E-MAIL [email protected]/files/22679/226798354.pdf · The crystal oscillator is a standard Colpitts type as used in several previous Homebrew projects. SW1 is

19

December 2008 ♦ RadCom

EAMON SKELTON

E-MAIL [email protected]

EI9GQ HOMEBREW

THE JOYS OF 80. Back in the good olddays, a typical home made transmitterwould only have two or three active stages.One common type of valve transmitter wasknown as the MOPA (master oscillator -power amplifier). The master oscillator wasusually crystal controlled and the poweramplifier would use a high gain tetrode orpentode valve. Simple transmitters of thistype were usually 'rockbound' (restricted to asingle frequency) because it would bedifficult if not impossible to achieve goodfrequency stability if a VFO was used in sucha simple circuit.

It is possible to build a solid-stateequivalent of the old MOPA transmitter. Allthat is required is a simple crystal oscillatorand a transistor PA. There are manypublished designs for very simple CWtransmitters. My first attempt at building asimple CW transmitter for 80m [1] can beseen in Figure 1. I built this 1.2Wtransmitter in a single evening about tenyears ago. It provided many hours of fun andsome surprisingly distant DX contacts. Iused a 3.579MHz crystal from the junkboxand a CB PA transistor in the final stage.3.579MHz crystals and ceramic resonatorsare widely available because they are used

in American colour TV receivers. As there isa lot of PSK31 and other digital modesactivity around 3.579MHz, I placed a smallinductor in series with the crystal to pull thefrequency down by a few kHz.

The transmitter is keyed by interruptingthe DC power supply to the oscillator stage.A Morse key is placed in the emitter circuitof the transistor so that one of theconnections to the Morse key can begrounded. This arrangement will work withall straight keys and most electronic keyers.

Apart from the lack of frequency agilityand limited output power, there are a fewother disadvantages to using such a simpletransmitter. This simple rig doesn't haveautomatic Rx/Tx switching. I used a double-throw switch with the common terminalconnected to the aerial and the otherterminals connected to the transmitter and aseparate receiver. It is a bit inconvenienthaving to manually operate the switch everytime you want to transmit. The lack of rapidbreak-in or semi-break-in switching alsomeans that you won't hear any signals thatappear on your transmit frequency until afteryou set the Rx/Tx switch back to the receiveposition. This can be a big disadvantage fora QRP station. When you run very low

power, there is a high probability that youwill be swamped by interference from highpowered stations that can't even hear you.

Break-in operation won't do anything toprevent the interference, but at least you willbe aware of it. There is no sidetone oscillatorin this simple transmitter. This is not really adisadvantage because the signal can beheard in the station receiver.

The five element half-wave type of lowpass filter in the original transmitter is barelyadequate for the job. The class-C PA stage isa very efficient harmonic generator thatproduces harmonics at significant levelsright up to the VHF region. The secondharmonic suppression of this transmitterwas measured at -35dBc, which is probablyacceptable for a 1W transmitter. Attenuationof the third and higher harmonics was muchgreater and no measurable harmonics werepresent above the HF region. Even so, Iwould recommend more filtering if youattempt to use this circuit.

A SIMPLE CRYSTAL CONTROLLED CWTRANSMITTER. Our first project for thismonth is a simple crystal controlled CWtransmitter. We will continue to use thesimple two transistor MOPA approach forthe RF stages of this transmitter, but we willadd a couple of transistors to the circuit toprovide automatic Rx/Tx switching. Thecircuit of the transmitter is shown in Figure2, and it's not fundamentally that differentfrom Figure 1. Q3 is used to key the powersupply to the oscillator, while Q4 providescurrent for the aerial changeover relay. An external VFO input is provided so that a VFO or frequency synthesiser can beused instead of the built-in fixed frequencycrystal oscillator.

The Rx/Tx switching circuit is verysimple. When the key line is grounded, Q3and Q4 are both biased on. Note that bothdevices are PNP transistors. The currentthrough Q4 operates the aerial relay thatdisconnects the aerial from the receiver andconnects it to the transmitter output when

HomebrewAfter a few months of more advanced projects, it is time to get back to basics and build a couple of very simple projects.

XTAL

T1

R4R2

R1

C2

C3

C5

Key

C1

R3

T2

R5

C4

RFCR6

C12

C7

C8

L1 L2

C9 C10 OUT

C6

+13.8V DC

FIGURE 1: This simple circuit is all you need to put your key on 80m.

20

RadCom ♦ December 2008

HOMEBREW

the key is down. Q3 acts as a gate for the13.8V power supply to the oscillator section.When the key is pressed, the oscillator ispowered up very quickly, when the key isreleased, the power to the oscillator is cutafter just a few ms. The 220μF capacitor inthe base circuit of Q4 keeps the aerial relayenergised for several hundred ms after the

key is released. This means that thetransmitter will switch back to receive mode any time that you stop sending for asecond or so.

Keying a CW transmitter is just anotherform of amplitude modulation. The processof keying the transmitter will generatesidebands at frequencies above and below

the carrier frequency. If the keying circuitwas a perfect mixer/modulator and thekeying waveform was a perfect sine wave,the spectrum of the transmitted signal wouldcontain only the carrier and a pair ofsidebands at ± the keying rate. Thewaveform produced by a straight key is asquare wave with rapid rise and fall times.Modulating a transmitter with an unfilteredsquare wave will produce sidebands in theform of key clicks. Restricting rise and falltimes to several ms keeps key clicks towithin acceptable levels. The 100nFcapacitor in the base circuit of Q3 and the100nF decoupling capacitor in the oscillatorcircuit limit the maximum rise and fall timeof the keying voltage. Using longer rise andfall times will produce 'soft keying', which isvery difficult to read, especially at highersending speeds. It is necessary to arrive at areasonable compromise between bandwidthand readability.

The crystal oscillator is a standardColpitts type as used in several previousHomebrew projects. SW1 is a double-poledouble-throw switch that is used to disablethe crystal oscillator so that the transmittercan be driven by an external VFO. Theoutput from the oscillator is taken from the

Q1

470`

560p

1n

10k

10k

int xtal

~50p

100`10n

Ext VFO

SW1

60p

300p*

T1

Q2

100`

0μ115μH

0μ1

10μ

0μ1

1n

23T

2μ645H

25T

3μ125H

23T

2μ645H

1n68 1n68 1n

Q3

10`

0μ1

1k5

1N4148

100k

1N4148

1k5

Q4

220μ

Key

IN4148

To RX

Aerial

+13.8V DC

34T 5T

T-50-2

T-50-2 T-50-2 T-50-2

Relay

FIGURE 2: More practical 80m transmitter with T/R switching and better filtering.

PHOTO 1: Oscillator and PA stages of the transmitter in Figure 2.

21


collector circuit of Q1. The simple resistivecollector load of the original transmitter hasbeen replaced by a resonant circuit. Thisproduces fewer harmonics than the originalcircuit and improves the efficiency andspectral purity of the final stage. The PA is azero bias class-C amplifier. As the optimumload impedance for the PA is close to 50Ω,an output matching network is not required.The original five element low pass filter hasbeen replaced by a seven elementChebyshev design. I took the lazy approachand used the filter design tool in QUCS(quite universal circuit simulator) [2] todesign this filter.

CONSTRUCTION. The circuit was builtusing dead-bug style construction on a sheetof copper PCB laminate. Photo 1 shows theoscillator and PA stages. T1 is wound on aT-50-2 powdered iron toroid core. Theprimary is 34 turns and the secondary is 5turns. As 300pF is not a standard value, youcan use a pair of 150pF capacitors inparallel. A 50pF variable capacitor in serieswith the crystal can be used for tuning of theoutput frequency. The copper ground planealso serves as a heatsink for the PAtransistor. The heatsink tab of the 2SC1237

(Q2) is internally connected to the collectorso it is necessary to use an insulatingwasher and heatsink compound betweenthe transistor and copper foil. As theoperating frequency is relatively low, thechoice of PA transistor is not too critical.Most FM or AM CB PA transistors shouldwork in this circuit. If you want toexperiment with transistors from thejunkbox, look for a TO-220 transistor withan fT rating of at least 30-40MHz and apower dissipation rating of several watts. Q1is a BC547 or similar general purpose NPNtransistor. Q3 and Q4 are BC557C PNPtransistors. If a different type of PNPtransistor is used for Q4, you may need tochange the value of the 220μF capacitor toadjust the break-in delay time.

The output LPF inductors are wound onT-50-2 toroid cores. The first and thirdinductors are 2.645μH (23 turns) and thecentre inductor is 3.125μH (25 turns). The1.68nF capacitors are made from a 1nF anda 680pF capacitor connected in parallel.The aerial relay is a DPDT type with a 12Vcoil (Maplin N17AW or similar).

TESTING. For the initial tests, a dummyload and power meter was connected to thetransmitter output. The only adjustmentrequired is the tuning of the trimmercapacitor connected to T1 for maximum RFoutput. I found that the required capacitancewas exactly 330pF and since this is astandard capacitor value, I removed the300pF capacitor and the trimmer andreplaced them with a fixed 330pF capacitor.Key down output was measured at 13.3Vpeak into 50Ω, which is just under 1.8W.

The PA transistor runs cool, even after longperiods of transmission. Figure 3 shows theoutput keying waveform. The upper traceshows the RF output, the lower trace showsthe keying signal. Monitoring the signal on areceiver shows that the signal is clean andfree of key clicks. An audio sample of thetest signal can be found at http://tinyurl.com/6onnva. The output spectrum of thetransmitter is shown in Figure 4. Secondharmonic output is at -55dBc and the thirdharmonic is at -57dBc. All higher harmonicsare lost in the noise at -70dBc. Testing overa wider span shows no measurable spurioussignals at VHF/UHF.

The receive aerial output from thetransmitter was connected to a home madereceiver and the dummy load was replacedby a HF doublet and ATU. The Rx/Tx relay isconfigured so that it is not possible toaccidentally operate the transmitter into thereceiver input. Even though the receiver isisolated by the relay contacts, thetransmitted signal produces a very strongS9+ signal at the receiver. This works finewith a superhet receiver that has a goodAGC system. It might not work so well witha simple direct conversion or regenerativereceiver that is not equipped with AGC. Aftera few minutes calling CQ, I worked G4HOMwho gave me a 589 report. Since that firstcontact, I have worked several other UKstations and one German station. I havebeen quite surprised by the level of activityin the CW section of the 80m band. Even atthis low power level, I have found that a CQcall rarely goes unanswered for more than afew minutes. As I mentioned earlier,3.579MHz is not an ideal frequency for low

FIGURE 3: Output keying waveform of the 80mtransmitter. Upper trace is RF output, lower trace iskeying waveform.

FIGURE 4: Output spectrum of the 80m transmitter.Harmonic suppression is -55dBc or better.

10n

10n

100k 100k 5k1

5k1 5k1

470p

IC1

22K

Speed

ZD5V1 47μ

1k2

12V DC

BC547

5k1

DASH DOT

RA 0

RA 1 RB 0

OSC 1

RTCC

+ MCLR

4

3 5

6

14

16

17

18

FIGURE 5: Circuit diagram of the simple iambic keyer.

22


HOMEBREW

power CW operation. If you are ordering acrystal for a homebrew transmitter, the QRPcalling frequency at 3.560MHz would be abetter choice. Since I don't have a suitablecrystal and I'm far too mean to buy one, Iused the DDS from last month's homebrewas an external VFO. This allowed me to workseveral QRP stations on 3.560MHz in oneevening. The PIC that controls the DDS wasprogrammed with the appropriate 32 bitvalue so that the output frequency is exactly3.560MHz. As I haven't yet fitted afrequency entry keypad or a rotary encoder to the PIC, I am still effectivelyrockbound, but at least I am now stuck onthe correct frequency. We will take a look atkeypad scanning and rotary encoders in afuture Homebrew.

A SIMPLE IAMBIC KEYER. Our secondproject is a PIC based iambic keyer. The PICmicrocontroller is well suited to this kind ofI/O and control application. I used an 18 pinPIC16F84 for this project. although most ofthe I/O pins are left unassigned. It should bequite easy to move this project to a smaller 8 pin PIC if you need to install it in a small space.

The hardware is very simple. The PIC isconfigured to use an external RC oscillatorso that no crystal is required. The keyingspeed is varied by adjusting the value of theresistor in the RC oscillator. To operate thetransmitter, the PIC switches on a transistorwhich is used to ground the keying line ofthe transmitter. The logic of the keyer is verysimple. When the dot paddle is pressed, thekeyer generates a dot pulse, when the dashpaddle is pressed the keyer generates a dashpulse which is three times longer than a dot.If the paddles are not released at or beforethe end of a dot or dash, another dot or dashis generated. If you hold the dot paddle, thekeyer will generate a continuous stream of

dots. Equally, if you hold the dash paddlethe keyer will generate a stream of dashes.An iambic keyer will generate an alternatingsequence of dots and dashes when bothpaddles are held at the same time.

The PIC software is extremely simple.The following few lines of code are the keypart of the program.

The logic of the program is very simple.Check the dot switch. If it is pressed,generate a dot; if it is not pressed, donothing. The same sequence is repeated forthe dash switch. If neither paddle is pressed,the entire process is repeated again in anever ending loop. The code that generatesthe dots and dashes sets a flag that is usedby the iambic routine to indicate whetherthe previous element was a dash or a dot.The PIC source code and PCB layout isavailable from http://tinyurl.com/5ctl7b. I have been using this keyer since wellbefore the turn of the century. I have neverfelt the need to add any new features. Mysimple keyer program only takes up a smallfraction of the program code space on thePIC. There is plenty room to add newfeatures like memories or an auto-CQfunction. Feel free to experiment.

CONSTRUCTION. The circuit is built on asingle sided etched PCB. The board wasmade using the toner transfer method [3].The keyer circuit is shown in Figure 5. Theoriginal design used a PIC16C84 but thiswas later changed to the more modernPIC16F84. As some of the newer deviceslike the PIC16F628 and PIC16F627 are pin

compatible with the PIC16x84, it should befairly easy to use a newer and cheaper PICfor this project. The PIC was programmedusing the simple PIC programmer from theSeptember 2006 Homebrew. As the circuitonly takes a few mA, there is no need for theusual 5 volt regulator IC. A simple Zener

diode regulator is used to provide the5.1V DC supply. None of the resistorvalues are particularly critical. You canuse 4.7k resistors in place of the 5.1kresistors if they are more readilyavailable. All resistors are 0.25W carbon

film except for the speed control which is alinear 22k pot. The prototype PCB foilpattern is shown in Figure 6, thecomponent overlay is shown in Figure 7.There is no reason why the circuit cannot bebuilt on Veroboard or even dead-bug style.

My keyer is housed in a homebrewenclosure made from double-sided PCB.The original version used a pair of paddlesmade from long strips of PCB laminate anda couple of surface mount push-buttonswitches as the paddle contacts. Thisarrangement has since been replaced by alovely brass twin paddle key made byG3GMN. The finished keyer PCB is shownin Photo 2.

FINALLY. As usual, many thanks toeveryone who sent comments andsuggestions via e-mail. All comments andsuggestions welcome at the e-mail addressabove. Next month, we look at highperformance receiver front ends.

REFERENCES[1] http://homepage.eircom.net/~ei9gq/tx.html[2] http://qucs.sourceforge.net/[3] Homebrew RadCom July 2006

NEXT BTFSS RA, DIT_SW; Is the DIT paddle pressed?

GOTO DOT

BTFSS RA, DAH_SW; Is the DAH paddle pressed?

GOTO DASH

GOTO NEXT; Loop until next paddle press

FIGURE 7: Keyer component overlay.

PHOTO 2: Completed prototype iambic keyer PCB.

FIGURE 6: Keyer PCB foil pattern.

46


THE BASICS. A few weeks ago myelectricity meter was changed, the originalhad come to the end of its service life. Thatwas a shock (excuse the pun), had we reallybeen in our home for over 30 years? Thenew one is a modern electronic unit, verymuch smaller than the one it replaced. Thechange prompted me to ask a colleagueabout electricity metering. He is a consultantto the electricity industry, very knowledgeableon metering, remote signalling and meterreading. When hestarted his response, Iwas hardly prepared forthe complicatedscenario that he setout, but since we are allvery dependant onpower for our hobby, I followed his everyword.

Deregulation of theindustry, he started,has split the industrybetween theinfrastructure (these arethe network owners),the suppliers (they arethe financial guys, whobuy electricity and gasand sell it to us) andthe producers (whooperate the powerstations). As he said,even after 19 yearsthere are few peopleoutside of the industrythat seem to understand the complex systemthat has been foist upon us. He explainedthat it is the ‘supplier’ that sells us ourelectricity who is responsible for seeing thatmetering is done, but that they normallycontract this out to a third party such asmetering company that perform meteringoperations (installing and maintainingmeters), data collection (meter readers) anddata aggregation (adding up customersreadings for each supplier). Sometimes,separate companies may provide all threefunctions.

At this point he apologised for the lecture,but I had found it very enlightening.

His observations comforted me aboutmetering generally. He told me that astatistical process determines the life ofmeters whether it is an old mechanicalversion or a new electronic unit. As metersare changed they are routinely submitted foraccuracy determination and they establishcriteria for service life by the deviation inaccuracy. If it is seen to change by more

than 2% then the service life for that batchof meter is reduced accordingly. Thearrangement is enshrined in Law, theStatutory Instrument (SI) that enacts the EUMeasuring Instrument Directive. Brusselsagain, I thought!

It is the certification board working forWeights and Measures that supervises theassessment process. So, somehow they andthe manufacturer had decided that the timewas up for our electricity meter, installed just30 years ago.

METERING AND RF FIELDS. I then askedabout RF immunity requirements formetering, he was already aware of myinterest; he knows that I am a radio amateur.Once again he was able to reassure me withhis information. The new style electronicmeters have been designed for immunity inan RF field of greater than 10V/metre and10V superimposed signal on theconductors. The accuracy of the meter mustnot be adversely influenced by more than

3%. He went on to explain that thissort of signal is virtually impossibleto achieve on a power circuit.Interestingly, the Standardsestablished for immunity ofmetering systems areinternationally recognised.

My only other question was,“what would happen, when wehave remote reading and themodem line or remote power linereading system is being corruptedby local RF signals?”. Once again Iwas comforted, by the fact that thecommunications systems plannedare designed to be extremely robustand, of course, very secure.

Commenting on electronicmetering and remote readingsystems he indicated that there hadbeen trials going back as far as1984, using power linecommunication. I hasten to addthat this all takes place a very lowfrequencies and in a veryrestricted bandwidth. It is not the

broadband PLT that we have becomefamiliar with in recent times – and havelearned to hate. However, it turns out thatvery little has actually been done aboutremote reading. For some time to come wewill see a meter reader every quarter.

There is an initiative referred to as ‘smartmetering’, which will permit variable tariffsto be applied on a time of day/day of weekbasis. This is called for by the ‘EnergyServices Directive’, (yet another Brusselsintervention!), but is only defined in vagueterms, thus BERR and similar ministries

Electricity, Meters, and a few Green thoughts.This discussion was prompted by questions raised by members in recent monthsconcerning the possibility of erroneous electricity reading and interference.

JOHN PINK G8MME-MAIL [email protected] G8MMTECHNICAL FEATURE

A typical modern electronic electricity meter. (Photo: Landis + Gyr).

across Europe are trying to define thefunction of a ‘smart meter’, without fallingfoul of the European Commission’s trading laws.

In addition, there are all sorts ofcomplications, such as what happens if yourelectricity supplier installs an expensive‘smart meter’ and then you change yoursupplier. Therefore, the industry is hangingback waiting for a mandate fromgovernment before going ahead. Obviously,upgrading 30 million meters is veryexpensive and, it has to be said, that therestructuring of the industry that was startedin 1989 does not necessarily lend itself tocross-sector initiatives, such as ‘smartmetering’. In fact, the network owners weresomewhat in the dark until a few monthsago, because they were not involved unlessthere was somebody that wanted to usetheir network for PLC. In fact, until recentlyGSM (cell phone) was seen as the maincontender for comms, until they worked outthe cost! The energy retailers have taken thelead in smart metering because it representsthe suppliers, but it is rapidly becomingapparent that the responsibility is splitbetween industry sectors.

Just to complete the picture, I askedabout the possibility of an electricity metercausing interference, I had in mind thatthere is a microprocessor imbedded deepinside the guts of the meter. His reply wasthat there is very little chance of this and,with well over a million meters already fitted,it would have come to the attention of themanufacturers long ago, added to which,the meter must comply with the relevantconducted and radiated emissionsstandards.

Before we left that topic, he did bring tomy attention the fact that new electronicmeters have considerably lower impedancethan older types. The measurements arezmade across a shunt, which is a very shortmetal strip. Thus there is the possibility thatnoises arriving on the supply companymains will be more evident. Equally, noisegenerated in the home will egress moreeasily. He did not consider this to be aproblem that would afflict amateur radioactivities.

POWER IN PRACTICE – THE GREEN BIT.Having passed my questioning so far, withflying colours, I decided that it would beinteresting to watch our household powerusage on the newly installed meter over thenext few weeks. Watching an electricitymeter is rather the opposite of watching aclock, whereas the clock seems to movemore slowly when being watched, theelectricity meter seems to break into agallop. The new meter is very easy to read,not that the old one was difficult, but thepresentation and clear numbers meant that Icould quickly record the reading every day.More obvious was the flashing LED on thefront panel, which illuminates more rapidlyif more power is being consumed. It flashesat a rate of once for every 1/1000th kWhused. Not quite a disco light, but sometimesit did seem to be blinking away quite rapidly.I was quite alarmed at first, and prompted todig out some of our old bills for comparison.I quickly found that the usage was, as nearas I could determine unchanged. It mademe think, where was all that power going?

I have a small plug-in energy monitor,which I have found to be reasonablyaccurate by using known loads. So, I wentaround the house assessing power usage ofeach appliance. Well, there are some, whichare not in the ‘class A’, low energy category;our old fridge/freezer for example is greedy.Most other things seemed quite reasonable.On the lighting circuits we have a highpercentage of CFL units, or fluorescentstrips. Then I discovered the penalty ofdevices with a ‘standby’ mode. I shouldhave known better, there has been so muchabout this in the popular press, which I hadnot taken seriously enough. What shockedme was the number of these appliances thatI found: three TVs, each with digital set-topboxes – and one with a satellite receiver andhard disc recorder. That was just for ourentertainment. Further were the computers,cell phone chargers and the power linelighting control systems, etc, etc.

THE SHACK. However, the biggest shock ofall came when I assessed the shack! I tendto visit the shack most days and I leavecertain things live. The computer is mosttimes hibernated, but the packet cluster

TNC, the telephone line modem and theinterface electronics for the rig are all left on.I found five other small PSUs that wereconstantly powered on, all neatly tuckedaway out of sight and out of mind. The lowvoltage task light over my operating positionhas a transformer that is constantlypowered. All this is of course before I turnthe rig on.

The FT-1000 runs a moderate power inreceive mode, but an alarming 700W on keydown CW. Even on SSB over the period ofan evening the usage mounts up. I also liketo monitor the 2m SSB calling channel, sothe FT-847 is on power, at least when I amin the shack.

Even more alarming was the use of thelinear amplifier. I have a homebrew amplifierusing a 4CX1000A and the first thing toremember about this is that the heatersalone require 12.5A at 6V (75W!) and thebleed on the HT supply takes another 80W.Then there are the bias and low voltagesupplies and, of course, the I2R losses inthe transformers. When the amplifier ispowered on, running 400W SSB, Imeasured over 2kWh power usage,operating in a contest over a period of about4 hours. No wonder it helps to warm myshack up rather nicely in the winter. I reallywill only use the linear when it is reallynecessary – 200W barefoot is just fine mostof the time.

This almost sounds like a strongargument for a solid state linear, with adreaded SMPSU, but I still like firing up andloading the big tube.

I am certainly not advocating QRT, but along hard look at the energy usage here hasmade me think again about ‘leaving thingson until I come back to them’. I really willuse THE BIG SWITCH.

Take a look around your shack – don’tspoil your hobby by being too enthusiasticwith power saving but, in these times of everincreasing energy cost, knowing what ishappening could be well worth a little effort.

I have not told the XYL about this, shewas an accountant and I might be forcedQRT! I also noted that the little front panelLED on the electricity meter flashes morequickly when she is in the kitchen preparingour meals – I don’t seem to mind that at all!

47


RECAP AND SWANSONG. It is getting onfor three years since VK6APH and VK6VZfirst wrote about Software Defined Radio inthe pages of RadCom [1]. Since then, SDRhas arguably moved from being somethingthat only a few technically-inclined radioamateurs were interested in pursuing intothe mainstream of amateur radio.

The inspiration behind ourSDR column was one written onSSB during the 1960s by thelegendary G R B “Dick” Thornley,G2DAF. From what VK6APH andVK6VZ can recall – both beingwell over fifty years of age andinto the ‘memory overflow’ stageof life – G2DAF set out to explainthe basic principles of SSB andhow signals of this kind could bepractically generated by radioamateurs, at a time when therewere relatively few amateursin the world who couldoperate in this mode.

Once G2DAF hadachieved what he had set outto do and SSB had movedinto the mainstream, heceased his column andconcentrated writing longerand less frequent articlesabout SSB and other radiosubjects that he wasinterested in. This is the pointwhere we now are with theSDR column. Havingexplained the basics of SDRand with it now becoming acore part of amateur radio,we feel the useful life of thiscolumn is now at an end.

The good news for thosewho have enjoyed what wehave written is our plan isalso to write less frequent butlonger articles on SDR, particularly from thepractical point of view, and a compacthandbook on the subject. As G2DAF wentonto to write about constructing state-of-the-art SSB transmitters and receivers, the firstsubject we intend to tackle is the homeconstruction of digital down

conversion/digital up conversion-type(DDC/DUC) SDR transceiver hardware,using HPSDR boards [2].

VK6APH has been using a prototype ofthis transceiver hardware for almost a yearnow and in the last two months VK6VZ hasmoved onto using DDC SDR receiver

hardware, based on the HPSDR Mercuryand Ozy boards. This is being converted intoa transceiver (running PowerSDR™ software[3]) similar to that used by VK6APH.

VK6APH and VK6VZ hope to publish theconstructional details of the HPSDRtransceiver during 2009. A picture of an

experimental version of the transceiver, in its prototype case, designed and built byour friend Bob Crowe, VK6CG is shownnearby. Constructional details of a VK6CGcase for the HPSDR are also planned to be published.

TECHNOLOGICAL CHANGES. Whenwe started writing this column, all theSDR hardware designs intended foramateur radio were based on aquadrature switching detector (QSD)followed by an analogue-to-digitalconverter (ADC) that used a PC soundcard or chip. The highly popularSoftRock [4] series of receiver andtransceiver kits and the Flex-RadioSDR-1000 (and its successor) theFlex-5000 [5] all use this design.However, a second SDR hardwarearchitecture of digital down- or

up-converter (DDC/DUC) is now emerging, where ahigh-speed ADC is connecteddirectly to the receiverantenna. Examples ofreceivers using this techniqueare the Perseus [6], SDR-IQ[7], Hans Zahnd, HB9CBU’sADAT ADT-200A [8], theHPSDR Mercury and theQS1-R [9]. These twotechniques were discussed in detail in the May 2008RadCom.

VK6APH was converted toSDR through the purchase ofan early version of the Flex-Radio SDR-1000, while bothVK6APH and VK6VZ havebeen greatly enthusiasticabout the SoftRock receiverkits. VK6VZ has used variousSoftrock receiver versionsextensively on 1.8MHz and

as the basis for an out-board bandscope forhis FT-1000 transceiver. However, time hasmoved on and, right now, both us areprimarily interested in the use of DDChardware-based receivers and companionDUC transmitters, configured as atransceiver.

48

Software defined radio Looking into state-of-the-art SDR hardware

STEVE IRELAND & PHIL HARMAN


VK6VZ/VK6APH

SOFTWARE DEFINED RADIO


PHOTO 1: HPSDR Mercury-based DDC receiver and Elecraft K3.

PHOTO 2: Production version of Mercury board.

The reason for this ‘sea change’ is thatdirect sampling Analog to Digital (ADCs),such as the Linear Technologies LT2208,are now available that provide sufficient bits-per-sample so as to give blocking dynamicranges over 100dB – comparable to, orbetter than, some top-of-the-rangeconventional HF transceivers. As anillustration of this high performance, thefigures are given nearby for the final ‘alpha’version of the open-source HPSDR MercuryDDC receiver, which is about to go intoproduction by the not-for-profit TAPRorganization – as you can see, it has a BDRof 119dB, which is basically independent offrequency spacing.

In the time that SDRs have movedtowards DDC/DUC hardware architectures,the world of analogue transceivers has alsomoved on, with superb performance(assisted by digital signal processing and

filtering) offered by radios such as the Ten-Tec Orion, Icom IC-7800, Yaesu FT-2000 and, latterly, from the Elecraft K3.

The blocking dynamic range of theseanalogue radios is similar to that achieved inDDC SDR hardware and they also offerbandscope facilities and easily-updatablefirmware – two of the major advantages thatSDRs have had almost to themselves in thepast. The radios also offer a very familiaruser interface – knobs and buttons, ratherthan a mouse and a keyboard. So whywould radio users actually want to use a trueDDC/DUC SDR rather than an up-to-the-minute analogue radio with a really goodDSP back-end?

The answer is very simple and comesdown to one of the most crucial features ofany radio receiver – because a DDC SDRsounds better. There is also another reason –for those of us that use computers every day,keyboards, mice, windows and pull-downmenus are now even more familiar andintuitive to use than knobs and buttons.

A few weeks before this column waswritten, three radio amateurs got together in

the VK6APH workshop– VK6APH himself,VK6VZ (long-time low-band DXer andSDR convert) and ourfriend Fred, VK6GE (anElecraft K2 owner/fanand very long-timeradio amateur andprofessional radiooperator). On the benchwere an Elecraft K3 anda HPSDR Mercury(using the open-source PowerSDR™software), both connected to VK6APH’sMoxon Claw HF beam antenna via anantenna switch.

For 45 minutes or so, we sought outweak SSB and CW signals on a noisy14MHz band and switched the antennafrom one radio to another – just simple A/B

testing. The verdict was unanimous – whilstthe Elecraft K3 was probably the best-sounding (and performing) analogue radiothe three of us had ever used, the HPSDRsounded ‘better’ and weak signals wereconsiderably easier to understand.

Why was this so? The performancefigures for the two radios were almostidentical! The answer is actually very simpleand relates to the crystal filtering used by theK3 and most other analogue radios.

All the radio frequencies that amateursuse are covered in noise – some of which isatmospheric, some of which is ionosphericand the rest is man/machine made. Whennoise pulses/spikes pass through a crystalfilter, the phase response of the filterchanges, depending on the noise frequency.However, when noise pulses/spikes passthrough an ADC with a linear response, thephase response stays the same, because theADC treats them in a linear manner.

That’s the theoretical explanation – whathappens in practice is that on a DDC SDRany noise actually sounds mellow and easy-on-the-ear, in a manner that has to be

heard to be believed. In the case of ananalogue radio, even one stage of crystalfiltering (such as is used on the K3) isenough to cause a phase response to noisethat eventually irritates/tires the user andmakes them want to switch the radio off.

In the case of the noise response of theDDC SDR Mercury, it was balm and musicto the ears of VK6APH, VK6VZ and VK6GE.VK6VZ now trawls around 7MHz all nightlong listening to weak DX on his HPSDRMercury and laughs cruelly to himself whenother radio amateurs complain how noisythe band is…

FAREWELL. Thank you for being such agood audience, we hope you have theopportunity to listen to a DDC SDR receiverin the near future and look forward to writing more articles for you in RadCom. Vy 73 es BCNU.


49

WEBSEARCH[1] ‘Byte-ing the Bullet’, RadCom. April 2006[2] [8] http://hpsdr.org/[3] [5] www.flex-radio.com[4] SoftRock – The SoftRock receiver and transceiver

kits are collated and sold by Tony Parks, KB9YIG. Check current availability by emailing him at: [email protected]. See also ‘The Peter Hart Review: SoftRock v6’, RadCom. March 2007

[6] Perseus - www.microtelecom.it/perseus/[7] SDR-IQ - www.rfspace.com/SDR-IQ.html[8] ADAT ADT-200a - www.adat.ch/index_e.html[9] www.philcovington.com/QuickSilver/[10] www.tapr.org

The Editor would like to thank Phil and Stevefor their wonderful column which hasintroduced SDR to so many people – anddemystified it to many more. We look forwardto reading their new articles as they come up.

ERRATAIn November's SDR the captions of Figures 4& 5 were accidentally reversed, whichwrongly implied the 3rd order intermodperformance was worsened by around 30dBby predistortion, when it is in fact improvedby some 30dB.

TABLE 1: HPSDR Mercury DDC receiver performance figures

ADC overload +12dBm (preamp on), +8dBm (preamp off)

MDS (500Hz) all amateur bands (1.8MHz to 50MHz) -138dBm (preamp on), -118dBm (preamp off)

MDS (500Hz) on 50MHz via HPSDR Alexaires preamp -146dBm

IP3 equivalent +33dBm (preamp on), >50dBm (preamp off).Note the IP3 is independent of tone spacing.

Blocking Dynamic Range (BDR) 119dB. Blocking Dynamic Range was measured at 100kHz and 5kHz for 1dB gain compression with similar results.

122.88MHz clock phase noise -149dBc/Hz at 1kHz spacing.

NOTE: The BDR is set by the overload point of the ADC rather than being phase noise limited.

PHOTO 3: VK6VZ’s partly-built HPSDR transceiver in VK6CG’s prototype case.

REVISITING OLD DESIGNS. I thought Iwould make a four valve regenerativereceiver 'just like the old days', built as wellas I could so it would be stable, and use it asa test bed to try out ideas and see how itwould perform. While this is not intended asa blow-by-blow constructional article and iscertainly not suitable for beginners toduplicate, it does give some idea of thechallenges and opportunities I faced whenplaying with this venerable circuit idea.

Looking back at my oldWireless World Diary frommy school days, I can see 69country prefixes ticked off.This was achieved using atwo valve battery receiver.These stations were heardmostly on AM, as SSB hadnot yet caught on very much.The method I used was tozero beat on the carrier withthe receiver gently oscillating,whereupon the receiverwould lock on and providesynchronous detection. Thereceiver was also able toreceive SSB and CW and, inthe gently oscillatingcondition, the regenerationsetting was not critical.

It doesn’t do to live in thepast. Back then, I wouldthrow stuff together in asingle afternoon. The receiver used to be acomplete lash-up and would wobble aroundin frequency, partly as a result of mechanicalflimsiness and also due to hand capacitance.But now, unlike my time at school in the50s, I have now learned to take my time and enjoy building theequipment slowly.

SPECCING THE DESIGN. Full of optimisticenthusiasm, I originally thought my newreceiver should be general coverage, DC to

light, with plug-in coils for simplicity but withplenty of bandspread. In practice, thecoverage is from 15 to 160 metres. Thereceiver has an untuned RF stage to isolatethe detector stage from the antenna and soavoid frequency pulling caused by theantenna swaying in the wind. I rememberedthat a single audio stage would not usuallybe enough to drive a loudspeaker, so I ended up with RF, detector, AF and output(Figure 1). Some time ago, I tried the little

6.3V valve type 6AK5, a hot VHF pentode,followed by a transistor audio stage. Thisworked very well with 6V for the heater plusHT. But this time I thought it would beinteresting to use 1.4V battery valves. Ifanyone is thinking of experimenting I wouldnevertheless recommend 6.3V valves, usingbattery power, as they have much betterperformance than the lower filament voltagetypes. It is easy to obtain 72V or so HT usingPP3 batteries in series. The heater supplycan use rechargeable batteries (with a

suitable current limiting resistor) or a D-sizealkaline battery.

PHYSICAL DESIGN CONSIDERATIONS.My original idea was to have the valvessticking out of the top, to provide visualinterest. But I wanted to avoid handcapacitance effects. The final constructionwas in a simple interlocking-U equipmentcase from Maplin, but after many hours'construction, I realised that a quicker way to

build the equipment wouldhave been in the traditionalway with a chassis and front panel.

The receiver is shown inPhotos 1 to 3. The tuningcapacitors, both with 6:1reduction drives, weremounted in a box, like atypical VFO. I used bandsetand bandspread capacitors of50 and 5pF respectively. Thebandspread was furtherreduced in coverage to 1% ofthe bandset, about 0.5pF,using the circuit shown inFigure 2. This circuit givesfairly uniform bandspreadaction and allows reasonabletuning of CW and SSB.

It is not possible with aTRF receiver to have uniformfrequency coverage on all

bands. Also, remember that when tuning anSSB signal on – say – 14MHz, the tuningcapacitor only varies by thousandths of apicofarad. This means we must be carefulwith mechanical rigidity. The coils werewound on old plug-in formers made byDenco. I found it very difficult to make sucha former at home, but it might be possible towind the coils on DIN plugs. [SeeHomebrew, December 2007, p22 – Ed.]The cathode tap was 1/6 of the turns,measured from the ground end. Use a tuned

86

Experiments with aRegenerative ReceiverSeeing some of the old radio components at a Kempton Park Rally gave me a burst of nostalgia. I soon found myself cutting holes in aluminium and collecting likely-looking slow motion drives and tuning capacitors…

DAVID SUMNER


G3PVHTECHNICAL FEATURE


PHOTO 1: The completed valve TRF receiver. The knobs (from left to right) are main tuning,bandspread, gain and regeneration. The toggle switches are for the 20dB attenuator andSSB/CW filter. The aluminium cylinder holds the band coil.

winding of roughly ¾ turn per metrewavelength at maximum tuning capacitanceand a coupling coil of about half this.

I have found that the 1.4V filament valvesare incredibly microphonic, so I used a foamstrip to mount the sub-chassis that carriesthe valves. This creates a mechanical lowpass filter, where the mass of the chassisrepresents a shunt capacitor and thespringiness of the foam a series inductor.This greatly reduces the microphonyproblem, though at a slight expense infrequency stability.

To avoid hand capacitance effects,where the frequency changes as you bring your hand near the receiver, it isnecessary to screen the circuits. It is alsonecessary to use an RF isolating stage,because your body provides an earth thatwill shift the antenna frequency. If you donot use an isolating stage, start off with very loose coupling to the antenna, to ensure oscillation.

If not using an isolating stage, a goodway to couple the antenna is via a trimmercapacitor of a few pF; this capacitor isactually carrying out an impedancetransformation as in an ATU. If you use aresonant antenna, it can cause dead spotsin the oscillation, so a random lengthdoublet is a better choice. A coaxial feeder ishelpful, but preferably not too long as thiscreates many resonances, which can alsocause trouble. If you use a wire antenna, justa simple short wire of 7m or so is a goodchoice. The isolating stage removes all theseproblems and I have not noticed any crossmodulation problems caused by it.

The isolating stage originally used a

resistor in the grid circuit, but I discoveredthat this was causing demodulation ofincoming signals by grid leak action, so to beon the safe side I changed to a ferritetransformer, which has nearly zeroresistance. It steps up the antenna voltagetwice, providing 6dB gain for nothing, so tospeak. The grid can be switched to -7V bias,

which provides some 20dB signal reductionfor strong signal operation. For 136kHz youmight also need to use a resistor in the grid.

FILTERING. I tried out different audio filtersfor SSB and CW, and these reducedbackground noise greatly. However, I haveyet to meet a filter that can beat the humanear. A good way to design filters is the

interactive computer program called ELSIE,on the ARRL Handbook CD. The inductors I used were little transistor transformers, but unfortunately the Q turned out to be very poor, increasing the losses andreducing the selectivity.

OUTPUT. The receiver gain is ample forearphones but on most amateur signals isnot enough for a speaker. I suggest using acomputer style battery powered extensionspeaker/amplifier or an additional audiostage if you want to use a loudspeaker.

VALVE ISSUES. The directly heatedvalves create a few problems. A choke isneeded in the filament circuit for theHartley oscillator, and the bias for theoutput stage has to use the back-biascircuit as shown. This also provides theswitched gain reduction bias for the RFamplifier. The other valves operate with no applied bias.

If you use a 6.3V valve the gain will bemuch greater. In this case it is a good ideato always use a 47Ω resistor in series withthe grid connection, even in oscillatorsand audio amplifiers, to avoid VHFparasitics. You might also find it useful totry a 30pF trimmer for the detector grid

capacitor. In effect, this taps the grid downthe tuning coil, giving lower gain and betterselectivity. Too much gain can make forregeneration problems, manifesting as asudden plop into oscillation rather than agentle transition. If using a triode, do not usean inductive anode load as it will causethreshold howl. It is only necessary todecouple the anode supplies for each pair of

V1IT4

2:1Toroid

10n

1n

ANT.

10n

4k7 Coil = 3/4 turn per metre wavelengthTap = 1/6PRI = 50%Dia. = 1cm

4k7

10n

50p

10p 3p

5p

3p

V2IT4

1M

100p1n

Longwire

100μHToroid

10n

100k 47k

Regeneration

39k

10n

1M

1n

4k7

47μ

V3IT4

10n

**1M 100k

10n

S3A S3B

150p

10n

1n 1n

10n

VOL100k

SSB SSB

CW CW

LT44 *

LT44

V43A4

8H

*

#

2.4VA9V

Mainstrans.

L.S. orcomputerL.S./Amp.

Hi ZPhones

0μ1

47μ

8 X PP372V

620`

S1A

Be carefulto wire

this areacorrectly

HP21.5V

S1B

+

S2Normal

-20dB

Attn.

Bandset Bandspread

Filament rail

Bias rail

Optional filters** Screen resistor shouldproduce about 50% HTon the anode of V3

* Inductors are LT44transistor transformers(Maplin HX82D)

# Output transformeris 240V - 9V mainstransformer such asMaplin YN15R

S1 = On/OffS2 = 20dB Atten.S3 = Filter CW/SSB

1

2

34

5

6

7

f(-)

a

g2 f(-)

g1

f(+)

1

2

34

5

6

7

f(-)

g2

a

g1

f(ct)

f(+)

a

Pin-outView from underneath

B7G holder

4A34TI

Bias-7V


87

PHOTO 2: Interior view. The whip antenna is suitable forbroadcast listening only. The valve sub chassis is mountedon foam.

FIGURE 1: Circuit of the experimental four valve TRF regenerative receiver.

88

TECHNICAL FEATURE


audio valves; however, three valves togetherwill oscillate.

It might be thought that an FET version ofthe receiver would be just as good, but myexperience of solid state regenerativereceivers is that they're actually not verygood, and I found that broadcastbreakthrough was greater.

RESULTS. When I switched on, the receiverworked right away. I was very surprised bythe quality of the short wave broadcastaudio. As there is no AGC, the noise does notrise between words, as is often the case withcommunication receivers, and the signal tonoise ratio often seems to approach that ofFM broadcasting. The background is silent.Maybe we have all forgotten that modernreceivers present us with continuoushiss, exacerbated by the AGC action.The oscillator was quite stable,suitable for SSB, and did not driftparticularly, as a result of the tinyamount of heating. However, I feel the1.4V valves are generally a bit flimsyand vulnerable to mechanical shock.

It was a fascinating pleasure totune the amateur bands with thereceiver. When I started hearing DXand could hear a pin drop, Iremembered an old colleague (whowas 'second across' to ZL) telling methat the 40m band was then emptyand sounded just like velvet, only afaint rushing noise being heard. Thesensitivity is very good with thesereceivers and you can easily hear thenoise from the RF amplifier andantenna.

IN USE. When first tuningthe band, I use the wideaudio filter so I do not misssignals. Set the regenerationcontrol so the receiver gentlygoes into oscillation toprovide BFO action. If youmeet a strong signal, you canincrease the regeneration togive stronger oscillation, butthe setting will usuallychange very little across theband. For AM reception, youcan adjust for zero beat,whereupon the receiver willlock on to the signal and providesynchronous demodulation. This is usefulfor weak signals which are affected byselective fading. Alternatively, you canreduce the regeneration until the set juststops oscillating. In this condition there is a

Q-multiplier action giving good selectivity.There are a number of good and bad

points with these receivers:

1) They are simply made and givereasonable results, especially on the

lower frequencies, with just a singletuning coil.

2) In common with simple direct conversionreceivers, there is no single-signalselectivity, ie signals either side of theBFO are heard.

3) If high stability is required, as much C as possible is needed across the tuned circuit, and this limits the tuningrange. It is asking too much to tune SSBat 30MHz near the minimumcapacitance setting of the tuningcapacitor. For broadcast reception this is not such a problem.

4) Calibration cannot easily be made thesame on all bands, but good bandspreadis easy to achieve, using a logging scale.A good plan is to make the bandspread1% of the bandset capacitance, andrestrict coverage to a maximum of about 8MHz per coil.

5) Breakthrough from strong broadcastsignals is a problem on 7MHz SSB; the20dB reduction helps eliminate this, anda wave trap also seemed useful. I have met the same problem with direct conversion receivers, even withbalanced mixers.

6) Compared with a direct conversionreceiver, there is no RF microphony(noises due to phase modulation of LOleakage when you touch the case orantenna), no 50Hz hum and arequirement for less audio gain. I thinkthe broadcast breakthrough is less. Thereceiver can receive AM, which a directconversion receiver cannot. Set againstthis, there can be slight pull-in at 14MHzand above, which can be avoided byusing at least 20pF tuning capacitance.

7) There are no obvious spurious responsesand birdies as found with superhetreceivers. In theory, signals at twice thetuned frequency could be demodulatedby harmonic mixer action but this doesnot seem to be a problem in practice.

FINALLY. I must mention the real enjoymentto be had from making and using a receiversuch as this and winkling out those weaksignals. The receiver reminds me of a prewar sports car - not as fast as the modernone and with a crash gearbox needing a bitof skill, but twice the fun.

SUGGESTED PARTS SUPPLIERS. Manysuppliers can provide parts for radios such as this, including several advertisers in RadCom.

Slow motion drives etc: MainlineElectronics, www.mainlinegroup.com.

Valves: Rod Burman,www.valves.uk.com; Wilson Valves (Jim Fish), [email protected].

Aluminium, screws etc:www.mechatronics-online.co.uk.

5pVariablebandspread

3p

3p

50pBandset

PHOTO 3: Interior view. By using quite a large case, internalaccess is reasonably good and there is room for modifications.

FIGURE 2: Bandspreading circuit.

kHz

dB rel. to peak

0

-10

-20

-300 1 2

SSBCW

3 4300 600

FIGURE 3: Audio filter responses.

92

IAN WHITEC/O RSGB HQ

E-MAIL [email protected]/G3SEK

GM3SEK IN PRACTICE


PAINT REMOVAL TOOL. Paintedmetalwork is the enemy of reliable earthcontacts and good RF shielding. Since manymetal enclosures and equipment covers arepainted all over, inside and out, we needsome means to remove the paint fromaround screw holes and other attachmentpoints. The lead photo shows a simple toolfor doing this.

It is obvious why you need to remove thepaint around a solder-tagged earth contact:otherwise, the only bared metal is aroundthe inside of the screw hole, which gives avery unreliable contact through the screw.Photo 1 shows what a difference a properpaint removal tool makes. Now there is alarge, flat contact area directly between themetalwork and the solder tag, and the screwis strictly a mechanical fixing to keep thosetwo surfaces solidly pressed together.

For RF shielding, the metal covers ofmost equipment do not need continuouscontact all the way along their edges. Youonly need to guarantee good contacts at aseries of closely-spaced fixing points. If thespacing between these points is muchshorter than the wavelength of the RFenergy that you’re trying to contain (or keepout), then the shielding will probably beadequate for all but the most criticalapplications. For example, good HFshielding can often be achieved using justenough screws to hold the covers on – butonly if there is good contact between bare

metal surfaces at every one of those fixingpoints. Unfortunately, there are countlessexamples of metal covers for RF equipmentcompletely coated in a durable scratch-prooffinish that allows no electrical contactwhatever. There’s an obvious conflict here,for unless the fixing screws do somehowmanage to scratch through the paint, thecovers may be completely ‘floating’,connected to the underlying metalwork bynothing more than capacitance through alayer of paint. Equally bizarre are the so-called ‘CE compliant’ PC cabinets that havespring fingering around all the openapertures, making firm contact onto… paint.

The reason for this commercial poorpractice is a combination of two factors. Oneis ignorance and the other is cost-cutting.Ensuring that certain areas of metalworkremain free from paint is much moreexpensive than painting the whole thing.Even when spraying only from one side, it needs care and attention to spray theedges while preventing the overspray fromdrifting onto the inside surface. Photo 2 isan example where this has been doneproperly and you can clearly see themasked-off areas around the screw holeswhere the black overspray mustn’t go. Butas you’ll see, even this item has beenimproved a little…

Photo 3 shows how the paint removaltool in the lead photo was made, by grindingthe conical point off a countersink cutter.

These cutters are meant to fit onto anordinary drill bit, allowing the hole for awood screw to be drilled and countersunk ina single operation. Sets of drills and cuttersare readily available and even ‘pound shop’quality will be good enough for thisapplication [1]. The original cutter should beground off to a flat, square end so that itdoesn’t wobble as you rotate it, but it isn’tdifficult to judge this by eye. (If you don’thave a bench grinder, these cheap cutterscan even be filed down by hand.) You willnotice from Photo 3 that the raked cuttingedge has completely gone, leaving a bluff90° edge that will scratch the paint offwithout cutting into the metal beneath [2].Do not try to drill a hole and remove thepaint in a single operation, for you’ll findyourself making an oversize hole. Instead,drill the hole first and then use the toolcarefully by hand, attached to a smallscrewdriver as shown in the lead photo. Itobviously works faster when used in anelectric drill, but that also makes it far tooeasy to dig into the metal. Remember thatyour aim is to remove all of the paint butnone of the metal.

What you’ll probably find in practice isthat it removes almost all of the paint. Beforeyou go too far and dig into the metal, youmay find it useful to switch to the differenttechnique shown in Photo 4. Here we use astainless steel scratch brush [1] to removethe last traces of paint, using a large washeras a ‘stencil’ to avoid damage to the rest ofthe rest of the paintwork. The scratch brushon its own is too slow for removing mosttypes of paint, but it’s ideal for finishing off.The result will be a nice clean surface and asolid electrical contact under every screw.

Other ways to remove paint includemasking off the areas to be protected andusing a shot blasting gun at a car body shop.A chemical paint stripper followed by wirewool might work, but take care to preventleakage through the screw hole to the othersurface. To remove a strip along the wholelength of an inside edge, you could clamp ona strip of wood and then use a wire brush.These methods all require correspondinglygreater levels of hand and eye protection andcarry greater risks of making a mess of thewhole thing.

In PracticeBack to the normal two pages this month, and I'm rounding up topics and correspondence from previous columns.

93


SMD ROUNDUP. Comments are stillcoming in about September and October’sarticles on soldering surface mount devices.Sam Jewell, G4DDK, points out that coredsolder as thin as 0.35mm/28swg is readilyavailable and is perfectly good for solderingchip resistors and capacitors in thetraditional way. The relative amounts ofsolder and flux are just about right formaking simple, straightforward joints. But,as Sam points out on his own website [1],only 0.35mm/28swg solder is suitable –0.7mm/22swg solder is guaranteed to makea mess! If any further rework is required, orif you want to solder large ICs, then the ruleschange: you must be able to applyseparately controlled amounts of flux andsolder. Rapid Electronics offer a small SMDrework kit that includes both the 28swg fluxcored solder (tin/lead/silver alloy) and asyringe of jelly flux [1].

Bob Henderson, 5B4AGN, is anotherexperienced SMD constructor and he prefersthe jelly or paste flux because it can help tohold components in place. ICs with veryclose-spaced pins can very easily slip offtheir narrow solder pads, so the paste fluxcan help keep the package centred on top ofthe pads while you’re tack-soldering the firsttwo pins. This suggestion also appears inthe excellent YouTube presentation on‘Surface Mount Soldering 101’ –recommended viewing, with something to learn for even the most experienced SMD constructor.

Geoff Darby, G7RTC, has a differentsuggestion for holding very tiny chipcomponents where lack of space may makepositioning and initial solder-tackingvirtually impossible with tweezers in theway. He handles the component using a tinyball of Blu-tack on the end of a smalljeweller's screwdriver. He also has atechnique for tacking the component intoplace using a minuscule drop of supergluebetween the solder pads on the PCB.However, the droplet of glue should be assmall as possible, carried on the tip of adarning needle, and good ventilation isessential to carry away toxic fumes when thecomponent is heated. Special heat-curingadhesives are available to avoid thesehazards and are widely used in production-line SMD manufacture.

Greg Smith, ZL3IX, recommends dentaltweezers with tips that are encrusted indiamond dust. “Sounds expensive,” he says,“and they do cost more than the standardtweezers, but are not as much as one wouldexpect. They really stop those 0805 and0603 parts from becoming airborne!”

Several people agree that it’s useful tohave two soldering irons on the go, one forfine work and a more powerful one forheavier work such as soldering togroundplanes. David Stockton, GM4ZNX,strongly recommends learning how to solderquickly using a relatively high tiptemperature and not letting the iron linger onthe joint. Although the initial thermal shockis greater, the overall stress on thecomponent is less than if it had been heatedfor much longer with a cooler iron. PaulHigginson, GW8IZR, also warns againstthermal shock and the risk of building

mechanical stress into components as theboard is heated and cooled. For example,when soldering a chip capacitor to a thintrack on one side and a solid groundplaneon the other, make the heavy joint to thegroundplane first and then let the board cooldown before soldering the other end.

David Stockton is a great fan of the RFheated Metcal irons but Phil Cooper,GU0SUP, says the Spanish JBC irons havenow raised the bar, in terms of tighttemperature control, fast response and arange of useful practical innovations [1].The prices of both the JBC and the Metcalrange are beyond our means as amateurs,but they do sometimes turn up at bargainprices. We can also pick up useful ideas thatcan ‘trickle down’ to our lower-budgetamateur level. For example, the JBC ironshave a feature that senses when the iron isresting in its stand, and automaticallyreduces the temperature by about 50°C toextend the lifetime of the tip. Good idea – ifyou already have a thermostatic iron with aconvenient temperature control, simply turndown the temperature when the iron isn’t inactive use, and then you won’t need toreplace old corroded tips so often.

In later columns I will come back to thetechniques of hot-air soldering and thebroader topic of de-soldering and removingSMD packages.

SEASON’S GREETINGS. As ‘In Practice’enters its 15th year, it’s time once more tothank everyone who has contributed thequestions, comments and criticisms that arethe lifeblood of this column. Season’sgreetings and very best wishes for 2009.

PHOTO 1: Bared metal makes a solid earthconnection.

PHOTO 2: Contact areas should be masked offbefore paint spraying.

PHOTO 3: Grind a woodworking countersink cutterto leave a flat, square end.

PHOTO 4: Remove final traces of paint using astainless steel scratch brush.

NOTES & REFERENCES[1] For more information, follow this month’s links from

the ‘In Practice’ website.[2] This is also why machining tools such as spotface

cutters and counterbores are not suitable.

82

BY GILES READ C/O RSGB HQ G1MFGBOOK REVIEW


Book reviewEverything you ever wanted to know about six metres and antennas.

The 6 Metre HandbookBy Don Field, G3XTT / NK1G

Don Field is an accomplished writer, familiarto most members through his HF column inRadCom but he has also written many otherbooks and articles over the years. Now hehas turned his considerable skills to Six. It isnot for nothing that six metres is called 'themagic band'. In 1944, the ARRL GeneralManager said of 6m that its performance is"erratic, unpredictable, unreliable andunexpected, a band whereanything can andgenerally doeshappen … its veryeccentricities give ita peculiar charm forus". And so it hascontinued to this day.

The book clearlydescribes how Sixshares properties ofboth HF and VHFpropagation, andbegins with anentertaining andinformative review ofhow we got where weare today. Then Donmoves on to examinethe equipment andantennas that arecommonly availabletoday, with some reallyuseful hints and tips onhow best to get on Six. There's a substantialchapter on propagation, which explains the

various common (and not-so-common)ways the band bounces around the planet.And the Operating chapter contains muchvaluable information about the operating

wrinkles that let you getthe very best out of thefrequencies.

I was quite taken bythe chapter on weaksignal working. Thisconfirmed that while CWhas its place, modernweak signal operating isdone using computersand the revolutionaryWSJT Weak Signal (by)Joe Taylor software,which can have a 10-15dB advantageover CW. I can imaginehow much it hurt Donto write those words,as his fist is well-known on the bands!

The book isn't justcentred on Britain or even Europe:

it has a world-wide perspective anddiscusses the band from several differentviewpoints. This not only means that it is

useful to all amateurs, but it also forms anuniquely comprehensive reference work ifyou're planning to DXpedition and want toknow what to expect in foreign climes. Theauthor's wide experience of operating fromvarious locales really shines through as heshares his knowledge.

It's worth pointing out that there is also achapter dedicated to 4m, the 70MHz band.This shares certain properties with Six and,while the band is by no means universallyavailable, this is one of the few good booksthat even mentions the band. I wassurprised to learn that Somalia has a500kHz allocation on 4m, with a powerlimit of 3kW. There's even a note that thisisn't a typographical error!

I learned a lot from this book andcommend it to anyone who has ever thoughtabout getting active on the 'magic' band.

The 6 Metre HandbookISBN 9781-9050-8647-4176 pages, 174 x 240mmNon-Members' Price £13.99Members' Price £11.89

83


Antennas for VHF and aboveIan Poole, G3YWX

"For something so simple to check out, andoften so simple to make, an antenna isremarkably difficult for many people tounderstand. That’s unfortunate, because formany radio systems the antenna is one ofthe most important elements, one that canmake the difference between a successfuland an unsuccessful system." So starts thiswonderful referencework, which subtitlesitself "Understandingpractical antennasand design". I think itmore than succeeds inmeeting this aim.

Joel's writing style isvery easy on the eyeand easy on the brain.He doesn’t fall into thetrap of assuming thatthe reader is a total idiotand put little happy-smiley characters everyother paragraph; insteadhe uses just the rightnumber of line drawings,graphs and photographsto illustrate his points. All the drawings areto a very high standard and clearly conveytheir messages to even the novice reader.

One of the things this book does really

well is to demystify antennas and theirsurrounding theory. Nothing is really

presented as frightening, or 'toocomplex for you to understandright now' – instead, the chaptersare progressive and build on yourknowledge stage by stage. Mostchapters end with a little one ortwo paragraph summary of whatit has told you and – usefully – asmall handful of reviewquestions to help you makesure you've actually understoodthe key points. This alonemakes it a really usefullearning tool. I did find some ofthe treatments a littlelightweight, but the fact is that

I was introduced to antennaconfigurations I'd not met before, not evenconsidered. Once I'd seen them in the book Iwas able to go out and find out a little moreif I wanted. Before I saw this book I didn'teven know what questions to ask; after, Icould look things up on the internet to my

heart's content. By the time you've read your way through

this book you will definitely know a heck of a lot more about antenna theory andpractice than you did before. You'll want to keep it handy, too, because there are lotsof little things in it that you can try on a rainySunday afternoon when the bands are closed.

The 27 chapters and two appendices ofthis book cover antennas with far moredetail than its 'basic' title suggests. There'severything from the Right Hand Rulethrough to advanced computer modelling ofradiation patterns, dipoles through tothousand-foot parabolics, all in one place.It's a treasure-trove of information, allcalculated to be easily accessible. I found itvery useful and informative.

IBSN 978-0-87259-999-4216 pages, 216 x 279mmNon-Members' Price £24.99Members' Price £21.24

It’s very difficult to review a book that has somany things done well within its pages.What bits do you pick out for particularattention? This was certainly the case withthis book.

One of the most interesting things aboutantennas is trying tobuild your own. It givesyou the chance tounderstand more abouthow they work, toexperiment and perfect adesign that suits you andyour location.

The theory in this bookis punctuation with plentyof illustrations making iteasy to understand andstarts from basic principlesof electromagnetic wavesand propagation. So, if youare new to amateur radio orantenna building, you’ll endup with all the buildingbricks needed to have a goodgrasp of antenna theorybefore you start constructing your antenna.The second chapter about feedlines isequally as important as getting themaximum amount of energy transferred

from the antenna to the feeder – or viceversa – is vital. It is important tounderstand impedance andstanding waves if you are tocreate an efficient antenna. Thedescriptions of the differenttypes of cables – andwaveguide – available and theiruses makes interestingreading. And if you are unsurewhich connector you shouldbe using with which type ofcable, you’ll certainly have theanswers at your fingertipswith this book.

There are chapterscovering six different types of

antenna; the dipole, Yagi, cubicalquad, verticals, wide-band antennas andmicrowave antennas. The chapter oncubical quads was interesting as there areinstructions for both internal and externalantennas, with everything you need to knowfrom materials to dimensions. The

illustrations are very clear and thephotographs give you an idea what thefinished article will look like in your backgarden. Having step by step instructions will help even novice antenna builderscomplete their projects. Experiencedantenna constructors aren’t left out thoughas there are some more complicated designs to try too.

If you are interested in trying your handon the microwave bands there is lots ofadvice on building parabolic reflectors andhorn antennas. Again, with step by stepinstructions, including templates, there arehorn antennas designs for 10 and 24GHz.

This book should keep any antennaenthusiast busy over the winter months,giving you a wealth of antennas for your station.

IBSN 9781-9050-8645-0144 pages, 240 x 174mmNon-Members' Price £11.04Members' Price £12.99

Basic Antennas By Joel R. Hallas, W1ZR

44


NEXT BIG THING. After a quick scanaround the repeaters and reflectors it’s clearthat D-Star is developing nicely with agrowing base of knowledgeable users. I’velistened to many QSOs where amateurshave been very happy to help newcomersget going and even arranged visits to helpthem setup their rigs properly. This is greatnews and seems to have rekindled a touchof the amateur pioneering spirit, which mustbe good for the hobby. D-Star repeatersaround the world are linked via the Internetand can provide transparent callingirrespective of distance.

DV DONGLE - WHAT IS IT? Essentially, it'sa way of getting in to the D-Star Internetbackbone without using a radio. D-Star usesa proprietary voice encoding system knownas Advanced Multi-Band Excitation (AMBE).Voice encoding and decoding is done by adedicated AMBE chip, present in all D-Starradios. The DV Dongle uses the sametechnology but with a USB interface so itcan connect to the USB port of your PC. D-Star signals from the Internet via the DVDongle for decoding/encoding asappropriate. Getting on air is achieved viathe supplied software and a standardInternet connection.

GETTING STARTED. The DV Dongle has asmart translucent plastic case with a shortUSB lead for connection to the computer.Also in the pack was a CD-ROM withsoftware and instructions. Whilst the

software was fine, the instructions in thereview copy were all in WebArchive formatthat can only be read on a MAC! However,all the instructions were also available onwww.dvdongle.com so I could get all Ineeded from there.

The first stage of the installation was toload drivers so the PC could see the DVDongle. For most systems and certainly my Vista setup, this was handled by thewizard that kicks in when you plug a newdevice into the USB port. If you have anyproblems with Windows not being able tofind a driver you just need to point thewizard at the supplied CD-ROM to pick-upthe necessary drivers.

If all is well with this stage of the setupthe DV Dongle’s green LED will be gentlypulsing (as mine was). The next stage is tocopy the DV Tools software to the PC. Withthe files transferred I ran the DVTOOL.BATbatch file that starts the software. Thisstarted a Command window and the mainDV Dongle control panel where I couldcomplete the testing and setup processbefore going on air.

D-STAR NETWORK & CONNECTIVITY.Whilst you can use D-Star for conventional1:1 simplex transmissions, the real power ofthe system comes from working through theever growing number of D-Star repeaters. Inaddition to providing a convenient point foroperators to work through, many D-Starrepeaters have a link into the main D-StarG2 network. This link enables calls to be

routed over the Internet to other repeatersanywhere in the world. Routing is controlledfrom the originating transmission byspecifying the repeaters to use in specialfields of the set-up message. This facility isextremely powerful and enables high qualitycommunications over vast distances.

One powerful addition to D-Star is theuse of Reflectors. These provide aconferencing facility where many repeaterscan be connected together so that everyonecan talk to each other. This has reallyopened up the system, particularly in theseearly days whilst the system is still growing.At the moment there are a total of 12 activereflectors globally and these are organisedby country. For example, Reflector 5A islocated in the UK and used to link UKrepeaters together. You will find that manyUK D-Star repeaters are left connected to the5A reflector as the default setting. As aresult, when you make a general callthrough a UK repeater, that call can berelayed via all the other connected repeaters.In these early days of D-Star that’s a realboon as it makes it easy to find someone totalk to. Let’s just take a quick look at the keyelements of the D-Star network:

D-Star Repeater: A locally providedVHF/UHF repeater that can operate on someor all of the following bands: 23cm (port A),70cm (port B) or 2m (port C).

Gateway: The vital link that connectsrepeaters to reflectors and the K5TIT TrustServer. The gateway runs on a Linux serverthat communicates via an Internetconnection from the local repeater orreflector to the D-Star G2 network.

CALL SETUP PROTOCOL. Although thisrelates to the use of a D-Star radio you needto understand this to appreciate the correctuse of the DV Dongle. There are four keyelements to the call setup that are sent at thestart of every transmission - MYCALL,URCALL, RPT1 and RPT2. I’ve shown anexample of a call in Figure 1. Here you cansee that MYCALL is set to your own callsignbut URCALL is set to CQCQCQ. This is themost common setting and is used for allgeneral calls. RPT1 is the local repeater that

DV Dongle ReviewWant to try D-Star but don’t have a local repeater or maybe you can’t erect a VHF antenna? The DV Dongle reviewed here could provide the answer.

MIKE RICHARDSC/O RSGB HQ


G4WNCPRODUCT REVIEW

The DV Dongle comes in a smart blue translucent case with just a single LED indicator.

45


you are connecting to and the suffix Bindicates that you are operating on the70cm port. The RPT2 port is the one thatseems to cause the most confusion. In theexample, you will see that I’m connecting tothe same repeater but using port G, which isthe gateway. This tells the repeater to relaymy call into the G2 network. Without this,you will only be heard on the local repeater.One of the common frustrations fornewcomers is not managing to get throughto the reflector and the cause is usually themissing second repeater with the vital Gsuffix in the right place. All suffixes have tobe in the eighth position to work.

LISTENING WITH THE DV DONGLE.With the software up and running itwas time to take a look around and seewhat could be heard. The old amateurrule of listen first certainly applies to D-Star and I would recommend that allnewcomers do this to make sure youunderstand the operating procedure.Making the initial connection to the G2network was easy as the DV Tools softwareautomatically retrieves a list of all the linkedgateways and presents the result in a drop-down menu. All I had to do was select arepeater or reflector to check for activity. Foreach connection you also have a choice ofmodules but when starting it’s best to select* as this lets you pick up calls from any ofthe modules. Once this has been set, I justpressed Start and the software completedthe connection and started retrieving anddecoding the data. In addition to providingthe decoded voice signals, the software alsodisplays the call set up parameters Idescribed earlier so you can see what’sgoing on. Rather than connect to a repeater,the best place to start is one of the morepopular Reflectors, either 1C for the US or5A for the UK.

For the review, Reflector 5A was my firstchoice and I was soon copying stations withno problems. The resultant audio was veryclear and the occasional reversion to R2D2type audio was invariably due to a poor RFlink into the repeater. The latest DV Dongle

Tool software includes a useful history tabthat lets you keep track of activity on thechannel by logging the date/time and callsetup data. It was quite an eye opener tohear mobiles in Kent chatting to othermobiles in Dallas!

TRANSMIT TIME. Before you can use theDV Dongle to transmit you have to registeryour callsign on the G2 D-Star network. Thisis both free and very easy as all you have todo is contact your friendly D-Star repeaterkeeper. Depending on their availability, youwill normally find registration is completewithin a day or so. Although you apply via a

single repeater group, the registrationpropagates through the G2 network and yougain access to all the D-Star nodes. This isgreat for DV Dongle users as it means youcan directly connect to any repeater orreflector on the network!

Next step was to connect a headset. It’swise to check your audio levels before yougo on air and the DV Tools software includesa useful loop-back test so you can listen toyour own audio. Loopback tests include asignificant processing delay so you hear youraudio a while after you’ve spoken.

You do need to make sure you enter yourcallsign into the MYCALL field before youstart. One very important point you need tobe aware of is that the DV Dongle doesn’tinclude an option to add the RPT2 field,which means that you can’t connect to arepeater and have your transmission routedthrough the gateway. Therefore, if youconnect to a repeater and that repeater islinked to a reflector, your signal won’t beheard on the reflector – very irritating foranyone trying to listen! The solution is

simple – you use the drop down gateway listto make your connection direct to thereflector. That way you will be heard on allthe connected repeaters. It is also bestpractice to use reflectors from your DVDongle due to limited upstream bandwidthon D-Star repeater ADSL circuits.

For my first transmission I connected toReflector 1C (USA) and put out a CQ call.After a couple of calls I was delighted to geta reply from Bill, AE5BK, who was in Dallasdriving home for lunch. Audio quality bothways was extremely good – a prettyimpressive start to DV Dongle DXing! Audioreports continued to be very favourable

throughout the review. One importantoperating tip: keep the PTT operatedfor about a second after you finishspeaking to allow the DV Dongle tofinish processing your audio.

The only real limitation of the DVDongle over a dedicated D-Star rig isthe lack of control of repeaterfunctions. Without the RPT2 field it isnot possible to control the repeater’s

routing via the gateway.

SUMMARY. The DV Dongle has greatpotential for a wide range of differentsituations. The DV Dongle provides anexcellent way to enjoy D-Star if you can’t getaccess to a D-Star repeater. Imagine howuseful this could be for those confined to anursing home, staying away at hotels or justabout any situation where Internet access isavailable but not a D-Star repeater. Theability to link into worldwidecommunications is extremely powerful andwell implemented thanks to some verydedicated enthusiasts operating in the truespirit of amateur radio. There are already alot of amateurs using DV Dongles on D-Starand I’m sure the numbers will continue togrow as more realise its potential.

The DV Dongle is available from Watersand Stanton priced £149.95. My thanks toWaters & Stanton for the loan of the reviewmodel and Declan McGlone, M0TMX, of theUK Interconnect Team (www.ukit.org.uk) fortechnical support.

DV Dongle – QSO in progress.

FIGURE 1: D-Star call routing parameters.

D-Star Network and DV Dongle connection.

Documents

E-MAIL [email protected]/files/22679/226798354.pdf · The crystal oscillator is a standard Colpitts type as used in several previous Homebrew projects. SW1 is