ANTENNA MATCHING UNIT ROUND-UPzs6wr.co.za/media_docs/antenna matching unit.pdf · 2010-06-19 · ANTENNA MATCHING UNIT ROUND-UP E..J. Van Loggerenberg ZS2LR The need The modern HF

ANTENNA MATCHINGUNIT ROUND-UP

E..J. Van Loggerenberg ZS2LR

The need

The modern HF transceiver usually has a broad bandedsolid state fmal amplifier designed to work into a resistiveload of approximately 52 ohm. SinCe most antenna sys-tems either present a slightly reactive load or are not thatexact impedance; such an output system seems to me tobe more wishful thinking than any thing else. Apparently,manufacturers have also had their thoughts along thoselines since the latest idea seems to be to include a match-ing unit inside the rig itself together with an S.W.R. in-dicator. Another consideration is the harmonicsuppression level of the average rig. The harmonics arerequired to be 40 db down by regulation and this is theaverage specification seen for most commercially avail-able rigs. As I have commented previously, harmonics at-40 db still present a problem to receivers in closeproximity to the transmitter. A suitable matching unit alsoserves the purpose of reducing those offending signals. Nostation can be considered complete today if it does notemploy a good low pass f1lter,a good S.W.R. monitor andan antenna matching unit. The matching unit should bechosen so that it has a low pass characteristic. Bear thesefacts in mind when you are experiencing TV! and BCIproblems.

The Ideal Matching UnitThe ideal matching unit would have the following

characteristics: -

(a) Able to transform from a high impedance to a lowimpedance and visa versa.

(b Able to tune out any reactance present and presenta purely resistive load to the transmitter. It is probablybecauSe of this "tuning" ability that the device is com-monly known as an "Antenna Tuning Unit".

(c) Able to match both balanced as well as un-balanced lines to the rig.

(d) Includes an impedance noise bridge as an aid tosetting up while off the air.

(e) Includes an S.W.R. indica~or as a monitor whileon the air.

(1) Constructed of components to present little lossand rates in excess of the transmitter output power.

(g) Includes an antenna switch which earths unusedantennas and has a position to disconnect and earth all

.connected antenna systems.

, (h) Includesa dummyload to facilitatetuningandloading of the rig off the air for test purposes.

Choice of Circuit

If you intend "rolling your own" as hopefully you do, acareful consideration of the circuit to use should be made.

6

Part 1.

Decide whether you would like to have all the facilities ofan ideal unit or if you only require those that fulfill yourparticular purpose and combine those into one unit. Asingle purpose unit will not be of much use ifyo~ are goingto play about with different antenna systems. On the otherhand, if you intend using only one antenna for the nexttwenty years, a single purpose unit should suffice.Circuits to choose from

There are many circuits to choose from and these will befound in the Radio Amateurs Handbook as well as in thevarious antenna handbooks. No originalityis claimed by thefollowing anthology of circuits since they are commonknowledge by now. All of them were taken from availablehandbooks. Each one has a different characteristic worthstudying.All the circuits shownand discussed havebeen triedby the writer.

Figure 1showsa simpleseriescircuitconsistingof a variable

Cl Lf

ANT

TRANSMITTER

>~GND

Figure 1

capacitor connected in series with a variable or tappedinductor. It is important to note that the unwanted portionof inductance must be shorted out and not simply left opencircuit in this application. The circuit may be used to matchan antenna of unknown impedance When the circuit is.tuned to resonance the reactances in the circuit canceleach other and the load is applied to the transmitteruntransformed. Either side of resonance the circuit isintended for matching an unbalanced antenna such as arandom length of wire. Its f1lteringcharacteristics can beeither high or low pass depending whether the final adjust-ment results in a capacitive or an inductive reactancerespectively. It is not recommended as a harmonic killer.

The circuit of figure 2 is that of an L-network which is

L

> ~ ' <ANT

TRANSHITTE:R ~£) GND

Figure2

0 ",.1;" 7f::. f),.t"h 1OSlO

sed to match an unbalanced high impedance antenna to alow impedance rig. Since it consists of a series of inductorand parallel capacitor, the circuit has good low pass char-acteristics. It is very suited to m~tr.hrngrec;nn~ntend fedlong wire antennas. Circuit values may be {~knl~tedfrom:-

where Rin = input impedance and Ra = ~ im-pedance..

Inductive reactance XL =J Ra x Rin -Ra2

Capacitiye reactance Xc = Ra x R.inXL

The circuit of figure 3. is that of an ~network which is

L

TRANSHIT.

ANTII

I

GND

Figure3

used to match an unbalanced antenna which has a lowerimpedance than the output of the transmitter. It has goodlow pass characteristics for the same reasons as given forfigure 2. It is suited for matching loads as a quarter waveantenna. Circuit values may be calculated from:-

Capacitive reactance = Ra x J RinRa -Rin

A

Inductive reactance Ra x RinXc

The question that may arise at this point is, "Where doyou get a value for Ra, the antenna impedance?" Simply,measure it by means of a bridge. On the other hand, itmay be estimated bearing the length of the radiator inmind and drawing current and voltage nodes begjnninRat the far end of the antenna.

The circuit of figure 4 is that of a Pi-section networkwhich is mostly used to match a high impedance sourceto a low impedance load. It has very good low passcharacteristics. On inspection, it will be seen that it is acombination of the previous two circuits and may beused as such. By setting either capacitor to zerocapacitance, the circuit may be used as a simple ~net-work for matching to either a low or high impedance. Itis therefore a very versatile circuit when working withmay types of antennas where the impedance may not beknown. There is, however,. one problem that must be

Radio ZS October 1989

>TRANSHIT

>

figDre 4

~ANTC I . GND

home in mind. If this circuit is used with a final~mplifiP.r 'III'bichalso uses a pi-section output tank cir-cuiI:znd an.S.W.lL indicator is placed between the rigand :Be JR:t~r.hfflgnnit, false indications will result on~ s..'?F.ll. indicator. Yon will be loading the input capof the ~~ ~ so to speak. The circuit is ofcc:::::rse~ fur an ftnh::!l~nced load. If the com-poIIdJs zre !A~rJ chosen and it is required to matcha.~ '=:iJich~.l2nCe is lower than the source, it isbeaec to do me ~dring adjustment with the aid ofan ~ ~ After- adjustment, ignore theS.W..R. ~.rc£ shoc!d rhe rig have a pi-section tankcircuit. It will n:Jt ~te me true sitnation. To calcu-mre dte 'I"alnesof die compooenlS. consult your Radiop. tn2tf'1ll"SHandbook..-bich gMs you fnll details. It isn~ as 1 as m: IDa;"at firs:: th.in.k.~ J J

~T-eeci A~DS~

Ire prenous circuits may easily be used to match3nte.nn='lC:that present a balanced load. Insert a balunof ~ ra::io between the antenna and the matchingu.niL.Us!lG.llya 4:1 ratio will be sufficient. Baluns arevery simple to construct and should present no dif-fiailiies to the constrnctor. Once again, details can befonud from the handbook or from one of the excellentarticles tha: appear from time to time. A word ofwarning. If the antenna being fed is a high impedancevoltage fed antenna, high voltages appear at the fedend and the insulation used in the construction of thebalun must be suitable. The writer has burnt up quitea feVI'during experimentation.

The prerious circuits are favorites with the writer.The circuit (as shown on page 8) is a well tried andnseful circuit.

The circuit of figure 5, (see page 8), is popularlyknown as the -Z Match- and is intended for use with abalanced feeder. It consists of two interconnected cir-cuits and has good bandpass characteristics. It is amultiband circuit which requires no inductance ad-justment. The writer has used this circuit successfullyto match a G5R V and a center fed co-linear array.Both antennas used parallel open wire feeders and nomatching stubs, baluns or coaxial cable feeders. Don'tbe frightened by stories of TVI and BCI. As long asyour harmonics and spurii are well suppressed, youshould have no problems.Circuit values are as follows:- .

L1 = 3,41lH : 7,35 turns 16 SWG 52 mm diameter.Length is 32 mm.

7

CI

T1<ANSMITTER> 7h

20/15!tOM£T£RS~.L3

L/

Figure5

L2 = 1,7.uH : 5,5 turns 16 SWG 52 nun diameter.Length is 41mm.

L3 = 2,35.uH : 6,5 turns 16 SWG 67 mm diameterconcentric with L1. Length 13 mm.

L4 = 1,8.uH : 4,75 turns 16 SWG 67 mm diameterconcentric with L2. Length 13 mm.

Cl -single gang 340 pf: C2 double gang or split stator250 pf per section.

Mount the two coil assemblies at the right angles to eachother to minimize interaction between the two.

Calculating the number of turns,My apologies for not knowing the metric eqqivalent, but

the number of turns for a special inductor may be calcu-lated from:-

N =JL x (9r + 10I)

roG

where:- N is the number of turns; L is the induc-tance in diicrohenry; r is the radius of the coil former;I is the length of coil in inches.

Mount the coils at least the coil diameter away from anymetal work. Keep all connecting leads as short as possible.

The circuit of figure 6 (below), is popularly known as a

TRANSMITTER

,",

'Figure6

j-~

L2

Coo & 40 METERSL4

"Transmatch". The writer found that the pi-section circuithad better low pass characteristics. The circuit is usuallyused for matching a low impedance source to a highimpedance load. It consists of two variable capacitors and,for multiband use, a tapped or variable inductor. It is anunbalanced circuit and the inductor, being in parallel withthe antenna, does a good job of keeping the static on theantenna out of your rig! On inspection, it will be seen thatthe source is actually tapped across a capacitive voltagedivider. The two caps resonate the inductor connectedacross the antenna. It does provide excellent matching tothe antenna. Values, taken from the handbook are asfollows.

Cl = split stator 200 pf per section.

C2 = single gang 350 pf

The inductor is a variable type having a maximumvalue of 28 mHo Adding a 4:1 balun at the outputmakes this a very versatile matching unit. It also suffersfrom the same problem as other units having an inputcapacitor across the input terminals when used with api-section output tank circuit in the transmitter

The circuit of figure 7 (see page 9) is that of a teesection matching unit. As will be seen, it consists oftwo variable capacitors in series with the load and aof the two capacitors. This configuration represents a highpass characteristic. It can be used for matching to both high

. and low antenna impedances. It does not suffer from the

CQ

ANTENNA

LJ

6ROUND

Radio ZS October 1989

13,3 vol t s (recurring), a meter intenZ ~TVP. of0,5 Megohm and a source ~~n~ of t.J5 ~.f~

Since Vm = 1m.x Rm and since Im = ERs .."..Rm

Vm = E x RmRs+Rm

therefore:-

E = Vm(Rs + Rm)Rm

Substituting the values given above:-

E 13,3 x 0,75 x 1060,5 x 10°

= 20 volts

So, you see, it is possible to find out what the true EMFreally is."

Fred: "That's more like it. Maybe someday we will be

able to have instruments with even higher int ernal resis-tances and better readings. They will probably cost an armand a leg too. hi.n

Joe: 8ffyou have no more questions, then let's see if wecan rustle up some DX - the twenty meter band is wideopen this afternoon. n

Fred: -Ioe, thank you very much, I must be off. I"!III2IJ(tc finish my project. Good luck with the DX."

Joe: "'Fine Fred. If only more feIrows were doing what~ are doing -some construction, the future of amateurrndio~ be more secure.Whenyouhavethat project

JUug.ha:wabout swottingup old Ohm's Law again;aiWe a! Xin:hoffwilldo no harm as well,but, leave theIf:n:tie~~'Ortonand Thevenin alone, they are bound:n ':JIJ~~hi.hi C1Jeecs,see you later Fred mymate."

;m:t "C1ieersJOf; thanks again.-

'The ~ mene rig OIlIoe's desk remained silent all thistime..A grea piIy.

I zs I

Sa,.- ~ saw it in Jhdie ZS

ANTENNA :\IATCIllNG UNIT ROUND-UPE.J.?aD. LoggerenheI'g ZS2LR

CO~~CLLSION

The circuitof figure7 is that of a Leesec6a& ~ As -=?i:De

seen, it consists of two variable ~~ in s:::ri::s...:!:. i:beload and a variableindnctO£I::I."~': - ~ z::dJejunction of the two capacitors. This 1::1...:-'...,...,..~ "qa ,(ba high pass characteristic. It does not::smra :n.-dJe~that other units have when feeding from z.~ ccp:.tank. The writer found it to be a ~ eifedire so---~matching unit. For multtl>anduse. the ~meC ~ ~citors of350 pF each and atapped inclnctonJf~ ";'aSSIdl.it is intended for use with an unb;il:mt'P.CtfeeGa:. ~~a 4:1 balun added at the output of the ~ ~ ~ ~ areused with balanced feeders.

---

> *TRANSf1/TT£R

>fI

Figure7

Construction

The first consideration when constructing any piece of eqmp-ment is of course the aquisition of components.. "The~problem facing the constructor today is the Q...db.!~J :£transmitting type tuning capacitors. They are obt~ -butat a steep price for what you get. There must be a great ~

. of these componentsstored awayin junk boxes~ ~ theworld. The most likely source of supplies of secood handcapacitors is from the old timers who used to roll.their 0'Vi'Dinyears gone by. Flea markets and swop shops proride'anothersource of supply. Roller inductors are worth their weight ingold today! The writer has not been able to lay his bands onone yet!

The matching unit may simply be constructed on a woodenboard (the original breadboard), but to inake full sense of the

Radio ZS November 1989

-:"aere are many other specialised antenna matching units~~beIDondinthehandbooks. Most of them are single~ m:Btst!Singcomponents suited ,mostly to only singlehmd...~ rfu:ingingof coils and adjustment of tappings.~~~f:romoneband to anotler a laborious task200 ~ me fun oar of chasing that elusive rare DX station.£~.::5Ie ~ ~ here can be constructedas multi-~ ":!IIits~ ~ roller type inductors or multi-tap'~brt...,...a;rl~ ~a..1 SP."iI:dJesto changethe valueof- ::;...,-,F

,... ---.L..

. * <ANT"'"

~~ ~..

~

,...

A . ..i. <GND..........

harmonic suppressing capability of the unit, it should beenclosed in a metal box with the case connected to the com-mon RF. earth. The writers choice of a multiband matchingunit would consist of the following:-

L P-section circuit using a roller inductor. Capacitorssuitable for use at the maximum legal power limit with theoutput capacitor plate spacing suitable for working intofairly high impedances. The input capacitor need not havea large spacing since the impedance is low.

2. The inductor would be a roller inductor of about 28,uH. In the absence of such a unit, an inductor tapped andselected bymulti position rotary switches. A large capacityseries variable capacitor could be used for fme adjustmentof the inductor.

5

002L2

S2

'Figure 8

3. A built in noise bridge for matching adjustment offthe ,air, switchable in or out of circuit with an interlockcontact on the selection switch to disable the transmitterwhilethe bridge is in use.

4. A built-in antenna switch capable of selecting at leastthree antennas, a position to disconnect and earth all theantennas when not in use. It would also have a position toselect a suitably rated dummy load for loading and testpurposes.

5. A built-in S.W.R. m~:)Oitoringbridge is desirable butnot essential.

6. A built-in 4:1 balun for use with balanced feeders. Itshould be selectable by means bf sockets and shortingbridges at the rear of the unit.

7. All controls to be mounted on the front panel and. carefullycalibratedto affordpre-setselectionfordifferent

antennas on different bands.

8. All sockets and plugs, ,including the common R.F.~arth terminal, to be on the rear of the enclosure.

9. Most important, the box should have a hinged topcover secured by one easily removable screw to facilitate.any quick modifications that may be required at a laterstage. Amateurs are experimenters!

If you have bothered to read this far, then you are probably ahome-brew type and not a ''buy-it-over-the-counter'' type.Good luck and have fun. Incidently, 150 watts at 53 ohmsrepresents a peak voltage of about 125 volts; at 300 ohms it isabout 300 volts and at 1000ohms it is 1100volts. The peak topeak value, which the capacitors should be I;'ated for, aredouble those voltages.

Getting back to earth, for the benefit of those that may wantto hame brew their own matching unit, but lack ideas, figure8 shows the circuit diagram of one of the writers matchingunits.

,Si and S2 are eleven position single wafer switches

arranged to progressively short out L1 and I2. The twoinductores L1 and L2 are Inounted at ninety degrees toeach other to minimise coupling between the two coils.

L1 consists of 16 turns of 1,18 millimeter bare tinnedcopper wire wound to a diameter of 50 mm and 50 mm

'" 6

LfSOl

l.

long. The taps are made at the following number of turns(including beginning and end).

0 - 2 - 25 - 3 - 3.5 - 4 - 4.5 - 5 - 6 - 7 - 16 turns.

I2 Consists of 40 turns of 1,18 millimeter bare tinnedcopper wire wound to a diameter of 50 mm and 120 mmlong. The taps, as before are:-

0 - 4 - 8 -12 - 16 - 20 - 24 - 28 - 32 - 36 - 40 turns.

A split stator capacitor of200 pF per section was used for C1with the two stator sections paralleled. A 350 pf capacitor wasused for C2. The rotors of the two capacitors were groundedto the common earth connection on the metal case of the unit.Coax sockets were provided at both input and output andbanana socket/binding posts were provided for the antennaand earth connection points. The unit is sufficient for the fulllegal power limit. It will match any unbalanced load such ascoaxial fed antennas or random wires. The unit may also bebuilt using one capacitor with a change-over switch to switchthe capacitor from input to output for matching to lower orhigher impedances respectively.

4:1

BALUN

9

lJ SAL.I

;JUI'1P£RI

]JRANDOMAHU

13ANANA/

. --:J.;- G N D

COAXFigure 9

Figure 9 shows a circuit arrangement for connecting a 4:1balun at the output of the unit so that balanced feeders maybe employed. In order to make the connections as short as

Radio ZS November 1989

S3~=rJS3b:r ;:Js3~Tr~S3d

each end to form a jumper between two a~ t.;...~~posts. The writer's balun is an air \yound unit usiog bHa'windings of 12 turns close wound on a 25mm rn~TT1ffP.rP"'-Cformer. A 42 meter long center fed antenna was ~fed on all bands from eighty meters to ten meters n:siDgd!isarrangement. If you use such a balun, keep it awayfrommea:work and use sufficient insulation for the highvoltage that mayappear on the balanced line.

~

AHU

AI

Figure 10

A2 A3

Radio ZS November 1989

!!::De ~ce of static charges on the antenna are a prob-an. sDnp.Tconnect an RE. choke across the input terminals= memUt.

~ 10 shows a switching arrangement that may be~parered in the unit to switch three antennas and a~;; loadmakinguseof agangedwaferswitch.The single...mx ~ shown is, of course, better.

@]

A3

A2

GND

DL SINGLE WAFER .sWITCH

7