OBSERVATIONS FROM SCOTLAND                         28/05/2008                                GM1SXX

The humble L-Match.

Sometimes you just can't be bothered .... either with spending money, or lugging too many things along, so for QRP use, the humble L-Match has a use.  You don't need to lug along a commercially made automatic  'Antenna Tuner' or the like. If you want to operate on the trail or in the hills, with not much in the way of equipment, a rig, a simple L-Match and a couple of lengths of wire will do the job.   The L-match is so simple, that anyone can make one. What's more, it does a good job of work and can be constructed cheaply using junk-box parts.

The L-match consists of nothing more than a tapped inductor in series and a variable capacitor across the feed to your aerial and ground. The matcher should go at the end of your feed-line, where it feeds the aerial system (yes really).

The aerial too can be cheap and lightweight. Nothing more than a pair of measured lengths of wire with markers (I like knots) at strategically important places. Any unused wire can just be rolled up.  Such an aerial can be used in a number of ways. If you don't know how, try Google.  I'm told that 'Google is my friend'.

So back to the L-Match.  It's nothing more than a simple L/C network used to match your bit of wire or whatever to the impedance of the feedline. It does absolutely *not* 'tune' the aerial.  Whenever you see the term 'Antenna Tuner'... think 'aerial matcher'.... unless you speak American.  If you speak Americanese, Think 'Antenna Matcher'... not 'tuner'.

Tuning aerials to resonance is generally done by 'pruning' or otherwise adjusting their length.  So, our cheap and cheerful L-Match is not tuning anything..... it's merely used to provide an acceptable impedance match between the aerial system and the feeder.  It does this by adding inductance in series with or capacitance in parallel with the aerial wires.  This in turn keeps your rig happy, because it most likely wants to see a 52R resistive load.

The coil was wound using some fine enamelled wire I had lying around. Since this matcher will only be used for QRP work, the small wire gauge is not a problem.  It's wound on a former made from a short piece of scrap PVC plastic conduit I found in the back garden.  I drilled a small hole and anchored the wire I then wound on the following turns.

0,2,4,6,8,12,16,24,36.  The exact number of turns is not at all critical, but it's common to pretty much place double the previous number at each increment.  At each tapping point, I simply looped the wire back on itself for about an inch and twisted it up tight to form a connection point before winding the next lot of turns. In this way, I wound the complete coil, slightly staggering the taps to make them easier to solder to. When the coil was wound, I 'locked' the wire in place with three strips of hot-melt glue run along the length of the former.  You can use tape or whatever insulating material you may have lying around.

I tinned the loops using a very hot iron to melt the insulating lacquer. My wire is of the self-fluxing type, easy to solder through.  I then soldered on some 'tails' to the taps. These are the red wires you see going to the switch. These I folded against the coil and coated with hot-melt glue to give them some rigidity.  The coil former doesn't have to be plastic pipe. If you are stuck, a length of varnished bog-roll cardboard centre will also work fine. I glued the coil former to the cabinet with a dollop of, yes you guessed it! Hot-Melt.

My tuning capacitor came from EBay. It cost a fiver plus postage and was a brand new (NOS) yaesu transceiver spare. It's a 330pf type with silver-plated brass plates.  You can substitute a variable from an old valve radio or even a cheap polyvaricon, provided you are using QRP power levels.  The capacitor value is not critical. 330-500pf would be about right. You could even use a lower value capacitor with a set of switched fixed micas or styrene capacitors paralleled with it.  Be inventive! Use what's in the junkbox!

The red and black terminals were purchased along with the plastic box (Maplin). The terminals can take plain wires or wires terminated in wander plugs.  The transceiver input is a 50R BNC 'thieved' from a scrapped DFM.

I used a switch made by 'LORLIN' that was a 4 pole three way type.  No damned good, cause I wanted a single pole switch with lots of ways!   I clamped a pair of small Mole Grips onto the shaft at the front (flush against the start of the fixing threads) and popped the back off the switch by prying the 4 plastic 'hooks' free. I ripped off three of the four wiping contacts, soldered a link between all '4 poles' and reassembled the switch. I removed the small Mole Grips (they were holding the shaft in place so the spring and two ball-bearings wouldn't fly out!)....... and hey presto... I had a 1 pole 12 way switch.  Of course YOU could do the clever thing and BUY a 1 pole 12 way switch instead!   The switch is arranged so that it can switch between zero and maximum inductance. Just blank off the unused 'ways' by moving the little tab that's under the fixing washer. Most rotary switches seem to use this arrangement now. If you leave unused 'ways' in place, your matcher will be open-circuit in those positions... since the coil is in series. Important to remember!

And no, I won't insult your intelligence by including a circuit, it's only a capacitor and inductor after all, but should you be interested in aerial matching and suchlike, you'd do well to visit Ray, G4NSJ's Website where you can read more about how such things work.

For the confused.

73 AL.
GM1SXX