Duplexer Manual

Understanding, Maintaining &Re-Tuning Antenna Duplexers

By: William F. Lieske, Sr.

Founder, EMR Corporation

Forward

A technical bulletin on this subject wasprepared in 1990. Since then, antennaduplexers have continued to increase innumbers and many new designs haveappeared in the marketplace. This bulletin willreview antenna duplexers with emphasis onthe theory of operation, methods of tuning andmaintenance of them.

Duplexer, Defined:

The terms duplexer and diplexer have beenused interchangeably for many years. Theprefix “Di” is defined as “twice, double or two-fold.” The prefix “Du” means two or dual.“Plex” from the latin word plexus has, amongother meanings, the definitions: “An inter-woven arrangement of parts; A network.” Thuswe can conclude that duplexer and diplexerhave the same literal meaning. It is noted thatduplexer has been used with regard towireless (land mobile) systems and diplexerhas been used in microwave systemapplication. We will stay with duplexer to referto the devices covered in this bulletin.

Duplexer Applications

A duplexer provides the means forsimultaneous operation of a mobile relay orrepeater station having separate TX and RXfrequencies when using a common antenna.The benefits of this include: Saving oneantenna and one transmission line, comparedwith using separate transmit and receiveantennas for a repeater, maintaining reciprocalreceiving and transmitting signal pathcharacteristics compared with separate, TX

and RX antennas, and providing sufficientfiltering to prevent both transmitter carrierpower and wide band noise from desensitizingthe associated receiver.

Duplexer Types

There are two basic types of duplexers, BandPass and Band Reject. The easiest one tounderstand is the band pass duplexer.

Figure 1

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Figure 2

Understanding, Maintaining & Re-Tuning Antenna Duplexers

A band pass duplexer using six band pass typecavity resonators is shown in schematic formin Figure 1. Note that each branch has threecavities connected together with cables. A“tee” junction and two more cables connectsthe two branches with the antenna feed line.Sufficient selectivity must be provided by thecavity filters to preclude transmitter carrierpower from desensitizing the relatively broadresponse of the receiver front end and to alsoreduce transmitter wide band noise to a levelbelow the threshold of receiver sensitivity. Inthis example we show a VHF duplexeroperating at a 5 MHz. transmit-receivefrequency offset.

Typical selectivity curves of the three cavityresonator groups in each duplexer branch areshown in Figure 2. Two other curves areshown to represent the transmitter wide bandnoise spectrum and the desensitization curveof the receiver front end. Note that each filterbranch provides attenuation of signal powerequal to or greater than the noise anddesensitization levels.

Most duplexers found in land mobile wirelessservice are pass-reject or pass/notch types,shown schematically in Figure 3. Note thateach cavity has only one coupling loop. Theloop has a series capacitor, which is adjustableto resonate the loop itself, providing a rejectnotch when adjusted to a desired frequency.

The loop also couples energy into the cavityat the desired coupling factor, producing arelatively broad pass band selectivity. Thenotch can be placed either above or belowthe resonant frequency of the cavity, asneeded. The typical performance is shownbelow.

Pass/Reject Duplexer Response Curves

153.5 154.0 154.5 155.0 155.5 156.0 156.5MHz.

dB

Receiver front- end desens. curve

Transmitter wide band noise curve

0

10 20

30

40

50

60

70

80

90

100

Transmit154.0 MHz.

Receive156.0.MHz.

Figure 4 shows responses of a VHF pass-reject duplexer with transmit to receivespacing of 2 MHz. Note that the passresponses are quite broad. The overall notchdepths are very sharply defined and sufficientin depth to equal or exceed the noise andcarrier “desense” requirements.

Performance Comparisons

There are good reasons for selecting bandpass or pass-reject types according to siteand/or system requirements. Each type hasbenefits and shortcomings compared with the

Branchcables

"Tee"connector

Junction cables

Toantenna

TX in RX outPass-Reject Duplexer

Pass- rejectCavit ies

Series tuned loops

Figure 3

Figure 4

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Fax: (623) 582-9499 www.emrcorp.com e-mail: [email protected]

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other type. Some of these are:

Band Pass Type:

♦ Generally will have higher branch loss thanpass-reject type, 1.5 dB per branch orhigher being expected.

♦ Far superior for dense site use. Themultiple cavity strings provide addedselectivity for the receiver and a high orderof spurious and harmonic rejection for thetransmitter.

♦ Requires larger, higher “Q” cavities, andmore of them, resulting in higher cost andneed for greater site space occupancy.

♦ Through use of correct branch cablelengths and careful loop couplingadjustments, this duplexer type can betuned for a broad “nose” response toaccommodate multi-frequency trans-mitters and receivers.

♦ Impractical for closely spaced TX-RXpairs, compared to pass/notch types.Higher costs than pass notch types dueto requiring larger cavities.

Band Reject (Pass/Notch) Types:

♦ Lower insertion loss than band pass typesfor same TX-RX spacings.

♦ Since pass band is broad, little help isprovided in receiver front end selectivityexcept for the transmit carrier notch. Thiscan be a real problem when placed at highdensity sites.

♦ Can use smaller volume cavities for agiven TX-RX spacing, saving space.

♦ Lower cost to manufacture; savings inmaterials and labor.

Special Duplexer Types

At today’s overcrowded land mobile sites,there are often conditions that require unusualpass band and notch responses. Thecharacteristics of the two basic duplexer typesmay be combined where system needsdictate.

Some examples are:

♦ Adding pass-reject cavities to one or bothbranches of a band pass duplexer to notchout a bothersome transmit frequency thatis close to the receiver branch frequency.

♦ Adding band pass cavities to a pass-rejectduplexer to increase the effective front endselectivity of a receiver or to help in theattenuation of transmitter wide band noise.

♦ Internal duplexers installed in portableradio and cellular units often employceramic resonators. A wide T-R spacingand broad range of transmit and receivesub groups can be covered in this mannermostly due to the low transmit power levelsconcerned.

Mobile Duplexers

Duplexers are provided for use with low powerbase stations such as control stations andmobile transceivers. Since they are isolatedby distance from multi-use sites the benefitsprovided by larger fixed station duplexers arenot required.

Both small size and economy are possible inthese designs. Most of these units employsimple band reject operation. Through lightercoupling factors from 20 to 30 dB of notchdepth per cavity is secured. This is adequatefor the lower transmit power involved in mobileand low power bases.


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♦ Because of the lower circuit “Q” there isvery little band pass effect, however, morethan 60 dB of overall notch depth can besecured by using three cavities in eachbranch.

♦ The relatively small overall dimensions suitvehicular trunk or under dash mountingand there is often space within a controlstation for a duplexer.

♦ Most mobile duplexers are limited to 50 to75 watts input maximum, and some arelimited to intermittent service above 50watts of power. A few can be found thatare rated to 100 watt input, continuousduty rating.

♦ Successful designs are “ruggedized” tosurvive under the vibration, heat, cold andhumidity that can be present in vehicularservice.

♦ Special models have been developed tosuit mountings inside the cabinet of desktop control stations that access remoterepeaters.

dB

MHz.

0

10

20

30

40

50

60

70

80

450 455 460465

RX TX

Mobile/Control Station Duplexer Response

Performance curves for a typical mobileduplexer are shown in Figure 6. Note that thenotch in the transmit branch is about 67 dB atthe receiver frequency; usually sufficient for30 to 40 watt mobile transmitter wide bandnoise rejection. At these transmit powers, the60 dB notch in the receive branch is usuallysufficient to reject transmitter carrier to ausable level.

Where full duplex communications isanticipated, an additional 8-10 dB of notchdepth per branch will tend to provide clean,noise free communications. Mobile duplexersare usually designed for a particular T- R split,e.g.: 5, 7, 10, 12 MHz., etc. Due to thesharpness of the response of the notches,modification of branching cables and tap pointon the resonators are required to change theT- R split.

Duplexer Maintenance

Most high quality duplexers will give manyyears of trouble free service after placementat your site “right out of the box” from themanufacturer. The advent of frequencychanges or problems due to interference canresult in re-tuning on frequency or on newfrequencies.

Transmitter Input

To Antenna Output to receiver

Tuning Screws w ith set nuts

1" square resonators using tubular 1/4 wavelength, rods, capacit ively tuned.Shunt or series coupled as needed. Branch cables

Mobile or control station duplexer

Figure 5

Figure 6

465


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About Cavity Resonators

Duplexers rely on the characteristics of cavityresonators to provide needed filteringperformance. Beyond duplexer applications,cavity resonators are used as stand alonefilters, in transmitter combiners and variousother filter applications.

Cut-away views of typical band pass and pass-reject cavities are shown in Figure 7.

The difference between band pass and passreject cavity couplingloops are shown in Figure8. The band pass loop extend down into the

end of the cavity to excite the TEM (transverseelectromagnetic) field and to retrieve powerin the case of the band pass cavity. A similarloop with a series capacitor is used for thepass-reject version.

Most loops are formed from heavy copperstrips, silver plated for reduction of skin effectlosses. Loop dimensions, and aspect ratio,e.g.: width to depth, size (volume) of thecavity, operating frequency and degree ofcoupling will all contribute to the impedanceof the loop. For those readers familiar withthe use of imaginary number notation, thecharacter of the loop is predominatelyinductive.

Figure 7

Adjusting Knob

Invar Tuning Rod

Adjustable Coupling Loop (typical)Sizes vary according toband segment involved.

Type N Connectors

Fix ed Element Section

Adjustable Element Section

Finger stock fastenedto fixed e lement, pro-vid ing contact w ith themovi ng element whentuning knob is turned.

Compression set nut

Cavity body - May be round, square orirr egular in shape.

Band Pass Cavity Pass - Reject CavityBand Pass Cavity Pass - Reject Cavity

Adjusting Knob

Invar Tuning Rod

Compressionset nut

Type N Connectors

Adjustable Coupling Loop (typical)

Sizes vary according to band segment involved

Fixed Element Section

Cavity Body - May beRound, square orirregular in shape.

Finger stock fastenedto fixed element, pro-viding contact with themoving element whentuning knob is turned

Adjustable ElementSection


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Admittance will vary from +j30 to +j50,according to how it is adjusted. Since the loopimpedance is not purely resistive, all cablelengths between cavities are critical. Thelengths of interconnecting cables must beselected to yield a suitable match with thetraditional 50 Ω system impedance used inland mobile systems.


Figure 8, above, shows the two typical loopconfigurations. Note that the “loop plate” canbe rotated to adjust the coupling factor intoand out of the cavity. For pass-rejectapplication, either a “T” adapter or a speciallyconstructed loop plate arrangement with avariable capacitor is placed in series with the“grounded” end of the loop. This capacitor isadjusted to position the resulting notchfrequency as needed. Cavity tuning as wellas notch frequency responses are somewhatinteractive, usually requiring several tuningsteps, each step bringing the cavity closer toan optimized tuning condition.

Cavity Comparisons

Some points of comparison between cavitiesare as follows:

Many manufacturers build round cavitiesbecause round forms of aluminum and copperpipe are plentiful or can readily be roll formedfrom sheet stock. Rectangular, square ormulti-

sided shapes will work fine if properlydesigned. These can be very space efficientcompared to round formats.

EMR Corp. uses square or rectangular formatdesigns exclusively, made from heavy gaugealuminum sheet or extrusions with TIG weldedend plates to provide a high order of

mechanical integrity. Some of the reasons forusing this design concept include:

1. Square or rectangular shapes fit better incabinets, racks and other enclosures than doround or irregular shapes. We secure a higher“Q” per cubic foot of occupied space usingour “Square Q” cavities, yielding betterperformance vs: site rack or cabinet spaceoccupancy. As an example, a 7” square cavityhas performance equal to an 8” round cavityand uses 20% less rack or cabinet space.

2. The square format for our cavities lendsitself to a variety of packaging and mountingmethods. Many round cavities must use largehose clamps on support rails to mount them.Our UHF and 800-960 MHz square cavitiescan be panel mounted using their sturdy 1/4”thick bottom plates.

3. The EMR Corp. line of “economical”integrated cavity duplexer bodies employheavy aluminum parts that are “dip-brazed”

Typical band pass loop assembly Typical pass/reject loop assembly

End plate of cav ity body

Type N connect or

Loop plateLoop hold down screw w it h f lat was her

Loop t uning capacitor:Resonat es loop to notch frequency .

Type N connector Loop hold downscrew w ith flat was herwas her

End plate of cavity body

RotatableLoop plate

Band pass Loop Pass-reject

Loop

Figure 8

Type N Connector

End plate ofcavity body

Loop hold downscrew with flatwasher

RotatableLoop plate

Band passLoop

Loop plateLoop hold downscrew with flatwasher

Type N Connector

Loop tuning capacitor:Resonates loop tonotch frequency.

End plateof cavity

bodyPass-reject

Loop


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together to insure absolute mechanicalintegrity.

These duplexers use “capacitive probe” tunedresonator elements. They provide outstandingperformance, stability and reliability at a veryattractive cost to the site owner.

4. High power handling cavities for continuousduty operation at 250, 500, 1,000 watts andhigher power levels are most often equippedwith heat sinks and thermostatically actuatedcooling fans to maintain thermal balance.

Tuning or Re-tuning Duplexers

Before attempting to tune a given duplexer tosuit a particular transmit and receive frequencypair, you must make several importantdeterminations.

Question: What specifications did the duplexermanufacturer place on the particular modelinvolved? Such as: (A) Input power rating,(B) Minimum T-R spacing, (C) Branchinsertion loss vs: T-R spacing.

Answer: Often old catalogs of the duplexermanufacturer will disclose the expectedperformance capabilities. If this can’t befound, measuring performance “as-is” canresult in a good opinion as to operatingcapabilities.

Question: What frequencies were theduplexer factory tuned for? Are these morethan 4 to 5% higher or lower than thefrequency pair that you want to tune to?Example: Originally factory tuned for 452.250TX and 457.250 RX. You need a duplexer towork on TX 464.925 TX and 469.925 RXchannels. Can the duplexer performacceptably at the new frequencies?

Answer: Measure and record performanceat the old frequencies, then retune. If the T-R split is the same as before and performance


is degraded sharply, you will probably needto modify cable lengths to get properoperation.

Question: What type of duplexer is it? Bandpass or pass/reject? A rule of thumbsuggestion as to probable performance ofvarious sizes and types of duplexers accordingto operating band will be found in Appendix#1 of this bulletin. These examples can beused as guide lines to determine whether theycan be successfully re-tuned and used for aparticular application.

Test Equipment Requirements

The following test equipment is considerednecessary to successful duplexer re-tuning:

♦ Preferred: A Dynamic Wave Analyzer withdual trace display, 100 dB (or more) in 10dB steps of vertical log display resolution,1 dB per division (or less) fine resolution,with integral transmission - reflectionbridge or “S parameter” test set.

♦ Acceptable: A dual trace spectrumanalyzer with integral swept generator andtransmission - reflection bridge, having atleast 80 dB of vertical resolution in 10 dBand 1 dB steps.

♦ Minimum: A spectrum analyzer with 80 dBor greater dynamic range and a stable wellcalibrated signal generator. If possible,an R. F. bridge should be available.

♦ It is possible to retune duplexers using twotypical service monitors, one havingspectrum display. Generate a calibratedsignal with one and use the other toindicate signal amplitude. Usually theaccuracy of readings provided aresomewhat lacking, making truly accurateadjustments difficult or impossible.

♦ Duplexer manufacturers and well equipped


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Signalsourcce

C1

C2

R1

R2

R. F.voltmeter

Typical Radio Frequency Bridge

Test Port

C1, C2 are also equal in capacitive reactancethe current and voltage flowing in all four legsof the bridge are matched and zero volts willbe indicated by the R. F. voltmeter. When allresistances and reactances are 50 Ω, resistorR1 can be removed and its position used as acomparison or test port. When the externalcircuit impedance is nearing 50 Ω theindicated voltage becomes less and less,becoming zero at a perfect match.

The amount of reflected energy comparedwith the energy applied to the bridge is thentranslated directly to dB and is known as returnloss. Return loss is a most convenient way tomeasure how well a device matches astandardized system impedance such as 50Ω .

A mismatched antenna or other device willreflect power in proportion to the degree ofmismatch. If no power is reflected, a 1:1VSWR or perfectly resistive match exists. Thisalso represents a return loss of infinity.Conversely, if all of the power is reflected thisrepresents a VSWR of infinity:1 and a returnloss of zero. The two methods of expressingimpedance compatibility are, in that sense,reciprocal.

full service land mobile service shopsusually have a range of spectrum and/orwave analysis instruments suitable for filtertuning work.

♦ In addition to the basic instrumentation,cables of known integrity, test terminationsand various adapters are needed toaccomplish the necessary hook-ups andto properly calibrate your set-up.

What Does SWR, VSR And Return LossMean?

The term VSWR or simply SWR is usedthroughout the wireless industry to denote the“quality” of a device relative to a statedimpedance. VSWR stands for VoltageStanding Wave Ratio. It is expressedmathematically as:

VSWR = Vo + Vr Vo – Vr

Where: Vo = incident voltage Vr = reflected voltage

VSWR is a ratio between incident (forward)power and reflected power. When none ofthe power is reflected VSWR = 1:1. If all ofthe power is reflected, the VSWR will beinfinity:1. Direct measurements of VSWR canbe made using wave analyzers or usingdirectional couplers with specially calibratedmeters. These instruments display reflectedpower in terms of VSWR in real numbers.

About R. F. Bridges: “Dual sweep, dual trace”wave analyzers have an internal sweep signalgenerator that drives a R F. bridge having a50 Ω impedance. If the analyzer does nothave this feature, a separate external bridgeshould be secured and connected such thatreturn loss measurements can be made.

A basic R. F. bridge is shown schematically inFigure 9. When R1 and R2 are equal and

Figure 9

Signalsource


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“A” resolution set for 10 dB per divisionobserve the resulting trace with the test cableunterminated. Set the trace position to showa horizontal line greater at the reference value.Connect the Type N “bullet” adapter betweenthe two free cable ends. Set the “B” trace tofull scale reading (top of the graticule).

Now, look at the “A” trace. It should be ahorizontal line at least 35 dB down from the“A” reference line. If it is less than 35 dB yourcable length should be modified.

Next, connect the cables to the cavity undertest. If no notch pattern is seen, expand thehorizontal sweep to 20, 50 or 100 MHz untilthe notch pattern is identified. It will probablylook like a distorted “W” response. Adjust thecavity tuning rod position to bring the patternto screen center. Reduce sweep width to 10MHz (1 MHz per division).

If your analyzer has markers, place Marker#1 at the pass frequency, in this case 154.0MHz and set Marker #2 at 157 MHz.Alternately adjust the cavity tuning and theloop notch tuning until minimum insertion lossand maximum notch depth are obtained.

If markers are not available, you caninterpolate between the screen divisions toarrive at the desired frequencies with about100 KHz resolution if you use care. Byreducing the resolution of Trace (B) to 0.50,0.25 or 0.10 dB you can now accurately seethe insertion loss with high definition. Theresponse patterns should be similar to thoseshown in Figure 10.

Tuning band pass cavities is more or lesssimilar. Depending on the particular need,these cavities are tuned for 0.5 dB to 0.75 dBof insertion loss each for duplexer service,although some are tuned for up to 3 dB ofloss as stand alone filters. You must be surethat the loop coupling factors are balanced.

Test Accessories.

You will need jumper cables with suitableconnectors to connect the analyzer to the DUT(device under test) and back to the analyzer’sreceive display input. Generally, a set ofcables made for each band will suffice. Thesemust be most suitably an electrical halfwavelength long. You can calculate this if youknow the velocity factor of the cable. Supposethat you are using RG142B/U cable having avelocity factor of 82%:

Example: The constant: 5,616 Frequency (MHz) 155 = ½ electrical wavelength = 36.23” To correct for cable velocity factor: 36.23 x .82 = 29.7” cable length

Be sure to use the cable and connector manufacturer’sinstructions to determine how much to shorten the cableto account for connector electrical lengths.

You can verify your cable lengths byconnecting one end directly to the analyzerbridge output and terminating the other endwith a known high quality test load termination.If the cable is good for your purpose, a returnloss of more than 30 dB (40 dB or betterpreferred) will be indicated at the desired testfrequency.

Tuning or Re-tuning Cavities

Figure 10 shows how a wave analyzer can beused to tune either band pass or pass/rejectcavities. Prior to doing any tuning, you mustset up the analyzer and calibrate it to insurethat your measurements will be meaningful.

Suppose that you wish to tune a pass/rejectcavity to pass 154.0 MHz and reject 157.0MHz. First set the center of the instrumentswept range to 155.0 MHz and the sweepwidth to 20 MHz wide.

With the channel “A” reference line set to thevertical center line of the display and channel


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exact coupling factors. Your cavity is properlytuned when you can reverse the cables to itand both the insertion loss and the return lossare the same in either direction.

Some manufactures place calibration markson rotatable loops. Beware: They are simplyfor guidance and not intended to result in

Figure 10

50 ohmTestTerminat ion

Type NBulletAdaptor

Type NMale-MaleAdaptor

Tuning & Calibration Accessories

Expected display traces (See text)

156

Swept Signal Input50 ohm Bridge With Swept Output

Wave Analyzer

Band Pass Cavity Under Test

Pass/ Reject Cavity Under Test

Cavity tuning under dynamic wave analysis metho d See text for purpose, set-up, calibration and adjustments.

Wave analyzer calibration settings

Reference Line"A"

Reference Line "B"

Return Loss dB

0

10

20

30

40

0

10

20

30

40

50

60

70

80

Insertion Loss dB

Return Loss

Response

Response

Respo nse

Return Loss

Cavity tuning under dynamic wave analysis method

Analyze

r Disp

lay

Reticu

le


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This makes the loop couplings equal throughthe cavity. Usually this will take several cablereversals and “touch ups” of loop orientationto arrive at the desired insertion loss withacceptable return loss at both ports. Mostcavities will show a return loss from 17 to 25dB when optimized. Remember, the cavity ispresenting a predominately inductivecharacteristic to an inherently resistive leg ofthe bridge.

Tuning Multiple Cavities, Tuning Duplexers

It is possible to develop a filter to pass a rangeof frequencies and having steep slopedresponses above and below the desired passband. The curves shown in Figure 11 displaysthe performance of four, 10” square cavitiesproperly phased together in a series configur-ation.

To secure broad band “nose” responses fromtwo or more cavities in series, the connectingcables between them must be of optimumlength. Before trying to tune a cavity “string”you must first tune each cavity individually,setting its pass or pass/reject to suit systemneeds. Usually the connecting cables betweencavities are close to ½ wavelength in electricallength, corrected to allow for the effectivelengths of connectors and loop configurations.Shorter lengths are often found to yield thedesired performance, according to band ofoperation.

If the “chained” cavities are to be used asbranches of a duplexer, one branch (or chain)will be tuned for the receive pass frequencyand the notch set for the transmitter frequency.The transmit branch is tuned opposite. Thejunction cables must be adjusted in length toeffectively maintain the reject characteristicsbetween the two branches while maintainingcorrect impedance matching with bothbranches and the antenna port.

Note that through critical coupling, the desiredpass band is essentially flat over a 1 MHz.range and the return loss is about 28 dB overthe range. Careful cable length adjustmentsand considerable tuning time is required toaccomplish superior filter performance. Thisinsures protection for a group of closelyspaced multicoupled receivers at a highdensity site.

Summary:

For the reader who wishes to becomeproficient at tuning or re-tuning antennaduplexers, it is hoped that this bulletin hasprovided enough insight to be of assistanceto you.

Appendix #1 lists expected performances oftypical cavity combinations in the VHF, UHFand “High UHF” bands. These are to beconsidered average, some makes and typesshowing better and some not as good as thesenumbers.

Errata

It is important that you have correct cablelengths between the duplexer cavityresonators and that proper tuning has beenaccomplished.

We are often asked for a list of generic cable

Figure 11

Reference Line "B"

Reference Line "A""

Return Loss dB

0

10

20

30

40

0

10

20

30

40

50

60

70

80

Insert ion Loss dB

152 153 154 155 156MHz.


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lengths to connect cavities together. Often,the user will be trying to make a filter orduplexer out of a set of mis-matched cavitiesas a duplexer or filter. Unfortunately, we canonly guess at the lengths, drawing from pastexperience.

We trust that this bulletin will be the basis fora better understanding of antenna duplexers,and bring out the reasons why care must beused in tuning them if acceptable performanceis to be secured.

Hopefully, this discussion of duplexers hasprovided enough insight to be of assistanceto you.


We will be pleased to assist you with anyduplexer problem. Call, FAX or E-mail us withyour duplexer type(s) and operating frequencyinformation.

EMR Corporation22402 North 19th Avenue

Phoenix, AZ 85027

Telephone: (623) 581-2875Toll Free: (800) 796-2875

FAX: (623) 582-9499EMR Web Page: www.emrcorp.com

E-mail: [email protected]

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Appendix #1Cavity Types vs: Performance in Duplexers

(1) Single frequency repeaters, only under 30 watts power.(2) Extended band pass, up to 15 MHz. wide, each response.(3) EMR Corp. Broad band SMR Duplexer, up to 15 MHz. per TX and RX response.


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Duplexer Manual

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