHz. The filtered signal is fed to thesecond mixer where it is mixed with thesignal of a quartz crystal oscillator withthe frequency of 10.245MHz (X2). Theresulting difference component is filteredby ceramic filter 455kHz (F2) with abandwidth is 30kHz. Due to frequencyswing of the WEFAX signal’s modula-tion of +/-17kHz the width of F2 shouldbe approximately 40-50kHz. Unfortu-nately, the only ceramic filter available isthe muRata/455/B. We have found thatthe narrower width of the filter has anunrecognisable impact on quality of thefinal image. Modulation of the first oscil-lator can have a substantial influence onthe quality of decoded image. That iswhy increased attention must be paid, inthis project, to the feedback loop of thePLL.The signal after the filter F2 is amplifiedin the internal limiter with the output tothe quadrature demodulator, which usesthe resonance circuit L6-C19. In order toensure only minor signal distortion afterdemodulation the linear characteristic ofthe demodulator must have width of atleast 40kHz. For this reason we havechosen the value of the damping resistorof 39k. For use with the METEOSATsatellite a bandwidth of approximately20kHz is sufficient.

3.4 Low-Frequency outputThe demodulated low-frequency signal isa tone of 2.4kHz that passes through asimple filter, formed by R19, C37, C38,which suppresses undesirable products.After the filter the signal is divided intotwo parts, one to the potentiometer P2which feeds the low-frequency amplifierIC2 with output to the loudspeaker, theother to the pre-amplifier IC6 for the2.4kHz tone decoder circuit IC7, and alsoto the output for the PC sound card.

3.5 2400Hz tone decoderA tone decoder [13] was included in thereceiver after considering possible modi-


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fications to the software for the originalreceiver from [2, 3, 8]. From the table oforbit times for individual satellites andfrequencies at which they transmit, it isapparent that the receiver must scan theband from 137 - 141MHz and stop onlyat signals with modulation by a tone of2400Hz and not at incidental interfer-ence. We have chosen a simple algo-rithm; the receiver performs a test afterswitching on and stops at the first chan-nel having a signal modulated by tone of2400Hz. When the satellite disappearsbehind the horizon the signal modulatedby a tone gets lost in the noise and thereceiver begins scanning again. It stopsat the next signal with 2400Hz tonemodulation. The tone decoding is reli-ably accomplished by the integrated cir-cuit NE(SE)567 (IC7). As soon as asignal appears on the input of the tonedecoder, it is compared with the fre-quency of the internal oscillator. When atone is detected the output, pin 8, of IC7is set to the a low level and diode D1 islit. The frequency of internal oscillator isset roughly by the capacitor C55 andaccurately to the value of 2400Hz by thetrimming resistor R25. The logic signalon pin 8 of IC7 is connected via thejumper JP3 to the input of the microproc-essor SQ OUT which controls mode ofautomatic searching for signals in thereceived bandwidth (SCAN). The jumperJP3 can be used to select control ofautomatic scanning for signals either onthe basis of presence of 2400Hz tone, orby active squelch.

3.6 SquelchA side effect of receiving weak FMsignals or operation of the receiver be-yond the tuned station is an unpleasantnoise in loudspeaker. That is whysquelch (SQL) forms an integral part ofany FM receiver. It interrupts the low-frequency signal to the amplifier at ab-sence of sufficient level of input signal.The DC component of LF signal from thepin 10 (MetDriv) of IC1 is fed throughR4 to the potentiometer P1, which is

used to set the sensitivity threshold of thesquelch. When the slider of potentiom-eter P1 is in the extreme left position thesquelch is disabled. Turning the potenti-ometer clockwise increases the level atwhich the squelch switches off. Pin 11 ofIC1 is carrier detect which is used tocontrol the squelch switch having a volt-age of approximately 0V for a signalwithout noise or a voltage of approxi-mately 2.8V for no signal, or a signalwith increased noise level. This is in-verted by transistor T2 and fed to pin 8of the loudspeaker amplifier IC2 thatmutes the signal path when the squelch isactiveWhen the squelch is switched off thecollector of the transistor T2 and pin 8 ofIC2 have voltage of 1.25V, and thelow-frequency is not muted. When atconstant signal is received at the antennaand the potentiometer P1 is turned clock-wise, we will reach a state where thesquelch activated i.e. a voltage 0V ap-pears at the collector T2 and the low-frequency path is muted. When signalvoltage on the receivers input increasesslightly the squelch deactivates andopens the low-frequency path.The squelch signal has been used also forautomatic scanning of signals. We haveadded transistor T3, which inverts thesquelch signal which can be connected tothe input P3.0 (SQ OUT) of the micro-processor IC3. The processor programthen takes care of the rest (see thechapter Setting-Up Of The Receiver formore details).Note: If you do not like the smallhysteresis of squelch, connect pins 10and 11 of IC1 using a resistor of 2 - 5M(R30) and connect a ceramic 100nFcapacitor (C61, preferably SMD) be-tween pin 11 and GND. Another optionto soften the squelch rise time is to use anelectrolytic capacitor C60.


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4.Experiment with an AFCcircuit

During the design of this receiver de-scribed in [2, 3, 8] experiments weremade for AFC. This tuned the frequencyof the reference oscillator using the DCcomponent of the voltage from the quad-rature demodulator at pin 13 of the IC1.This was connected to the inverting inputof an operational amplifier TL071 withit’s output connected to a pair of varactordiodes, KB105G, which replaced thetrimming capacitor C21 in the circuit ofreference oscillator. Thanks to the verygood stability of PLL we did not noticeany change in quality of the final imagewhen AFC was used, that is why wedecided to exclude the AFC circuit inorder to make the design as simple aspossible. For people interested in AFC,the circuit diagram is available at theauthors homepage.In relation to the use of AFC, it isappropriate to mention Doppler shiftingof frequency. This phenomenon is ob-served if a signal source, in our case ameteorological satellite, is approaching,

you perceive it’s frequency as higher,and when it moves away, you perceiveits frequency as lower than it in fact is.The magnitude of the Doppler shift fororbital satellites is a maximum of 5kHzwhich is still in the pass-band of thefilters and does not cause visible distor-tion of final image.

5.Antenna

A requirement for assuring high qualityreception of signals from meteorologicalsatellites is the use of a high qualityantenna. Polar meteorological satellitesare rotation stabilised and transmit circu-lar polarisation. It is therefore impossibleto use ordinary Yagi or ground planeantenna. When you listen to the signalfrom the loudspeaker it seems to be noisefree, however when you observe thepicture after it is decoded you will seethat it is unusable. Anyone can build highquality antenna. Two basic types areused: Turnstile and Quadrifillar Helix.The turnstile consists of two crosseddipoles (Fig. 2) phased for circular po-larisation. This antenna should be situ-

Fig 2 : General viewof a Turnstileantenna.


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ated as high as possible above the hori-zon, preferably above the house roof orin an open air space. Experiments madewith a turnstile antenna located on thebalcony of a blockhouse, satellites flyingover at a low elevation angle wereshielded by building or balcony. In short,

it is only possible to receive signals thatare “seen” by the antenna. Instructionsfor building several types of turnstileantennas are on the authors homepage.Drawings describing the construction ofa simple antenna made from plastic tubeand 8 - 12 mm aluminium tube are givenin the literature [18].We have tested the antenna shown in Fig3 with the receiver. The antenna wasinstalled on a roof at 40m above theground and gives high quality signalreception. The feeder connection for cir-cular polarisation is shown in Fig 4. Fig5 shows the polar diagram for this an-tenna, particular attention should be paidto the dipole to refelctor spacing becauseit changes the polar diagram, the authorchose 3/8λ.The manufacture of the Quadrifillar He-lix antenna, which is shown in Fig. 6 canbe done only in a well equipped machineshop. This antenna has slightly betterreception of signals and moreover it canbe used also in moving objects, such asyachts cruising in Mediterranean Sea.The article [20, 21] contain many de-scriptions of simpler mechanical con-structions suitable, however, only for ashort-term seasonal use, or for antennasmade of copper heating tubes. If thedistance of your receiver from the an-tenna will exceed 10m, I would recom-

Fig 3: Practical design of a turnstileantenna.

Fig 4: Detailsof feeder forturnstileantenna.


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mend use of selective pre-amplifier forfrequency range of 137MHz, preferablyusing bipolar transistor. Experience hasshown that summer storms have a ratherbad impact on MOS-FET transistors. Inan environment with industrial interfer-ence it is often desirable to use a bandpass helix filter in front of the pre-amplifier.

6.Power supply for the receiver

The receiver requires a stabilised powersupply adapter with a voltage of 9 - 12V.It is highly that you pay special attentionto the selection of a power supplyadapter. If you have an oscilloscope, lookat it’s output when on load at 150mA andcheck that there is no ripple. The low-frequency amplifiers IC2 and IC6 are feddirectly from the adapter. The othersupply voltages, 5V for receivers circuitsand 5V for synthesiser and microproces-sor, are stabilised by IC5 (LM7805). Thesupply voltage for the analogue part ofthe receiver is also isolated by choke L6.The input of the power supply is pro-tected against reverse polarity by diodeD2. Bridging jumper JP2 enables the usethe feeder cable to supply the antennapre-amplifier or Meteosat converter. Thisrequires a higher capacity power supply

adapter, for connection of the OK2XDXMeteosat converter [16], I recommend apower supply adapter of 12V/500mA.

7.Construction of the receiver

Building the receiver is very simple, itcan be done by any beginner, who has aknowledge of rf techniques, and is ableto use a multimeter. If care is taken therewill be no need for special rf measure-ment equipment. The secret of success isto put the correct value components inthe right place on the printed circuitboard and solder them in properly.If you build the receiver from the EMGOkit you will have all of the components

Fig 6 : General views of QuadrifillarHelix antennas.

Fig 5: Polar diagram for turnstileantenna.


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needed. First of all visually check thecomponents against the parts list. Thevalues of the resistors and capacitors canbe checked, the resistors can be measuredand the markings checked on the capaci-tors. Do not forget that the marking 470on some capacitors does not mean470pF, but 47pF. It is important that youpay special attention to this initial meas-urement of components and visual in-spection, including printed circuits. Afterfirst construction guide was published [3]I agreed, out of curiosity, to completeseveral almost finished kits. Althoughthese receivers seemed to be assembled,they did not work. In all the cases I foundthat this was caused either by mistakes,negligence or bad soldering of compo-nents. After minor repairs all of thereceivers worked perfectly.After visual inspection of the printedcircuit board (PCB) fit the four pillars inthe corners, they will simplify the inser-tion of components (Fig 7). Start byfitting the 9 SMD capacitors and oneSMD resistor using a small quantity of1mm diameter SnPbCu solder. Next in-sert and solder the remaining resistors,capacitors, semiconductors and the con-nectors for the loudspeaker and powersupply starting with small componentsand continue with larger ones. Socketsare used for the integrated circuits IC3and IC4. Before soldering the two quartzcrystals, X1 and X2, fit a 0.5mm paperpad and removed it after soldering. Simi-larly the 5 TOKO coils with metal coversshould be fitted into PCB with smallspace of about 0.5 mm., this prevents thecase shorting to other tracks on the PCB.The tank circuit of the discriminator, L6,also has a metal cover and should befitted approximately 0.5mm above thePCB. If L6 does not contain a capacitor,fit C19. Finally fit the switch SW1 andJP3 and connectors LINESB and LIN-EREP. If you use your own printedcircuit board and it does not have platedthrough holes, do not forget to solder thetop and bottom of components leads andfit fed through wires where required. Fit

the pre wound coils L1 to L5 for theinput band-pass filter, these are highquality coils made by Japanese manufac-turer TOKO.If you manufacture the coils yourself,you must wind 2.75 turns of 0.215mmenamelled copper wire on 5mm formers.Solder the wire ends to the metal pins onthe bottom of the former and cover thecoil with a droplet of beeswax. Turns onall the coils must go in the same direction(e.g. clockwise). Insert the coils to thePCB and check orientation before solder-ing. Put a metal cover over the formerapproximately 0.5 mm above the printedcircuit board. Finally fit ferrite coresmade of N01 (150MHz) into the formers.The following components will be fittedduring set-up using rf measuring equip-ment: capacitors C11, C12, ceramic fil-ters F1 and F2, integrated circuits IC1(microprocessor AT89C2051 with theprogram RX137DIP4X ) and IC4 (PLLSAA1057). If you do not have rf measur-ing equipment, insert all the componentsin accordance with the component lay-out.Fit the components onto the front panel(Fig 10) including the support plate forthe LCD display and its fixings to thereceivers main PCB. First fit the buttonsTL1 and TL2 to the panel from the frontand the 100k trimming resistor, for set-ting of display contrast, to the panel fromthe back. If you use a back lit LCDdisplay, you must also fit the 120Ωlimiting resistor. The panel will be fas-tened to the receivers PCB by solderingat the bottom corners, at places without aprotecting layer of solder mask. This isstrengthened by installation of two thepotentiometers P1 and P2, and the angu-lar connector. After checking that thepanel is perpendicular to the receiversPCB you can solder all terminals ofpotentiometers and angular connector.Insert the 16 pin connector into the toppart of the panel from the front andsolder it from the back. Finally insert theLCD display form the front and solder its


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16 pins to the front panel. Secure thepotentiometers P1 and P2 by fitting theshaft nuts.After putting the receiver RX-137-141into operation you can fit it into asuitable plastic or metal box with aper-tures for the display and controls. Theantenna and power supply connectors areat the rear of the box. Fig 13 shows thereceiver mounted in a suitable box.

8.Setting-up of the receiver

Connect a stabilised power supply, of 9 -12V, to the input connector U12. Ensure

the centre connector is positive and theouter is ground. Using a voltmeter meas-ure +5V at the stabilised output of IC5.If there are problems with the noiseproperties of the receiver, check thestabilised voltage using an oscilloscope.If there is noise on the supply thestabiliser must be replaced.The procedure for commissioning andtuning of the rf components will dependon the instruments available to the indi-vidual constructors [22]. The receiversboard is already fitted with all compo-nents except the intermediate frequencyfilters F1 and F2. Do not fit the micro-computer IC3 and synthesiser IC4 intotheir sockets. Turn the potentiometer P1(SQL) completely to the left, this will putthe squelch out of operation. Do not

Fig 7 : Componet layout of the main receiver PCB.


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Fig 8 : Top side of the main receiver PCB.

Fig 9 : Bottom side of the main receiver PCB.


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connect the shorting pin on the switchJP3. For tuning L1 - L4 at the receiversinput and the demodulation discriminatorL6 it is best to use a wobbler. Almostsame result can be achieved using an rfgenerator (even an improvised test oscil-lator 137 - 141MHz with one transistor)a counter and a simple rf diode probeconnected to an analogue voltmeter.Connect a signal with frequency of455kHz, preferably frequency modulatedby tone of 1kHz, with frequency swingof 30kHz to pin 7 of IC1 using a 1nFcapacitor. Connect an oscilloscope to pin13 of IC1 and tune L6 for maximumamplitude of demodulated signal. Byadjusting the value of R6 (a lower valuewill broaden the linear part of the curve)at least 30kHz of linear demodulationcan be achieved. If a signal generatorwithout frequency modulation is used,adjust it in steps 1kHz and plot a graphof the voltage at pin 13 of IC1. It is alsopossible to determine the value of R6experimentally by monitoring the imagequality (minimum noise, the highestloudness, sharpness of image details,etc.). The recommended value of R6 is33-56K.The next step is to fit filter F2 andconnect the output signal from a wobblergenerator or analyser to the receiversantenna input, connect the wobbler to pin19 of IC1. This will enable the inputselectivity to be adjusted without beingaffected by capacity of the signal source.Shunt the input coil L1 using a 50Ω

resistor and tune the band-pass filter L2,L3 and L4 to approximately the centre ofthe band (139MHz) and set the width ofband-pass to 4MHz. If you do not intendto use the METEOSAT converter withyour receiver, set the input band-passfilter to centre at approximately137.6MHz. If it is necessary modify C6,C7 and C9, C10 (0.5-1pF) in order toadjust the coupling of the resonant cir-cuits to critical or slightly supercritical.Remove the shunt resistor from L1 andalso tune it to the centre of the selectedreceived band, i.e. to 139 or 135.6MHz.Now fit filter F1 and to insert thesynthesiser and microcomputer ICs intotheir sockets. Switch on receivers powersupply and use the trimming resistor onthe front panel to set the LCD displaycontrast so that characters are legible. Ifturning of trimming resistor to both ex-tremes does not help and no charactersappear on LCD display, use an oscillo-scope to check communication betweenPLL IC4 and microprocessor IC3 (pins 8(CLB), 9 (DLEN) and 11 (DATA) TTLlevels). Push any button on the displaypanel and look for a sequence of im-pulses, when microprocessor sends newdata to the PLL synthesiser. If this isunsuccessful, check that the microproc-essors reference oscillator is working.With the LCD display showing a fre-quency of 137.5MHz, connect a voltme-ter to the pin 23 of IC1 or even better tothe test point “UL” and check that thefirst oscillator PLL is functioning cor-

Fig 10 : Component layout of the display PCB.


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rectly. Start by adjusting the core of L5using a non-metallic screwdriver andwatching the voltmeter for changes in thecontrol voltage of the PLL. The meas-ured voltage should not remain fixed at

either extreme, i.e. 0.2V or 4.3V. If thePLL is working, the tuning voltageshould vary smoothly between 0.2V and4.2V as the position of the core in L5 ischanged.

Fig 13 :Completedreceiver in aplastic case.

Fig 12 : Bottom side of the display PCB.

Fig 11 : Top side of the display PCB.


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If you have wound your own coil L5,you may have to adjust the value of C33.If the tuning voltage reaches the maxi-mum of 4.2V with the core almostunscrewed (minimum inductance), re-duce the value of C33 by one step. If thetuning voltage reaches the minimum of0.2V with the core completely screwedin (maximum inductance), increase thevalue of C33 by one step. In order toavoid such problems we recommend theuse of the specified TOKO coil.For example, for a received frequency137.50MHz - satellite NOAA 15 (oscilla-tor of the receiver oscillates at126.80MHz) set the voltage at the junc-tion of R16, C31 to approximately 2.5Vby adjusting the core of the L5. Checkthe exact frequency of the oscillatorusing a counter and tune it by slightchanging the trimming capacitor C21.The majority of constructors will prob-ably not have a wobbler or rf signalgenerator at their disposal. Neverthelesseven the modest ham shack equippedwith just a probe, a multimeter and“common sense” it is possible to tune theinput circuits to the lowest possible noisein the output LF signal. You just have tomake a test Colpitts oscillator, working at137.5 MHz. If you do not have a suit-

able circuit, I will gladly send it to youtogether with a PCB. You can then tunethe resonance circuits, by the followingmethod, to a minimum noise in theoutput signal. It is not necessary to makedirect connection of test oscillator to thereceivers input. it is sufficient to insert acut wire into the antenna connector, apaper clip formed to an “L” shape will dothe job. Set SW1-DIP4 to 137.5MHz andtune the frequency of test oscillator tothis frequency, i.e. when the noise disap-pears from the receivers loudspeaker (orat least its intensity is considerably re-duced). By touching the coil of testoscillator you can introduce “frequencymodulation”. At pin 13 of IC1 you cansee the signal using an oscilloscope, orby listening to the loudspeaker. Firstadjust the core of L6 for the lowest noisein LF signal and the loudest volume.Then tune the input circuit, L1 - L4, andgradually shorten the improvised wireantenna (or move the test oscillatoraway) to give the lowest noise in low-frequency. Do not use a metal screw-driver, make a non-metallic screwdriverfrom a piece of hard wood (preferablybamboo), or from suitable plastic mate-rial.Note: set the squelch off using potenti-ometer P1 set to its minimum value.

Fig 14 : Picture ofthe completedreceiver.


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When this procedure is complete you canlisten for received signals and set thecorrect squelch level. The synthesiserPLL must be set up before the alternativetuning method is used.Connect a 137 - 138MHz turnstile an-tenna [17] to the receiver’s input and setthe frequency for the NOAA or ME-TEOR satellites by using either switchSW1-DIP4 or buttons TL1/TL2. Thesesatellites should soon appear, consult thecurrent orbit timetable to verify thetimes. The switch SW1-DIP4 serves assimple memory to store pre-set frequen-cies after switching on of the receiver.When all the switches are set to theposition OFF, the PLL tunes the oscilla-tor frequency to 126.80MHz, thus thereceived frequency is 137.500MHz (sat-ellite NOAA15). Set the required fre-quency for the receivers oscillator byswitching some of the four switches ofthe selector switch SW1-DIP4x to theposition ON. Table 3 shows the setting ofthe 4 switches, however all 16 combina-tions in binary code can been used.To commission of the low-frequency partof the receiver all that is required is to setthe gain of the amplifier for the loud-speaker and sound card using an oscillo-scope, or by just listening to the audiooutput. Set the amplification of IC2 tothe required value by adjusting R28(3R3=74dB, 10R=70dB, 33R=54dB,105R=44dB, 820R=34dB) with C59100uF. Set the tone decoder IC7 byadjusting R25 so that the LED diode D1lights up whenever a tone of 2400Hz isdetected in the received signal. The opti-mum input sensitivity of the decoder hasbeen chosen during development. Should

you have any reason to change it, choosedifferent ratio of resistors R22 and R23.The output from pin 8 is connected usinga jumper on JP3 to the input SQOUT ofmicrocomputer. SQOUT can also be con-nected to the collector of transistor T3,which has a logical value depending onthe setting of the squelch and on magni-tude of input signal.

8.1 The Receivers control functionWith a jumper on JP3 position 2-3(TON), after switching on the receiverwill perform a first test for the absence oflow logical level on SQOUT. The test isthen repeated and if a 2400Hz signalabove the set threshold level is detectedon any channel, the processor stops tun-ing. When the signal disappears i.e. whenthe satellite sinks behind horizon, the testre-starts and re-tunes until another signalis captured. Tuning can be interrupted bypressing the push button UP (TL1) orDOWN (TL2), the receiver is set to thefrequency according to the selectorswitch SW1-DIP4. It is then possible totune manually from 137 - 141MHz insteps of 10.0kHz. Simultaneous pressingand holding of both push buttons re-starts scanning again. The LCD displaywill show the current received frequencyin MHz. With a jumper on JP3 position1-2 (SQL), the squelch output is con-nected to the input of processor SQOUT.The scan function still operates but isnow controlled by the level of thesquelch, and not the presence of a2400Hz tone.

9.How to connect a LF output ofthe receiver to your personalcomputer

After demodulation the receiver’s low-frequency output is an amplitude modu-lated tone of 2400 Hz, which can be

Receive freq. Oscillator freq. Switch

K0 137.500MHz 126.800MHz all OFFK1 137.300MHz 126.600MHz SW1 ONK2 137.400MHz 126.700MHz SW2 ONK3 137.500MHz 126.800MHz SW3 ONK4 137.620MHz 126.920MHz SW4 ON

Table 3 : Switch settings forsynthesiser.


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processed by several methods. WEFAXsignals used to be processed on oldercomputers without a sound card usingJVFAX 7.1a. (operating system Micro-soft MS-DOS 3.0-6.22). For those whowould like to reminisce, you can studythis issue on the web [19]. You will finda description of simple method, whichconsists of converting the amplitudemodulation to frequency modulation. Themaximum change of brightness is thenexpressed by a change of frequency ofapproximately 1500 - 2300Hz. This sig-nal is connected to the computer’s serialport using a simple comparator (EasyIn-terface) and processed by JVFAX 7.1a.Thanks to the negligible price of obsolete

computers you can make a receiver anddecoder at minimum cost and have con-tinuous receiption of pictures form me-teo-satellites.Modern fast computers with sufficientlybig operation memory give us a possibil-ity to use the most advanced programs.The demodulated 2400Hz signal is con-nected directly to the input of sound card.

9.1 Program for decoding of WEFAXsignals using JVComm32Decoding of pictures by personal com-puters is supported by many modernprograms [26]. I have tested a demon-stration version of JVComm 32. This is

Fig 15 : JVComm32 in action.


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an elegant program without the need foran EasyInterface, just connect the low-frequency output of the receiver to theLINE input of your sound card. I finallybought a full licence for use of thisprogram [27]. This program can be,however, only be used on a powerful PCworking under Windows 95/W98/W2000/W-NT. The following paragraphdescribes reception of an image and itsdecoding by JVComm32 version 1.0 or1.1.In 1998 the German author of the JV-FAX, Eberhard Backeshoff, DK8JV (e-mail address: [email protected] andhomepage at http://www.jvcomm.de/)wrote JVComm32 for decoding of WE-FAX, FAKSIMILE SSTV and othermodern modes, he continues to enhanceit and extend it. The program requires acommon 16-bit sound cards with it’sLINE input connected to the low-fre-quency output of the receiver. A disad-vantage of this program is that it requiresat least a 75MHz Pentium PC with 16MBof memory, operating system Windows95, W98 or Windows NT 4.0 and highquality graphic card (High or True Col-our) with resolution at least 800 x 600pixels. JVComm32 on the other hand canwork at the background and allows si-multaneous processing of received im-ages (viewing, cropping, sending to yourfriends by e-mail, etc.). The author, nev-ertheless, recommends for multitaskingat least 90MHz Pentium and 32 MB ofmemory as a minimum.Connecting the receiver to the input ofcomputer sound card is very easy. Fromconnector LINESB of the receiver con-nect the low-frequency signal to theconnector LineIn or microphone input ofthe sound card. Fig15 shows JVComm32in action, when a map is received inbackground for, the pictures receivedfrom the satellite NOAA are loaded fromthe folder, Picture files, to the desktop:

9.2 Description of softwareConfiguration of the program for recep-

tion of satellites NOAA or METEOSATis very simple. Set the mode to NOAA orMETEOR, and the Interface type toSound Card. The help with this programis also very user friendly, you will findall the details of setting and operation.

10.Kits

The author has produced kits for hamsfor several years and therefore knowsvery well that successful completion of aproject is often hindered by tiny problem.No matter how good a verbal descriptionis, it’s information capability is alwaysmuch lower than visual and/or audioinformation. That is why kits are sup-plied with a CD containing completeinformation. Kits contain a set of printedcircuit boards and parts. Fully assembledand tuned modules are also available fora slightly higher price.You can get more detailed information atthe following contacts:[email protected] or [email protected] or http://www.emgola.cz/

11.Literature

[1] Ing. Radek Václavík OK2XDX,Pøíjímaè and interfejs WXSAT (pøíjemsnímkù z orbitálních meteosatelitù) [Re-ceiver and interface WXSAT (receipt ofimages from orbital meteorological satel-lites)]. A-Radio Praktická elektronika,series of articles in issues Nos. 2-6/1997.[ 2 ] G ü n t e r B o r c h e r t D F 5 F C ,Funkamateur 2/1995, str. 153 - 156 DerW e t t e r f r o s c h - e i n 1 3 7 M H zSatellitenempfänger, continuation inFunkamateur 3/1995, p. 274


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[3] OK2UGS.: Pøijímaè FM v pásmu144 - 146 MHz s obvodem MotorolaMC3362 [FM receiver for the band 144 -146 MHz with the circuit MotorolaMC3362]. A_Rádio_Electus99, pp.73-79[4] Maršík, V.: Kmitoètová syntéza os-cilátorového kmitoètu rozhlasovýchpøijímaèù [Frequency synthesis of oscil-lator frequency of radio receivers], Am-atérské Radio B3/1987, p. 88.[5] Motorola, Linear/Interface ICs De-vice Data, Vol.II, str. 8-82[6] Philips Semiconductors, SAA1057 -Radio tuning PLL frequency synthesiser,November 1983.[7] ATMEL, AT89C2051 8.bit Micro-controller with 1kbyte Flash, cataloguesheets August 1994.[8] DF2FQ: VHF Empfanger, CQ DL1/1994[9] Borovicka, Jiri, OK1BI. Personalconsultation history of receipts from theMeteosat, Antenna Turnstile[10] Our location database includes mosttowns and villages in the entire world(over 2 million places!) http://ww-w.heavens-above.com/[11] OZ1HEJ, Pedersen, Michael: Re-ceiver with LCD readout: http://ozon-.homepage.dk/eng/elcd.htm[12] OK2UGS.: Pøijímaè FM v pásmu144 - 146 MHz s obvodem MotorolaMC3362 [FM receiver for the band 44 -146 MHz with the circuit MotorolaMC3362.]. Elektroinzert 5/97 p. 6.[13] Philips Semiconductors: SE567 -Tone decoder/phase-locked loop, , 4/92[14] OK2XDX.: http://www.qsl.net/ok2xdx/[15] OK2UGS.: http://www.emgola.cz/Info.htm[16] Ing. Radek Václavik, OK2XDX:Prakticka elektronika No. 7/1999 con-struction guide to building of converterLNC1700 MHz

[17] L. B. Cebik, W4RNL: http://ww-w.cebik.com/turns.html The Turnstile AnOmni-Directional Horizontally PolarizedAntenna[18] Martin DH7GL: http://www.emgol-a.cz/Ant_GLdipol/GL_dipol.htm[19] Ruud JANSEN’S PA0ROJ: QFHAhttp://www.hshaarlem.nl/~ruud/ Skládacíanténa na cesty za poznáním [Foldawayantenna for exploration trips][20] Steve Blackmore: QFHA http://ww-w.pilotltd.net/qha.htm (step-by-step con-struction guide to building and QHFA)[21] Borre Ludvigsen: http://abdallah-.hiof.no/~borrel/QFH/ QFHA - More pic-tures and construction picture above isone of the designs on this site.[22] Josef Danes et al: Amatérská radio-technika and elektrotechnika, 3. èást ,Mìøení na pøijímaèích [Ham radio andelectrical engineering, 3rd part, Measure-ment on receivers], pp. 190 254, Naševojsko, Praha 1988.[23] Gola, Miroslav: http://www.emgol-a.cz/jak_zacit_meteo.html[24] Kucirek, Petr: Program SatWinpøedpovìdi doby pøeletù satelitù nadzvoleným územím [Program SatWin pre-diction of time of passages of satellitesover the selected territory][25] Gola, Miroslav: http://www.emgol-a.cz/fapcsimile.html[26] Eberhard Backeshoff, DK8JV e-mail address: , homepage[27] Eberhard Backeshoff, DK8JV Whatdoes it cost and how can I get the fullregistered version?, http://www.jvcomm-.de/indexe.html[28] Vaclavik, R, OK2XDX: Easy down-converter for Meteosat, VHF Communi-cation 4/1999, page 196 - 207. http://www.vhfcomm.co.uk/[29] Vaclavik, R, OK2XDX: Small re-ceiver for Meteosat, VHF Communica-tion 4/1999, page 208 - 217. http://www.vhfcomm.co.uk/


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12.Parts list

Capacitors1pF 4 ceramic (SMD 0805) C6,C7,C9,C103p3 1 ceramic C25p6 1 ceramic C126p8 1 ceramic C310pF 1 ceramic C3312pF 3 ceramic C5,C8,C111.8 - 22pF 1 trimming capacitor CKT1 8-22pF

C2133pF 3 ceramic C22,C23,C2447pF 1 ceramic C1447pF 1 ceramic C19 (used if

not included in L6)150pF 1 ceramic C132n2 1 foil WIMA C254n7 1 foil WIMA C3710nF 1 ceramics C3510nF 2 foil WIMA C26,C4947nF 5 ceramic C4,C30,C32,

C36,C3947nF 1 foil WIMA C38100nF 9 ceramic C1,C15,C16,

C17,C18,C34,C43,C44,C47

220nF 1 foil WIMA C31330nF 1 foil WIMA C282M2/50V 1 radial electrolytic C2010M/50V 1 radial electrolytic C4547M/12V 2 radial electrolytic C27,C29100M/10V 1 radial electrolytic C46100M/15V 1 radial electrolytic C401000M/15V 1 radial electrolytic C48

Resistors and potentiometers2R2 1 R2047R 1 R3180R 1 R14120R 1 R10 (for LCD with back light)3k3 3 R8,R9,R114k7 2 R17,R1810k 4 R5,R7,R16,R1922k 1 R439k 1 R6 (39K - 56K see text.)47k 2 R12,R1350k/G pot 1 TP160, shaft 4mm P2100k/N pot 1 TP160, shaft 4mm P1100k 2 R1,R2100k trim 1 PIHER PT6VK100 P3 (setting of

contrast of LCD display)180k 1 R15

Transistors and diodesBF981 1 Dual gate MOS FET T1BC238 2 NPN universal TO92 T2,T3

Red LED 1 Any LED diode D11N4007 1 Rectifier diode 1A D2

Integrated circuitsMC3362 1 RX FM 2x MF IC1LM386 2 LF amplifier IC2,IC689C2051 1 With program RX137141DIP4LCD

IC3SAA1057 1 PLL up to 160MHz IC4LM7805 1 Stabiliser +5V IC5SE567 1 Tone decoder IC7LCD-DV-16100 Single line display LCD1

Coils7MC455kHz 1 TOKO 455kHz/600µH L6Choke 560nH-1 µH 2 axial, TLM20.1µH inductance 5 TOKO or 2.75 turns, see

text L1, L2, L3, L4, L5

OtherX-TAL 10.245MHz 1 Quartz X2X-TAL 4.000MHz 1 Quartz X1F 455 kHz/30kHz 1 Ceramic filter F210.7MHz 1 Ceramic filter F1Socket DIL8 1 low Pro IC2Socket DIL18 1 low Pro IC4Socket DIL20 1 low Pro IC3BNC connector 1 Antenna connector FSpacer 4 M3x5 mmNut M3 4 Fe/CdCon. SCD-016A 1 power supply socket

12V-2.5mmCon. SCP-2009C 1 power supply plug

12V-2.5mmCINCH SCJ-0363 1 GM Elektronic, REP

connector.Switch DIP 4x 1 GM Elektronic, DIP1

connector.Push butn. P-B1720 2 GM electronic, display

board TL1. TL2,Push butn. P-B1720 2 GM electronic, main PCB

TL1Knob 2 GM electronic, for 4mm

shaft for P1 and P2Pin S1G11W 1 GM Electronic, to connect

the base of the receiverto LCD display

Loudspeaker 1 Conrad ElectronicPCB1 DISPL 1 138 x 37mmPCB2 M-BOARD 1 138 x 88mm


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The waveguide is considered to be themost important transmission elementfor low loss transmission of electro-magnetic waves and signals from1GHz upwards. At frequencies below1GHz, coaxial cable can be more eco-nomically used for the transmission ofbroad band signals, however as thefrequency increases the transmissionlosses play a larger and larger role.The article below takes a closer look atthe transmission of electro-magneticwaves in waveguides.

1.Introduction

In principle, the transmission of highfrequency signals should take place withas little loss as possible. For signals of upto 1GHz, this can be dealt with entirelyby using modern, low loss coaxial cables.Above 1GHz, however, transmissionlosses play a larger and larger role, dueto cable attenuation.This disadvantage is often accepted foramateur radio use on grounds of cost.This means a high degree of attenuationin feeders to antennas (parabolic reflec-tor, horn radiator, etc.) if the coaxialcable used is not designed for highfrequencies.

For this reason, waveguides are predomi-nantly used in commercial equipment forfrequencies above 1GHz (e.g. radar andbroad band directional radio), in order toachieve the lowest loss possible.

2.Wave propagation in waveguide

When using waveguides, you must takeinto account the fact that the cross sec-tional area and the shape determine thefrequency response of waveguides. Mis-matches between the waveguide inputand output are particularly important, asare discontinuities, e.g. poor flange cou-plings. These can lead to reflections andto the formation of standing waves, andcan make it increasingly difficult topredict the wave propagation pattern.

2.1. Significance of lower limitingfrequency for waveguidesWhen signals are transmitted usingwaveguides, the frequency, the power ofthe signal being transmitted and thebandwidth, determine the shape andwaveguide cross sections. Rectangular,round and ridge waveguides are of im-portance here.So that we can understand wave propaga-tion in waveguides, we must now intro-

Wido Schäk

The Transmission Of Electro-Magnetic Waves In RectangularWaveguides


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duce a few simplifications, but these willnevertheless produce sufficiently accu-rate results.

• The dielectric material in thewaveguide is always considered asbeing loss free, so that ε = 1 (e.g.air).

• Only Joules heat loss due to the skineffect occurs, reflection losses due tomismatches between the transmitteroutput and the waveguide do notamount to anything significant.

2.2. Wave propagation in rectangularwaveguideFig. 2 shows the wave propagation in arectangular waveguide.The following assumptions are made in

the analysis below. A linear polarisedelectro-magnetic wave acts on thewaveguide, and the transmitted wave ispolarised parallel to the smaller side wallb.It is further assumed that the wave actingon the waveguide always has a magnetic,H field component, Hx and an electrical,E field component, Ey that are perpen-dicular to one another. This gives rise toa resulting directional vector, Sz, whichmakes the wave move into thewaveguide and generate an interferencefield in its interior, depending on thefrequency of the signal.Let us assume that a linear polarisedwave, parallel to side b, is acting on theaperture of the waveguide, with the fre-quency at the aperture being lower than

Fig. 1: Attenuation curve of coax cable in comparison to waveguides:Curve 1: RG 55B/U 50-Ohm coax cable, material solid PECurve 2: Rectangular waveguide type 40 ALCurve 3: Rectangular waveguide type 18 AL


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the limiting frequency, fg, of thewaveguide. The current generated in thewaveguide wall due to the low wave-length simply short circuits the electricalfield. This field is called an attenuationtype.If we now slowly increase the frequencyof the wave until the critical limitingfrequency, fg, of the waveguide isreached, a stable electrical field wave isfinally created, and a displacement cur-rent flows between the a sides. At thislimiting frequency, the wavelength of the

wave corresponds to double the length ofthe a side of the rectangular waveguide.

From this frequency onwards, this elec-tro-magnetic wave can enter thewaveguide. The zig-zag reflection of thewave on the side walls of the waveguidenow generate an interference field (Fig.3).Due to the reflection of the incident waveon the side walls of the waveguide, aninterference field arises in thewaveguide. There are now field areas inthis interference field in which the fieldsof the incident and reflecting waves areadded together. There are likewise fieldareas in which the amplitudes of the twowaves cancel each other out. To makethis easier to understand, the illustrationin Fig. 3 looks at the special case inwhich λE=2a. In this case, this frequency,fo, derived from λE, is the limitingfrequency, fg, explained above.

fo=fg in Fig. 3It is also significant that a resulting newwavelength, λH, with the directional vec-tor z, arises in the waveguide, due to thewave reflection of the incident wave.The value of λH depends on the angle of

Fig. 2: Incidence of a wave into arectangular waveguide. Sz = resultantdirectional vector, vertical to plane ofdrawing.

Fig. 3: Interference of a wave in waveguide.

a2≤λ


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incidence, E, with which the incomingwave is reflected on the waveguide wall(see also Fig. 3).Angle of incidence αE=Angle of reflec-tion αRThe wave component, λH, can easily bederived from the geometry shown in Fig3:

The section AC = λHAnd the section BC = λ0

The angle of reflection of the wave frontis dependent on the frequency, fo, of theincident wave.The new wave, λH, arising from theinterference field, with direction z, has analtered propagation speed, vg, comparedwith the wave, λ0.

3.Classification of wave types inwaveguides

Dependent on the excitation of the inci-dent wave entering the waveguide (po-larisation of the wave), we distinguishbetween two basically different wavetypes:Wave type 1 TM waveWave type 2 TE waveWe refer to a TM wave if the directionalvectors of the electrical field lines arevertically to the direction of incidence ofthe incident wave (see Fig. 4). Theresulting vector product, E x H, thengives an E field strength in the propaga-tion direction of the wave, and we there-fore refer to it as an E wave. This wouldbe clearly visible if we split thewaveguide in the longitudinal directionand examined the two resulting fieldcomponents, E and H.An example field pattern is shown in Fig.4. The resulting H field lines are circularand vertical to the plane of propagation.The electrical fields are parallel to the

Fig. 4: TM wave in rectangular waveguide.

EH α

λλcos

0=

Eg cv αcos⋅=

[ ]Efc λλλεµ =⋅=⋅= 000001


154

longitudinal axis of the waveguide. Thismeans that the resulting E field strengthcauses an E wave to be propagated in thelongitudinal direction of the waveguide.We can see the circular magnetic H fieldin the cross sectional representation inFig. 4. By contrast, the electrical fieldsterminate at the internal walls of thewaveguide.However, in the middle of the crosssection there is no conductor where theelectrical E fields can terminate, theytherefore bend round in direction z andthus determine the propagation directionof the E wave. This is called an E11wave.The following statements are thus validfor understanding wave propagation inwaveguides:

• Magnetic H field lines are closed andrun tangential to the waveguidewalls.

• Electrical E field lines are eitherclosed or terminate vertical to theinner walls of the waveguide.

• Electrical and magnetic field lines arealways vertical to one another.

The following naming convention hasbeen adopted and applies to both maingroups H waves and E waves:

H waves TEmn wavesE waves TMmn wavesWe can therefore determine the type ofwave within the two main groups, thenumber of closed field areas of the Hfields and/or the E fields in the crosssection of the waveguide is counted (seeFigs. 5a and 5b).

3.1. E wavesThis does not pose any problems with thevarious TM waves, as can be seen inFigs. 6a and 6b.The number of H fields is counted alongside a (direction x) and along side b(direction y) of the waveguide.In Fig. 5a we see two closed H fieldsalong side a (direction x) and one row ofH fields, in relation to side b (directiony).This gives us:

m = 2n = 1

Therefore were dealing with a TM21wave here.In Fig. 5b we observe three H fieldsalong side a and two complete field areaslying on top of each other along side b.This gives a TM32 wave.

Fig. 5a:Field image of a TM21 wave inrectangular waveguide.

Fig. 5b:Field image of a TM32 wavein rectangular waveguide.


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3.2. H wavesLets go on to the H waves (TE waves).Figs. 6a and 6b show an H10 wave. It canbe seen from Fig. 6b that the field linesof the H fields are orientated in direction

z of the waveguide, and we can thereforerefer unambiguously to an H wave. Figs.7a and 7b give an example of an H20wave, here two closed H fields areorientated in direction x, and shift intodirection z with the changing polarity.

Fig. 6a:H10 wave in rectangularwaveguide.

Fig. 6b : H10 wave in rectangularwaveguide.

Fig. 7a : H20 wave in rectangularwaveguide.

Fig. 7b : H20 wave in rectangularwaveguide.


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4.Wall currents in the waveguide

So far we have examined only the vari-ous field distributions of the magnetic Hfields and the electrical E fields in thewaveguide. For a comprehensive under-standing of wave propagation inwaveguides and of their construction, wemust also consider the wall currents inthe waveguide.The occurrence of electrical wall currentson the surface of the internal walls of thewaveguide can be explained by the factthat alternating magnetic alternating (tan-gential H fields) cause a surface currentin a conductive body. Here the currentflow is orientated vertically to the polari-sation of the magnetic field and thusvertically to the concentric field lines ofthe H field. If we now look once again atthe field distribution in the H10 wave(Fig. 6), we recognise that H10 wallcurrents can thus always be found in thevicinity of H field lines, with their direc-tional vectors being orientated vertically

to the circle of the concentric field lines.In accordance with this definition, thecomplex wall currents can also be de-rived for any other wave types of Ewaves and H waves. What is of interesthere is that the wall current generatedpenetrates only to a depth, δ, into theinternal waveguide wall (skin effect).Thus, particularly in very high frequencyapplications (f>20GHz), the waveguidesinternal walls are often electro-platedwith a layer of gold, silver or copper inorder to obtain a high degree of conduc-tivity for the waveguide surface.

5.Phase velocity and frequencyresponse in rectangularwaveguides

For the transmission of high frequencywaves in waveguides, the high degree ofdependence on the frequency (in contrastto coax cables) must be taken into ac-

Fig. 8: Frequency and application ranges of rectangular waveguides.


157

count. The signals fed into thewaveguide display a phase velocity thatvaries with the frequency. The phasevelocitys dependence shows itself, inparticular, in the vicinity of the lowerlimiting frequency, fg. For broadband,high frequency transmission, this leads toa frequency dependent distortion of thesignal in phase and amplitude. So an“unambiguous range” is defined with aspecified upper and lower transmissionfrequency for the waveguide section. Thelower frequency is set to exceed 1.25 xfg and the upper cut off frequency is setat < 19 x fg.

6.Guide wave impedance inwaveguides

The fact that waveguides display a highdegree of dependence on the frequencyhas already been mentioned. This prop-erty must also be taken into account forthe matching of waveguide sections tostandard terminating impedances, e.g. for50Ω power amplifier stages. By contrastto coax cable, which is matched to thetransmitter through its constant charac-teristic impedance, complicated circum-

Fig. 9: Basic forms and constructions of waveguide flanges.


158

stances are present for waveguides, sinceE fields and H fields are transmitted. Thepeak values of these fields now deter-mine the maximum transmitted rf output,P.Note: The maximum transmittable con-tinuous wave power and rf peak powerare dependent on the cross sectional area,A, of the waveguide, and are also de-pendent on the breakdown field strength,which is dependent on the pressure,temperature and relative humidity.For waveguides the wave impedance isintroduced as a replacement for the char-acteristic impedance, defined as the quo-tient of the peak value of the electricalfield, Ey, and the peak value of themagnetic field, Hx,. But its level is alsodependent on the frequency of the signal,feeder wave.Formulae:

Where wavelength of signal:

And characteristic wave impedance:

7.Coupling high frequency signalsinto rectangular waveguide

The rf input is fed to the waveguideeither by means of a special waveguideflange connection on the transmitter out-put or through a broad band 50Ω coaxialconnection, which then feeds the rf into awaveguide adapter. These adapters can

be obtained for all types of cable and allstandard coaxial plugs.Finally, a few other basic waveguideflange constructions are illustrated in Fig.9 in airtight and non-airtight versions.

8.Literature

Strips and waveguides, Walter Janssen,Hüthig Verlag, 1992

−

==

a

F

X

YF

ZHEZ

21 0

0

λ

0

00 fC=λ

Ω== 3770

00 ε

µFZ


159

The article below describes a projectto construct an amplifier for the47GHz microwave band. The twostage amplifier uses semiconductorchips from United Monolithic Semi-conductors, S.A.S. and gives at least26dB gain. Anyone actually construct-ing this power amplifier must haveaccess to bond technology.

1.Introduction

The non-thermal effects of pulsed highfrequencies are being examined as part ofcertain research projects [4]. Of particu-lar interest are the effects of high fre-quencies on the activity of protein basedbodies (enzymes). Since molecular reso-nances are to be expected in the rangefrom approximately 42 to 46GHz, theexperiments began in this range.In addition to a generator and measuringequipment for this frequency range, weneeded (among other things) a goodstable power amplifier. This articleshows that the amplifier designed andassembled is also suitable for use on theamateur radio frequency of 47,088MHz.This description could prove a stimulusfor people with projects of their own.

2.Selection of semiconductors

The use of discrete semiconductors in theGHz range referred to is always veryexpensive for radio amateurs and is com-bined with considerable design expendi-ture. An alternative is the use of suitablesemiconductor chips, which are gluedonto a carrier material, and their connec-tions bonded.Unfortunately, there are no ready madechips for this frequency range, so someDIY work is called for.After an examination of the data sheetsfrom various chip manufacturers, theCHA3093c from UMS (United Mono-lithic Semiconductors, S.A.S.) was cho-sen.In the data sheet, this chip is specified forfrequencies between 20 and 40GHz, butwhen the S parameters were studied, theywere listed for up to 50GHz, the amplifi-cation value for 47GHz was found to beat least 17.3dB.However, this applies only to measure-ments on the wafer but with a goodconstruction, values of approximately 14to 15dB can be attained. The inputmatching is -11.1dB, which is an accept-able value. A saturated power of 22dBm,approximately 150mW, is specified (3dB

Sigurd Werner, DL9MFV

Amplifier For 47GHz UsingChip Technology


160

compression). Fig. 1 shows the chips,greatly magnified. The four groups ofcascaded semiconductors can be clearlyrecognised.

3.Circuit design

The design is relatively simple. A driverchip feeds two chips in parallel through aWilkinson divider. The power is com-bined again through a second Wilkinsondivider. The same chips (type CHA3093c) are used for all three devices.

4.Mechanical and electricalconstruction

The housing for the prototype was milledfrom brass (60 x 30 x 9 mm.) and goldplated. The depth of the cavity is 4.8mm.K plugs (2.4 mm.) were mounted at theinput and output of the amplifier circuit.Fig. 2 illustrates the construction of theamplifier.The carrier substrate is a thin ceramicplate made from aluminium nitride (AIN,ε=9.0) which is only 0.254 mm. thick.

Fig. 1: Magnified illustration (x 120) of CHA 3093c chip from UMS.


161

The microstrip lines have each beenconnected to the chip surfaces through aco-planar spacer. Thanks to the goodthermal conductivity of the aluminiumnitride, the chips can be glued directlyonto the substrate.The power leads for the gate and thedrain are decoupled using 100pF capaci-tors, for longer paths there are also 1nFsingle layer capacitors and 100nF ce-ramic capacitors. These are fed throughthe housing base using soldered infeedthrough capacitors.The chips were bonded by means ofthermo-compression (including ultra-sound support), using 17.5µm goldthread. Chips, capacitors and substratewere attached using a single componentsilver conductive adhesive [2] hardenedat 150°C. Figs. 3 and 4 show details ofthe construction.

The gate bias of the first semiconductorstage on the driver chip was providedseparately. Its drive is intended to test theuse of the driver chip as a multiplier. Aconnection to the monitor diode of thechip was dispensed with for the pototype.

5.Results

The drain current per chip (at 3.6V) wasset at 360mA. This requires a negativegate voltage of approximately 0.6 -0.5Volts. A heat sink is necessary forcontinuous operation, since the powerconsumption is almost 4W.The amplifier was initially driven using aCW signal (47,088MHz) of 200µW. Thepower level of approximately 20mW

Fig. 2: Assembly of two stage microwave amplifier in gold plated milledhousing; the Wilkinson dividers can be easily recognised.


162

measured at the output which could beraised to 80mW by rough optimisation.In the output area of the chip there was amarked mismatching (the S22 parameteris only 5.8 dB!).The calculated amplification is approxi-mately 26dB, i.e. 13dB per stage. The -3dB compression point is at approximately20.7dBm.With suitable drive, a saturation powerexceeding 120mW can be attained (Fig.5). This means that the values specifiedin the data sheet were not attained. This

applies, in particular, to the saturationpower reached for one chip of only17.8dBm. This could be because thesemiconductors are designed for pulsedmode operation.The amplifier is particularly sensitive toheat, even with a moderately warm hous-ing, the output drops by 15 to 20%. Agenerously dimensioned heat sink musttherefore be used. To counteractwaveguide effects, absorption materialshould be applied in the input and outputareas before the metal cover is fitted.

Fig. 3: Details of driver assembly. Fig. 4: Details of second stage withparallel wired amplifier chips.

Fig. 5: The output of the amplifier plotted against the driving power,measured with a CW signal at 47088 MHz.


163

6.Outlook and acknowledgements

This article indicates the options formodern chips, and is intended to act as astimulus for further experiments.Some other interesting types of chip haverecently come onto the market, these arejust waiting to be tested. The price of achip is somewhere around one Euro,depending on the source of supply. Atthe VHF/UHF Congress in Munich at thebeginning of March, Michael Kuhne(DB6NT) introduced a project for a47GHz preamplifier. A particularly lownoise chip is used, type CHA 2157 fromUMS [3].In conclusion, I would like to thankseveral helpful people who have sup-ported me in word and deed, namelyKonrad Hupler (DJ1EE), Walter Ludwig(DL6SAQ), Mrs. Astrid Habel (Techni-cal University, Munich) and Mr. Wil-helm Hohenester of Rhode & Schwarz,Munich.

7.Literature references

[1] Data sheet from United MonolithicSemi-conduc to rs , S .A.S . ; Ref . :DSCHA30930207, 26. 7. 00[2] Technical Data Sheet, Ablebond 84-1LMI from Ablestik[3] Michael Kuhne, DB 6 NT Manuscriptfor VHF / UHF 2002, 14th Congress,Munich 2002[4] Institute for Physiological Chemistryof University of Munich

The UK Six Metre Group

www.uksmg.org

With over 1000 members world-wide, the UK Six Metre Group is theworld’s largest organisation devoted to 50MHz. The ambition of the group,through the medium of its 60-page quarterly newsletter ‘Six News’ andthrough it’s web site www.uksmg.org, is to provide the best informationavailable on all aspects of the band: including DX news and reports,beaconnews, propagation & technical articles, six-metre equipment reviews,DXpedition news and technical articles.Why not join the UKSMG and give us a try? For more information, contactthe secretary Iain Philipps G0RDI, 24 Acres End, Amersham, Buckingham-shire HP7 9DZ, UK or visit the web site.


164

Directional couplers are frequently re-quired in high frequency measurementtechnology. They are used, not only instanding wave measurement equip-ment, but also in high quality test rigsand network analysers. A directionalcoupler is described below, it has highdirectivity and can be used from ap-proximately 120MHz up to 1,500MHz.

1.Introduction

If we want to carry out wobbled match-ing measurements, then, among otherthings, we need a good directional cou-pler with high directivity. The directivityis the measurement of how well thedirectional coupler can distinguish theforward transmission from the reversetransmission.To be able to carry out meaningfulmeasurements, even on high qualitycomponents and good DIY circuits, thedirectivity should be as high as possible,exceeding 30dB. However, as industri-ally manufactured directional couplersare used in high tech equipment, theprice of a new one is too high for mostradio amateurs, and they are scarcelyfound on the surplus market.There are numerous publications dealing

with the construction of directional cou-plers, but some of them are designed forsimple applications only, e.g. for SWRmeters, the directivity is frequently only25dB.

2.Requirements

If you want to build a directional couplerwith a high directivity, then the follow-ing points must be given considerationduring construction:

• Maintenance of 50Ω impedance inthe main and secondary lines

• Precise 50Ω termination for all lines

• Discontinuities should be as slight aspossible where there is a transitionfrom the lines onto the coaxialconnectors and the dummy load

• Trimming option for secondary linefor maximum directivity

Experiments with coupled semi-rigidlines have already provided usable re-sults, but they have not led to the desireddirectivity of 40dB. Precise impedancecan be achieved using the solid sheathcable, but it is badly influenced by thelong windows (λ/4).

Bernd Kaa, DG4RBF

Precision Directional CouplerFor Matching Measurements


165

On the basis of these experiments, at-tempts were made to find a way toproduce two coupled lines running paral-lel, with 50Ω impedance.Here are some reflections on this. A 50Ωline can consist of a round externalconductor with a round internal conduc-tor, or of a square external conductorwith a round internal conductor. There isalso a formula for calculating a 50Ω linethat consists of a U shaped external

conductor with a round internal conduc-tor:

ZL = 138 x 1.17 x Ig D/dIf we use two of these U shaped coaxiallines and join them at the open sides,then we obtain two coupled 50Ω lines(Fig. 1).This led to the directional coupler illus-trated in Fig. 2, with a directivity exceed-ing 45dB together with a good bandwidth. The measurement of the forwardand reverse transmission in the frequencyrange of 40MHz to 1,500MHz was car-ried out using a Wiltron network analyser(Fig. 3). The good directivity values of45 to 48dB can be read off from themeasurement curve.Fig. 4 shows the measurement results upto 3GHz, which were measured using aRohde & Schwarz network analyser.Here too, we can see the extremely high

Fig. 1: U-shaped coaxial line.

Fig. 2: Ready-to-operate specimen of directional coupler, made from brass,with SMA jacks for main and secondary branches and with calibration screwsin cover.


166

Fig. 3:Measurementof forward andreversetransmission inrange between40 and 1,500MHz shows adirectivity ofapproximately45dB.

Fig. 4:Measurementresults up to 3Ghz.


167

Fig. 5: Good directivity (44db at 120MHz) even in lower frequency range to200 MHz.

Fig. 6: Specimen assemblies of various directional couplers in different sizes,with SMA or N jacks in the main branch.


168

directivity levels, which go all the wayup to 1,500MHz. If calibrated for narrowband, even higher values of directivitycould be attained, exceeding 50dB (!).Fig. 5 shows the forward and reversetransmission for the directional couplerin the lower frequency range up to200MHz. It can be seen from this that thedirectional coupler, in conjunction withsensitive wobble systems, can be usedwith a good directivity even at lowfrequencies (44dB at 120MHz).Several directional couplers with variousconnection jacks were assembled in ac-cordance with this concept (Fig. 6). Itbecame clear from this how important itis to use high quality coaxial jacks. It isextremely important to have as few rfdiscontinuities as possible in the transi-tion onto the coaxial jacks. The bestresults were attained with good SMA andSMC jacks. Somewhat lower directivityvalues were attained from assembliesusing N jacks (Fig. 7). So only precisionjacks with small flanges from reputablemanufacturers should be used.

If we move into directivity values ex-ceeding 30dB, we likewise need a highvalue 50Ω termination on the secondaryand main lines. It is best if two equaldummy loads are used. You can not dowithout high quality industrially pro-duced dummy loads but they can beobtained at acceptable prices.

3.Assembly

The mechanical components of the direc-tional coupler are all made of brass (Fig.8). The main body consists of an 18mm.x 18mm. square bar with a milled grove6.1mm. wide and 12.2 mm deep. An18mm. x 5mm. piece of brass flat mate-rial is used as a cover. Incorporated here,among other things, are 4 tuning screws(M3), to tune the secondary line.The side components are made from3mm. thick brass. The cover and sidecomponents are each screwed onto the

Fig. 7: Themeasurementcurves showthe somewhatpoorer valueswhen N jacksare used.


169

Fig 8 : Details of the mechanical components.


170

main body with M3 brass screws.The internal conductors of the main andsecondary lines are each made from3mm. round pieces of brass. Small holesat the ends support the internal conductorof the SMA jack. The bore diameter isdependent on the type of the jack used. A1mm. transverse hole is used to improvesoldering.The dimensions and units are shown onthe drawings shown in Fig 8. The twointernal conductors should be polished,to reduce skin effect.

A thread is cut in the cover and in eachside component for the SMA jacks.These threads are described as ¼" 36UNS-2A. But it is difficult to get hold oftaps with this thread. A ¼" 32 NEF tapcan be used for short threads. These canbe obtained in many model shops, sincethis is the thread size for heater plugs inmodel glow plug engines. Fig. 10 showsthe directional coupler when opened.

3.1. Coaxial jacksQuality SMA single hole jacks are usedas connecting jacks. In order to keep the

Fig. 9: Individual components of directional coupler: 18 x 18 mm. coupler,with side components, main coupling line made from round bars, cover withtuning screws and holes for SMA jacks, secondary coupling line and SMApanel jacks.


171

rf discontinuities as small as possible, theTeflon inserts, which project at the rearend of the jacks, are shortened to 0.7mm.This compensates for the discontinuitiesthat arise during the transition from the3mm. thick internal conductor of thedirectional coupler to the thinner internalconductor of the SMA jack (Fig. 11).

3.2. Final assemblyOnce all the components have been

manufactured and matched, the mainbody is assembled, with the side compo-nents and the straight internal conductorof the main line. The precise position ofthe internal conductor can be alignedagain while the side components arebeing screwed on. The four tuningscrews must be filed flat before they arefastened in the cover with a lock nut.The curved internal conductor of thesecondary line is fastened to the coverwith the two SMA jacks. A guide dimen-sion for the distance from the lower edgeof the internal conductor to the cover is5mm. (see Fig. 12).The distance between the two couplinglines can be adjusted, depending on howfar you screw the two SMA jacks into thecover. This influences the coupling. Thebest alignment was obtained with a for-ward transmission of approximately20dB at 1,200MHz. The SMA jacks arefixed externally using lock nuts.With this construction it is possible toalter the distance of the coupling lines,even after assembly. Arranging the linesat an angle is also possible.

Fig. 10: Thefully assembleddirectionalcoupler openedup; the tuningscrews in thecover can beeasilyrecognised.

Fig. 11:SMA jacks with unshortened(top) and shortened Teflon edge(below).


172

4.Calibration

The fully assembled directional coupleris best calibrated with the wobble systemwith which it is to be used. But calibra-tion can also often be carried out using afixed signal source (radio equipment)and a sensitive mW meter.Calibration is carried out with the 4trimming screws at the best directivity,i.e. with minimum reflection and adummy load of precisely 50Ω.In order to guarantee a broad bandcalibration, the directional couplershould be calibrated in at least one lowerand one upper frequency range. A broadband calibration can naturally be carried

out most easily using a wobble system.

5.Range of application

Directional couplers are designed, inparticular, for matching measurements inconjunction with wobble measurementsystems. The directional coupler is verysuitable, in conjunction with the rf syn-thesiser, up to 1,450MHz [2], for spec-trum analysers with tracking generatorsor for scalar network analysers in fre-quency ranges from 140MHz to1,500MHz.This directional coupler concept can bemodified and assembled for other fre-quency ranges by changing the length.

Fig. 12:Detailsfor assembly ofbody, cover,side compo-nents andcoupling lines.

Fig. 13:Measuring rigfor measure-ment offorwardtransmission:

Generator atport 1, detectorat port 4, port2 and port 3terminatedwith 50ΩΩΩΩ.


173

6.Set-up of measuringinstruments

First the forward transmission curve isdetermined and stored. To this end, port2 is terminated with a 50Ω dummy loadand the detector is connected to port 4(see Fig. 13). The curve determined inthis way is the start point for the subse-quent reflection measurement.To measure the reverse transmission, thedetector connected to port 3 and the testobject is connected directly to port 2(Fig. 13). The difference between theforward transmission and the reversetransmission can then be used for cali-brating the test object.

7.Literature

[1]Carsten Vieland, DJ4GC Microwavedirectional coupler with high forward /reverse ratio made from semi-rigid lines,VHF Communications 1992/3 pp 130 -139[2] Bernd Kaa, DG4RBF, HF synthesiserup to 1,450MHz, VHF Communications1998/2 pp 103 - 121 and 1998/3 pp 159 -181

Fig. 13:Measuring rigfor measure-ment ofreversetransmission(return loss):

Generator atport 1, testobject at port2, detector atport 3 and port4 terminatedwith 50ΩΩΩΩ.


174

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175

This article describes a CW modulatorfor radio applications which is simpleand uncomplicated from the rf tech-nology point of view. The call sign andadditional data are encoded and storedin an EPROM.

1.Circuit description

The pin diode modulator originates froman application note by Hewlett Packard[1]. The combination of pin diodes(BA479) and resistors offers optimalmatching (SWR) of the input and outputin both switching positions, “On” and“Off”. This guarantees good matchingfor the master oscillator and the laterfrequency multiplier stages.Because of its attenuation behaviour andthe impedance matching is dependent onthe diode current, the circuit is usableonly as an rf switch and not as anamplitude modulator. The graph in Fig. 1gives a visual representation of the at-tenuation performance. The circuit wasused as a broadband modulator for anSWR indicator [3] as long ago as 1984by Wilhelm Schürings (DK4TJ).The 3 pole high pass filters at the inputand output, attenuate the pulses from thecontrol circuit so that they do not affect

the oscillator and the frequency multi-plier. It is a Chebyshev filter with 0.01dBripple in its transmission range. With thespecified values, at 2 x 1µH and 270pF, alower limiting frequency of approxi-mately 10MHz is obtained.The control circuit consists of anEPROM with the text data and a drivestage (IC5, T2, T3). The operationalamplifier controls the switching currentfor the pin diodes (D1 to D4) through thetwo transistors: + 40mA in the switchingposition “On” and -40mA for “Off”. Thetransistor T1 acts as an inverter.The text data is stored in the 27256EPROM (IC2). This memory chip isorganised for 32k x 8 bits. Only the most

Wolfgang Schneider, DJ8ES

CW Modulator Using Pin Diodes

Fig. 1: Attenuation behaviour asfunction of diode current.


176


177

significant data bit, Q7, is used to savethe CW text in the above circuit. If thedata bits Q0 to Q7 are switched round, 8different CW texts can be stored.The CW characters are serially fetched,the EPROM is addressed through itsaddress circuit. This is achieved by aCMOS IC 4020 (IC1). This is a 14 bitbinary counter, in which the 9 low valuebits are used to address the EPROM.The programmed CW text should beoutputted at a rate of 60BpM. The clockfrequency can thus be calculated to make

available a storage area of 512 bits as273Hz. The address area begins at0000H and ends at memory address01FFH.A standard circuit using an NE555 timerIC (IC4) is used as a clock circuit. Theoutput frequency is defined by the RCcombination of 2 x 4.7kΩ and 0.47µF.One of the two resistors can be replacedby a potentiometer for the precise settingof the speed if applicable.

Fig. 3: Layout of CW pin diode modulator (printed circuit board underside).

Fig. 4: Earth side of CW pin diode modulator printed circuit board.


178

2.Assembly instructions

The CW modulator with pin diodes isassembled on a double-sided epoxyprinted circuit board with the dimensions54mm. x 108mm.. The circuit thus fitsinto a standard tinplate housing with thedimensions 55.5mm. x 111mm. x 30mm..The earth surfaces are soldered to thehousing all round on both sides.The components are inserted in no par-ticul

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