IEEE TRANS CTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. EMC-10, NO. 1, MARCH 196S

Resonance Properties of the Shield of a Coaxial

Cable over a Ground PlaneM. D. DEMILT, MEMBER, IEEE, YUNG-PING LOH, AND RALPH Mr. SHOWERS, FELLOW, IEEE

Abstract-In situations where several high-power transmittersand their antennas are to be used near one another, a certain amountof mutual interference can be expected. An instance of particularinterest is that of high-intensity radiation inducing standing wavesbetween the shields of nearby coaxial cables and a metal deckof ground plane. Standing waves induced may cause high potentialsand possible breakdown at the ends of the cable, damaging con-nectors and antennas. There may also be some reduction of the

Man-uscript received August 1967. For complete paper see IEEEElectromagnetic Compatibility Symposium Record (Washington, D. C.,July 18-20, 1967), pp. 447-453.M. D. DeMlilt and R. M. Showers are with the Moore School of

Electrical Engineering, University of Pennsylvania, Philadelphia, Pa.Y. P. Loh was with the Moore School of Electrical Engineering,

University of Pennsylvania, Philadelphia, Pa. He is now withPM\IC Colleges, Chester, Pa.

shielding effectiveness of the coaxial cable when high-voltagestanding waves are present in the shield. It has been common prac-tice to eliminate such standing waves by periodic grounding of theouter conductor of the coaxial cable. This, however, requires pene-tration of the insulation material on the cable and formation of metal-to-metal joints on the shield. This is not only an inconvenientmethod of installation, but is also undesirable around salt water. Cop-per shielding will corrode, and corrosion at the joint of the dissimilarmetal can cause nonlinear interference effects.The standing waves induced in the transmission system formed

by the cylindrical shield of a coaxial cable and a conducting planeare examined theoretically and experimentally as a function of theshield-to-ground impedance at the end points only (Z, and Z2 OfFig. 1). Ordinarily, standing waves are eliminated by terminating aguiding system in its characteristic impedance. In this situation,however, the exciting source (i.e., incident radiation) is distributedalong the length of the transmission system.

Measurement of RF Leakage in Multipin

Electrical ConnectorsFERD WV. SCHOR, SENIOR AIEMIBER, IEEE

Abstract-In the past measurements of connector RFI leakagehave evaluated combinations of the connector and cable so that ithas been difficult to separate the effects of the different leakageareas. The methods presented here enable the designer first toevaluate the leakage at the interface coupling with a variety of con-nector characteristics and then, separately, to obtain the cable RFleakage. Thus the effects of the varying parameters of connector de-sign may be determined without being affected by the connectingcable leakage. RFI performance data are presented, covering therange of 150 kHz to 1000 MHz. A description of the measuring tech-nique and its development is also included. A series of recommenda-tions aimed toward reducing RFI leakage in multipin electrical con-nectors are given.

INTRODUCTION

W ITH the rapidly increasing use of the RF spectrum,the higher output of transmitters, and the vastly

improved receiver sensitivities combined with the in-creasing size of electronic complexes, radio frequencyiniterference (RFI ) betwx eeu equipment has become a

Manuscript received August 29, 1967.The author was with ITT Cannon Electric, Los Angeles, Calif.

major problem. Careful shielding of the individual equip-ment reduces local radiation but it is still necessary tocarry circuits into and out of each equipment with the useof electrical connectors and cables.The areas of leakage in an interconnection system are

generally at the interface coupling between the matinghalves of the connector, the attachment of the cableshield to the connector, and the cable shield itself.

Past measurements of connector RFI leakage haveusually evaluated combinations of the connector anid cableso that it has been difficult to separate the effects of thethree leakage areas. Using the RFI measurement methodspresented in this paper, the leakage at the interfacecoupling only is first measured w-ith a variety of connectorcharacteristics. The cable RF leakage is then separatelyevaluated. In this xvav, the effect of the varying parametersof contnector design may be determined without beingaffected by the connecting cable leakage.The RFI performance data are presented in a series of

graphs covering the range of 130 kHz to 1000 M\IHz. Adescription of the measuring technique and its develop-

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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, MARCH 1968

Stoddart Field Shielded BoxStrength Measturing Set

Fig. 1. Measuring setup for determining RFI shielding capability of connectors.

ment is also included. The paper conieludes with a series ofrecommendations aimed toward reducing RFI leakage inmultipin electrical connectors.

\IEASURENIENT PROCEDURE

Past Mleasureinent MlethodsM\1ethods of measuring the RFI shielding capabilities of

connectors in the past usually involved the combinedleakage measurement of the shielded cable, cable attach-ment, and the interface coupling of the connector undertest. The result, therefore, did not always represent thecapabilities of the connector alone. Thus, if the connectorwere used with another cable or different attachment de-vice, the answer would be different.

In the measurement method to be described below, theRF leakage at the connector interface only will be mea-sured first. Following this, the attachment leakage will bemeasured separately. The cable leakage depends, ofcourse, on the type of cable used and has little to do withconnector leakage. This cable leakage evaluation, there-fore, belongs in a separate category.

Measurement Precautions

The shielding capability of any enclosure is a measure ofhow well it confines the field wvithini it. Thus, if a loop ofwire carrying RF current produced a field at a certaindistance away of X AV/m and if this loop were enclosedand the field were reduced to Y,V/m, then the ratio of thetwo field strengths or XIY would be a measure of theshielding capability of the enclosure. Expressed in deci-bels, the result is equal to 20 log1o X/Y.

In the investigation conducted, it was necessary toobserve certain precautions to insure duplication ofmeasurements. These are as follows:

1) The shape and size as well as the exposed length ofthe loop carrying the RF current was the same with theenclosure (connector) on or off.

2) The measuring anitennia w.as in the same relativeposition each time, and the same anteinna and instrumen-itxvere used over aniy specific frequency range. To obtainmeaningful results, the antenna was oriented to givemaximum output each time. Since the measurementswere made in a large shielded enclosure, reflections are, ofcourse, a source of error. The effect of these were minii-mized by taking readings at a large number of frequenciesand then averaging these out by drawing a smooth curvebetw-een the plotted points.

3) It Nxas found necessary to prevent any of the shieldsused from carrying conducted RF current. This was truealso of the connector shell. Early experiments used asingle conductor through the pipe attached to the con-nector. It' was found that the return current producedstray fields resulting in measurement errors.

4) All joints in the system not being measured werechecked for RF leakage. Leakage holes were pluggedwhere necessary using conductive gaskets or steel wool.

Measuring SetupThe entire setup was enclosed in a large shielded room.

The ambient field in all cases was below the sensitivitylevel of the measuring set.

Signal Generator: The signal generator consisted of aseries of General Radio oscillators enclosed in a shieldedbox. A vacuuin tube voltmeter was used to monitor theoutput. The output of each generator was connected to a50-ohm transmnission line through a 20-dB pad. Thistransmission line theni went through a termiinal panel onthe shielded box and then out into a half-inch iron pipeextendinig outside the box for a distance of about 20 inches.(See Fig. 1.)The RF field produced by the signal generators was

below any measurable level when the shielded box wasclosed and the 50-ohm transmission linie was not connected.

Connector: The connector was attached to the iron pipeusing an adapter wvhich mated with the connector at one

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SCIIOR: RF LE.KkGE IN MULTIPIN CONNECTORS

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80 (SI:FRIES MS-51 _5) .-__ B N_

70

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20

. 1 i 5 10FREQUENCY MHz

20 50 100 200

Fig. 2. Comparison of the RFI shielding at the interface coupling of various types of ITT Cannon connectors.

end ancd the half-inch pipe at the other. The tran-smissionline v-eint through the multipin connector UsiIng onie con-tact for the center conductor and another for the shield.Two centrally located contacts were used, and the spacingbetween these contacts was noted. Both mating sectioiis ofthe connector were in place.A 50-ohm resistor was then wired to the connector con-

tacts as showin to provide a termination for the tranismis-sion line anid complete the circuit. Another adapter wasthen screwed in place, and its opening was closed with a

half-inch threaded plug. This adapter and plug provided acompletely shielded enclosure for the rear of the connectorand the load resistor.

Shielding Instruinentation: All joints were tested with a

small pickup loop to insure no leakage at all connectionsexcept the one under test. When leakage occurred at anyof the pipe joints, this could usually be cured by addi-tional tightening. RF leakage at the other joints was

reduced by using gaskets made of conductive elastomers or

woven wire. As an extra precauition, all iron pipe used was

silver plated.Mtleasuring: A series of Stoddart field measuring sets

coverinig the range from 15 kHz to 1000 MHz was used formeasuring the external field. The antenna in each case

was placed a distance of one foot from the center of theconnector.

MeasuweTiient Technique: External field measurementswere first made on the connector pair setup as describedpreviouslyv and shown in Fig. 1. The connector then was

removed, and a pair of leads having the same spacing anddiameter as the connector terminals was connected to thetransmission line. These were again terminated in a 50-ohm resistor. The length of the unshielded leads was madeequal to the lenigth of the connector. Thus, the internal

circuit of the connector was duplicated as accurately aspossible.An additional set of field strength measurements was

then made at the same frequency settings. Care was takento make certain the VTV output readings of the signalgenerators remained the same as before.

TEST RESULTS

Connectors (all ITT Cannon) as Normally Supplied

Fig. 2 show s a comparison of a typical threaded coupling(KY series), bayonet coupling (PV series), and thethreaded and bayonet coupling of the MS-5015 and M\IL-C-26482 series, respectively. The popular rack and panelDPD series is also included. As can be seen, all of theabove connectors offer considerable RFI shielding over thefrequency range of 100 kHz to 1000 MHz and will giveadequate performance in many environments.

Connectors with RFI ModificationsSince most of the RF leakage at the interface coupling

takes place through tiny orifices between the matingcol)nectors, any means of completely sealing these open-ings will still further improve the RFI shielding capa-bilities. Fig. 3 shows the inmprovement in the RFI shieldingof a AIIL-C-26482 connector when peripheral spring fingersare used to provide contact between the shells of the mat-ing connectors. Note that while the standard olive drabfinish connector provides from 100 dB of isolation at 500kHz down to 30 dB at 200 MHz, the RFI type gives afurther improvement of 10 to 40 dB over this frequencyrange. The peripheral spring fingers are made of silver-plated beryllium copper and are permanenitly attached tothe barrel of the mating plug.

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IEEE TRAtNSACTIONS ON ELECTROMAGNETIC COMPATIBIL'rY, MAtcH 1968

110

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80

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20

40

30

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MIL-C-26 482 STAIDEARO Ol IVE DRAB F INISH-\( BAYONE T COU PL INE )X

.2~ ~~~ ~\5I1 0 s oo20 So1

FREQUENCY IN MHz

Fig. 3. Comparison of the RFI shieldinig at the interface coupling of a MIL-C-26482 connector having stan-dard olive drab finish with the same connector using peripheral spriiog contact fingers over cadmiium plat-ing (RFI type).

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Fig. 4. RFI shielding characteristics of Canniion standard MIS-5015 (24-28 size) imiated pair with olive drab)chromate over cadmium plating compared with the same connector with cadmium plated finish anld periph-eral spring finger contacts on the barrel.

Fig. 4 shows the effect of adding spring fingers to theM1S-5015 connector w hich uses threaded coupling. Notehere that the use of threaded coupling gives adequate RFIshielding for most applications and the improvement dueto spring fingers is not significant except above 300 MHz.Although not designed specifically for RFI isolation,

the ITT Cannon DPD rack and panel connector gives over60 dB of RFI shielding up to 20 MIHz. Its RFI shieldingcapability can be still further improved as shown in Fig. 5with the use of a woven-wire mesh (1/8-inch thick) gasket

between the initerfaces. This gasket is compressed wlhenthe coninectors are mated. Also metalastic gaskets (wovenwire and rubber) canl be used between the receptacle anidthe chassis mouniting surface and also betweeoi the plugand the aluminium cast endbell, thus effectively producingan RFI seal around this large rectanigular conniiector.Thr-eaded Coupling Versus Bayonet CouplingAs would be expected, similar conniiectors using threaded

coupling exhibit slightly better RFI shielding tha.n those

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SCHOR: RF LENKAGE IN MULTIPIN CONNECTORS

RAFK AND PA NEL WITH RFI MODIFICATJO

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4C ______ ~~~~~~~~S'ANDARD R CK AND PANEL CON ECTORS____ ____

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.1 .2 .5 1 2 5 10 20 50 100 200 500 1000

FREQUENCY IN MHz

Fig. a. RFI shielding characteristics of a standard DPD rack and panel connector compared with the sameconnector using RFI shielding gaskets (RFI type).

Olive Drab Chromateover Cadmium Plating

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Fig. 6. RFI shielding characteristics of series KV ITT Cannon connector with various finishes.

with bayonet coupling. This is illustrated in Fig. 2 whencomparinig threaded type (KV series) with bayonet type(PV series) connectors. Also, the MS-5015 (threaded type)is 10 to 15 dB better than the MIL-C-26482 (bayonettype) above 50 M\1Hz. In many applications, this is a smallprice to pay for the added convenience and field servicefeatures of the bayonet type connectors. This difference inRFI shielding can be more than made up, however, byincorporating the modifications described on the PV or

M\IJL-C-26482 series.

Comnparison of Surface FinishesFig. 6 shows the RF leakage of the KV series connector

with different surface finishes. Note that the conductingfinishes, olive drab chromate over cadmium, electrolessnickel, and stainless steel, all gave essentiallv the sameperformance.

While an aniodized finish over aluminum produces a hardand practically impenetrable coating of aluminum oxide,which meets most environimental requirements, the use ofthis finish also detracts from the RFI sh-ielding capability

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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, MARCH 1968

110

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o60

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30

20

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.1 .z .5 1 2 5 10 20 50 100 200 soo 1000

FREQUENCY IN MHz

Fig. 7. Variation of shielding effectiveness with respect to conductor position in a multipin connector: topcurve taken with a pair of conductors near the center of the connector, bottom curve with conductors near theshell.

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50

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Fig. 8. Comparative RFI shielding characteristics (electric field only) of galvanized iron pipe, sinigle shieldingbraid, and double shielding braid measured over a length of 20 inches.

as shown in the lower curve. Note, however, that thecurve rises at the higher frequencies due to the decreasingreactance of the capacitive coupling.

Effect of Conductor Position

The position of the conducting pair within the connectorcauses only a slight difference in the RF radiation. Fig. 7shows an improvement of about 5 dB when the conductorsare in the center rtather than being niear the edge of the

connector shell. Although specific tests were not made, it isknown that the RF pickup of a pair of conductors inicreaseslinearly with the separation between these conductors.Hence, in a multipin connector RF pickup is minimizedwhein adjacent contacts are used for a circuit pair.

Conductor ShieldingAs previously poiinted out, the connector forms only a

small portion of the interconnection circuit, atnd even the

Double Shi*1ding BrAid Gii[vanized Iron Pipe

single Stielding Braid

-1 I- I- - I- I 11 I 11 1; . 11 11 p ,.

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SCHOR: RF LEAKKGE IN MULTIPIN CONNECTORS

40__

CO D ROLL STEEL CONNECTOR PAIR30

0 20ILESSTEzlo 00:1~~~~~~~~~~~~~~~~~(0 SERIES)CCNET10

10 ELECTROLE SS NICK L ON A UMINUMSHELL CONNECTOR AIR

60 100 200 500 1000 2000 5K 10K 20K 50K 100K

Frequency in Cycles per Second

Fig. 9. Magnetic field shielding characteristics of ITT Cannon stainless steel (400 series) connector pairKVOR22T55PN98 and KV6R22T55SN98 compared with a similar connector pair with electroless nickelfinish over an aluminum shell KV06G22T55SN and KV02G22T55PN. Leakage was measured onily atinterface coupling. Also shown is a cold rolled steel KV connector pair.

best connector will not provide proper system shieldingwhen used wx ith a poorly shielded conductor. Fig. 8 showsthe relative shielding capability of galvanized iron pipe,double shielding braid, and single shielding braid. Singleshielding braid can only be recommended for frequenciesbelow 20 M\IHz or where the RF shielding requirements areat a mininmtm. Double shielding braid is recommendedwhere higher frequencies are concerned. Note that doubleshielding braid is nearly equal to galvanized iron pipeexcept betweeni 50 and 500 mHz where the RFI suppressiondips down but is still 40 dB better than single braid. Thisdip is probably caused by imperfect shielding plus cablelength resonance.Wherever a cable shield is attached to the connector out-

let, electrical contact must be made around the entire 360-degree periphery. This can be accomplished with cableclamps or cone shaped clamping ends.

Alagnetic Field ShieldingMagnetic field shieldinlg is important at frequencies

below 10 kHz. Fig. 9 shows the relative shielding capabili-ties of KV-type connectors made of cold rolled steel, 400series stainless steel, and aluminum. At 400 cycles, coldrolled steel gives 21 dB, 400 series stainless steel gives 14dB, while aluminium gives 6 dB of shielding. Above 10kHz, aluminum offers ample protection. Stainless steels ofother types wthich have no iron content will give the sameresults as aluminium.

APPLICATION RECOMMENDATIONS FORMLINIMIZING LEAKAGE

The following conclusions applicable to connectorapplication can be drawvn from the work performed on thisproject:

1) A conducting surface finish is necessary where RFI iscritical. The use of nonconductive finishes will necessitatea trade-off with RFI performance.

2) Threaded coupling provides better RFI protectionthan bayonet coupling.

3) Optimum shielding is achieved only when there iscomplete peripheral contact between mating shells. (Usespring fingers or conducting gaskets.)

4) Conductor pairs requiring the most shielding shouldbe located near the center of the connector and as closetogether as possible.

5) Double shielding braid should be used where RFI is aproblem.

6) The shielding braid should make complete peripheralcontact with the connector outlet.

7) MIagnetic shielding is only important below 10 kHz.Steel or 400 series stainless shells are recommended formagnetic shielding.

ACKNOWLEDGMENT

The author wishes to acknowledge the assistance of A.T. Parker and associates for their many helpful suggestions.

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