In the past when we've discussed Transmatches, someVHF experimenters have felt left out. This month W11CPdiscusses Transmatches for 2 meters and shows us howto build one.

How To Build A TransmatchFor 2 Meters

BY LEW McCOY' , WlICP

Here is the trensmetcn described in the text.

I 've had many requests to do an arti­cle about construc tmg a Transmatcnthat can be used on 2 meters. I find thatmany amateurs wish to experiment w ithrhombics or v -beams or just oddballantennas on 2 meters , To do so, be­cause the feed impedances of suchantennas can vary wid ely, it is neces­sary to use some type of adjustable im­pedance transformer .Such a device wi lltake the unknown impedance of theantenna and convert it to 50 ohms-avalue required by modern transceivers .This device is usually an adjustableTransmatch.

The only reason I have held back indescribing such a circu it is that it isalmost impossible these days to buysmall variable capacitors, Or if suchcapacitors are available, they are veryexp ensive. Whenever I mentioned thatfact to would-be constructors, however,the reply was always "Let us worry aboutthat. "

That may sound like a simple answer,but I still wasn't sausueo. I made it apoint last year to see what was availableat tleamarkets. and to my surprise therewere plenty of small variable capacitorsto be had. While not required. anotheritem that p roves very helpful is an old­fashioned g rid dip meter,

For the scores of newcomers to ama­teur radio, a grid d ip meter requiressome explanation. In those good olddays when amateurs built their ownequipment (receivers and transmitters),most of the work consisted of makingcircuits that would either be fixed tunedor tunable to a desired frequency. Thiswas usually accomplished by using acoil of a certain inductance ,and that coilwas resonated to various desired fre­quencies via a variable c apacitor.

"Technical Editor, CO, 1500 West IdahoStreet, Silver City, NM B806'

40 • co • June 1993

To try 10 make this clearer to the neo­phyte. I have shown two circuits at fig .l(A) and (B) . In the case of (A), the coil(LA) is "fixed" tuned by use of a fixedvalue capacitor. At B we use the samecoil and a variable capaci tor so that wecan cover a "tunable" range of fre­quenc ies. One of the more vexing prob­lems with such c ircui ts is knowing, aftermaking one , what the frequency hap­pened 10 be.

Just a spot of history is appropriatehere. The first grid dip meter wasdescribed in the early 1930s in OST. Ibel ieve that this first model is still in theARRL museum. The circui t was simpleenough. It consisted of a tunable tube­type oscillator that would be coupled toanother c ircuit. A change in the grid cur­rent would occur when the two coupledcircuits were resonant to each other .When metering the grid current in the

tube. the meter pointer would -op.' indiocating a resonance spot. A frequencymeter was used to calibrate the grid dipmeter. We therefore ended up with aknown frequency checking device thatcould be used to check an unknown Cir­cuit.

The OST device was so large, how­ever, that it was impractical to use. Abrilliant amateur named Bill Scherer,W2AEF. one of my predecessor 's hereat CO (he is now a Silent Key), thencame up with a small grid dip meter. Itwas so small it could be held in one handand brought very close to the circuit tobe checked, For years the Millen com­pany manufactured the Scherer g rid dipmeter, calling it the Millen Grid DipMeter,

One of my prob lems- if it can becalled a problem-is t tee! strongly thathistory should be preserved. I was

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twin lead, but it will work with coax feed.Let's face it' coax in long runs, say 150feet or more. can be quite lossy at 2meters so some amateurs may wish touse a low-loss line, such as open wire.A lso. one doesn't need a lot of room 10put up a 2 meter rhombic or v-beam 10obtain a d irectional 10 dB gain signal.Such antennas can be fed with the insu­lated-type open-wee line with great suc­cess. I am thinking now of amateurs whomay be marqmat into a given repeater,and the subseq uent answer is a high­ga in d irectional antenna (that doesn'tcost muc h!).

Th is Transmalch also can be usedWith random end -fed wires, or lor exam­p le, a m ultiband. center-led. tow-band

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and discuss antennas and matchingToday most of the amateurs on 2

meters use a coax-fed vertical , whileothers use beams, All of the current 2meter rigs are designed to work into a50 ohm load. Otherwise. either the rigwon't put out rated power. or it won'tload at all . Th is is done to protect thesolid-state circuitry . This Transmatchwas not really designed for these 2meter verticals or beam types of anten­nas simply because these antennas arealready matched and do not need atuner. However, in the event one has aproblem, I have provided fo r coaxialfeed matching.

Essentially, the Transrnatch is lorantennas fed with open-wi re line or TV

C,

(8)

' _ ..." Antennacr r feed

L2

C,

(A)

J>To ng Ll

and SWRbndg'

C2

Fig. 2- This is the circuit diagram of theTransmatch. L 1 and C2 comprise theinput variable link circuit. and C, and

L2 are the primary circuit.

Fig. 1- At (A) is the fixed-tuned circuitusing a fixed value of capacitance andresistance. At (B) the capacitor is veri­able. permitting tuning of the circuit to

various frequencies.

involved in many of these events . so for­g ive me if you feel I tend to ramble on .I do feel amateurs are Interested in theirhistory, however ,

When it comes to constructing thisproject, where can you gel a grid dipmeter? Probably your best bet is to asksome of the old timers at your radio club ,or any old timer for that matter. By oldtimer I mean anyone who has been inthe hobby twenty years or so. You canstill buy grid dip meters and they areuseful for solving many amateur prob­lems. They can be coupled to antennasto determine resonance, or to feed linesto see if the lines a re resonant. Howabout checking tower guys to see if theyare resonant and upsetting radiationpatterns? A grid dipper will do the job,If you lind one at a tleamarket . check tosee that all the plug-in coils are presentand also check to make sure it works.How much are they worth these days?Anything under $100 should be a buy.

Meanwhile , back to the 2 meterTransrnatch . The Circuit I used wascooked up by Ed Tilton . W1HDO, andwas described in many of the earlyARRL Antenna Handbooks. I d idn'thave a c hassis o r box handy when Islapped together this unit. However, Ido have a lot 01 copper-clad circui tboard available, so I cut up some p iecesand made a chassiszbox. As you cansee from the photo , it is nothing laney.It took me about two hours to cut up thecircuit board . solder the p ieces togeth­er, drill the holes, and mount the com­porents. Let me break away again here

say You Saw It In CO June 1993 • CO • 41

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Fig. 3 Single-section variable versionof fig, 2. (See text for details.)

dipole such as an 80 meter dipole, Oneof the antennas I use lor tests is an 80meter extended Zepp red with open­wire line. Using this antenna on 2 metersI was surprised to find I could triggerseveral repeaters that my verti cal wouldnot access, I tried all kinds of "2 meterantennas," I ran a wire out to my tower,connected the single end 10 the Trans­match. and found I had a reasonablygood antenna system. I also tried my 20through 10 meter multiband beam,which also worked on 2.

All this experimenting was a lot of funand provided interesting results . TheTransmatch doesn't cost much, de­pendinq on what you pay for the vari­able c apacitors and the coax fittings.The two variables I found at a tleamar­ket cost $3.00 for both.

Fig. 2 is the circui t I used. and fig . 3

J1To ng Ll

andSWR_.C2

L3

C3Antenna

teeo

is another configuration that will workand does not require a dual variablesuch as I used in the shown unit. Dualvariables may be harder to find,

Study fig. 2. The output 01 the trans­ceiver or transmitter is connected to J 1where the signal is rou ted through L1and C2, For newer amateurs, this is anold-fashioned method of couplingwh ich provides considerable matchingflexibility. L 1 is a link that is link-coupledto L2. L2 is tuned via C 1. The unknownfeeder/antenna load is lapped onto L2via two clip leads. An SWR indicator isconnected in the line from the rig to theTraosmatch. The SWR indicator is set Inthe renectec reading mode. While feed­ing power 10 the Ttansrnatcb from therig, both Cl and C2 are adjusted for anull, or zero, reading , II a zero readingin the reflected mode is not obtainable,then the clip lead taps are either movedin or out of the coil until a zero readingIn the reflected position is achieved. Theentire procedure is really quite simple.

In tne event of single-wire feed or end­feeding a wire, tne procedure is simple.Connect the single-wire terminal feedvia the clip lead to the coil. starting atthe outside of the coil or hot, unground­ed end of the coil. (In the fig. 2 config­uration the grounded end is at the cen­ter , or rotor, of the stator.) Try matchingand if you are not successful, gradual­ly move the tap towards the grounded

end, trying to match at each point. I didnot find an antenna-system load I couldnot match perfectly, but mat is not to saythat such a condition doesn't exist. Iwould simply add 19 inches of wire 10the feed end at the Transmatch, whichwould change the load, and in all prob­ability woul d put it within matchingrange.

Construction Details

As I stated earlier, I made the "box' thatholds the Transmatch from copper-coat­ed circuit-board matenat. Any metalchassis or box can be used. My only rec­ommendation is that you keep the twovariables as close together as possibleto avoid long lead lengths, My home­made box is 5 '/2 inches wide and 5 inch­es deep. The front and back panels are2 1/ 4 inches high. (None of these dimen­sions are cnncat)

If possible, in your search for variablestry to lind a small dual variable. The oneshown is a Hammarluncl 35 pF per sec­tion unit To be used on 2 meters. how­ever, you need to remove plates. USingneedle-nose pliers, carefully bend theoutside plates out and back a few timesand they Will come loose, Do this until youleave only four rotor and four stator plateson each capacitor secuon

If you happen to get a slightly largervariable from a fteamarket. then you

42 . CO • June 1993 Say You Saw It In CO

may have to remove more plates. Yourgoal in the plate removal is to get acapacitor that covers 144 to 148 MHzwith the coil described below. None ofthis is as difficult as it sounds. particu­lariy if you can beg or borrow a grid dipmeter. Marean this in a moment . C2 andC3 should also have a min imum valueof 35 pF.You may have to remove plateson these as well.

Coils L1 and L2 were made from ordi­nary No. 14 copper insulated housewire. I had some Romex No. 14 handy.so I stripped Ihe insulation off a leng thto make L2 . L2 consists of four turns ofthis wire (wound on a 1/2 inch diameterdowel and then slid off the cowen. Theturns were then spread 10 cover a totalof 1'/2 inches. This coil was then mount­ed to the stator connections of the dualvariab le. one lead going to one statorand the other coil lead going to the otherstator. I must emphasize that none ofthis is critical if you have a grid meter.

Once the coil is soldered to theca pacitor. take the grid d ip meter andcouple the grid d ip coil close 10 L2. andthen tune the grid d ipper to 144 MHz.Tune C1 and L2 through their range andwatch for a dip m the reading , indicat­ing the circuit has hit the 2 meter bandYou can determine the range of this tun­able circuit by selling C 1 to maximum(plates fully meshed) and then tunmqthe grid dip meter , looking for a d ip .

C!..... v ..... c:.",_ .. I ... ,...n

Likewise, open the plates fully (mini­mum capacitance) and again use thed ipper to l ind the other end 01 the range.

Next you need to make the link. It ismade from a piece of insulated No. 14solid wire. The link is also '/2 inch indiameter , and the turns are inserted atthe center of L2 (this isn't critical). Oneend of the link goes to the stator sec tionof C2. and the other end is connectedto the coax input litting _

II you make the version shown in lig .3, then the only change would be tomake the L3 three turns instead of four.Spread the turns as described above,and use a grid d ip meter to check 10make sure the CIrcuit is hilling 2 meters.The link is coup led to the bottom, orground end , 01 L3.

Some newcomers may be confusedby circuit-diaoram symbols- particu­larly ground connections. Fig . 2 showsthe rotors of both capac itors beinggrounded. This ground indicates thechassis. and in rea lity the rotors aregrounded to the circuit -board case viatheir mount ing points Ihrough the frontpanel.

You may find that in actually match­ing an antenna system the tuning ofeither capacitor may be too sharp, Indi­cating too much capac itance in eithervariable. You can remove more plates.I cannot tef you how much simplybecause there are so many different

types of variables to be found . Use thegrid d ip method. (And pleasedon'twnteasking me, as I don't have time toanswer and probably would not be ableto help anyway.)

Sometimes, but not always, whenusing certain types of wire antennas onemay encounter tuqh-impedance loads,which could lead to RF getting into therig via the Transmatch. A simple cure isto add a quarter wavelength of feeders(19 inches) in series with the feed lineat the Transmatch. Or if it is a single-wirefeed , then add 19 inches of wire as Imentioned earlier. This changes theload of the system to a low or d ifferentimpedance. and in all likelihood gets ridof the problem. Another thing to do is touse a complete enc losure for theTransmatch box.

The important thing here is that withthis Transmatch you can try any anten­na on 2 meters, and I do mean anyantenna. Anyone for a high-gam morn­b ic? A rhombic lor 2 meters would onlyhave to be 12 teet on a side to produceone heck of a lot 01gain. to trigger thatremote repeater, and I am talking 12 to14 dB of good. honest gain. Like I said ,however. don't write. Use the antennabooks. I'm getting too darn old, and timeis become more and more important tome. I will , though , add this: For repeaterwork and wire high-gain antennas, thinkvertical polarization. Good luck ! •

June 1993 • CO • 43