Can a Manual L-match Get the Job Done?

Or can it be made just a little bit better?

In part 3 of my manpack series of posts I discussed how and why I replaced a perfectly good LDG Z-11 Pro autotuner with a home-made manual L-match. To summarize, the autotuner was doing a great job but was using too much space inside the plastic “ammo case” enclosure. So how is the “QROp” rebuild working out so far?

Removing the LDG unit created lots of space inside the plastic case enabling me to place my Bioenno 12AH LFP battery inside the case, and store headphones, paddles and associated cabling inside the case too. Now it’s a grab’n’go pack.

But what about the L-match? Is it doing a good job? Actually it is doing a fine job. I tested some of my field antennas using the L-match and obtained very encouraging results.

• End-Fed Half-Wave (EFHW) for 40,20,15,10m: the EFHW was connected through a very short (~12 inches) length of RG-58 coax. The feedline of an EFHW is often used as a counterpoise and, since this isn’t possible with a very short feedline, a counterpoise wire was added (0.05 lambda is recommended; I used 6 feet). An EFHW is “resonant” on its design frequency and its harmonics. Nonetheless, some touch-up is often required even on the design frequency, but especially on the harmonics. This was an easy task for the L-match:
  • 40m SWR 1.5:1
  • 20m SWR 1.0:1
  • 15m SWR 1.2:1
  • 10m SWR 1.8:1
• End-Fed Random Wire (EFRW): this test involved an 84ft radiator in inverted-V mode. The radiator was directly connected to the manpack without using any kind of unun/balun/transformer. Two 17ft raised counterpoise wires were added. An EFRW is, by design, not resonant on any band so this was a real test for the L-match. Here are the results:
  • 80m SWR 1.1:1
  • 40m SWR 1.2:1
  • 20m SWR 1.7:1
  • 17m SWR 1.5:1
  • 15m SWR 2.0:1
  • 12m SWR 1.0:1
  • 10m SWR 2.0:1
• Other antennas were also tested with very similar results. a 28.5ft EFRW tuned up on all bands from 40m up to 10m, but the L-match failed to find a sweet spot on 17m. My 20m Zepp antenna comprising 17ft of window line with a 33ft radiator tuned up lickety-split.

Tuning with the L-match is fast and easy. First, the 12-position rotary switch attached to the variable inductor (18 microhenries max) is set for maximum noise, then the variable capacitor is used to peak the noise. Finally, a quick check by keying up and making final adjustments for lowest SWR reading on the radio (in my case that’s a FT-891).

Antenna impedance all around the Smith Chart

An L-match can be configured for matching either a low impedance or a high impedance antenna. The basic architecture for an L-match comprises a series inductor and a capacitor that is parallel to either the input or the output. I wired mine to be switch selectable. Inevitably some antennas, such as an 84ft random wire that can cover a very wide range of bands, will have sweet spots all over the Smith Chart. Sometimes the low impedance setting produces the best match while on other bands the high impedance setting is better.

Why a very short – or zero – feedline?

We are often lulled into thinking that if the radio is happy with the SWR it is seeing, then all is well with the antenna. This is only true under two conditions: if the feedline is electrically a half-wavelength long (or multiples) – or is zero length. It is possible – indeed likely – that under any other conditions the SWR at the antenna will be quite different to the SWR at the radio end of the feedline. Analysis using a Smith Chart will show that the value of R+jX (or R-jX) moves around the chart according to the length of feedline used. And who uses a half-wave feedline anyway? Taking the popular RG-8X as an example, calculating a half-wavelength for 40m using a velocity factor of 84% gives a coax length of about 56 feet. Of course that would change for every band, but a zero length feedline has the same electrical length on every band, period.

Alright, a zero length feedline is only practical for field expedient portable antennas, and even then it isn’t always convenient. In some circumstances (like when operating inside a vehicle in winter) a coax cable is needed. We just need to be aware that, if operating conditions necessitate a compromise, then it is better to be outside and on the air than at home fretting about perfection.

But what if …?

Okay, the L-match is a success; if only there was peace in the world and I had an unlimited ham radio budget then life would be a bunch of roses. So what am I thinking of tinkering with next? Well, there is one design “flaw” with my L-match. It relates to the tapped inductor. The total inductance is 18 “michael-henrys”; there are 12 taps and I chose to space them evenly around the coil. That places a limit on the precision with which I can select an inductance. The 12-position rotary switch could be replaced with a crocodile clip to select individual turns on the coil, but that is not very convenient when the L-match is inside a weather-protected case. There is another solution that I am considering.

Many years ago I participated in a club builder project. The result was a QRP tuner that uses a a series of toggle switches to select the required inductance. There are 7 inductors with values of 8, 4, 2, 1, 0.5, 0.25, 0.12 uH. The very interesting thing about this tuner is that the total inductance can be selected with a precision of 0.12 microhenries. Selecting the inductance is achieved by setting each of 7 toggle switches in binary fashion where the least significant switch selects the 0.12 uH coil and the most significant switch selects the 8.0 uH coil. If you understand binary notation this will be self-explanatory.

There are 10 types of people; those who understand binary notation and those who don’t.

Setting all those switches sounds complicated and a very slow way to achieve a match, but in fact it is not. The slow bit is only performed once and the binary word for the correct switch settings for that band recorded. Minor adjustments may be needed in the field as every field antenna erection is different, but that is a very fast and easy procedure.

Will I make this mod on my QROp manpack? Probably not, but somewhere in the future – if the great spirit in the sky grants me that much time – I will take delivery of my bought and paid for QRP Labs QMX QRP radio, then this old club project tuner will come in very useful. And that is a good segue to …

And now for something completely different

I have been bleating about my QMX order delay for a while, but there is progress. My order is now only number 368 on the waiting list. That is after waiting 4 months already. QRP Labs is shipping on average about 30 orders per week, so my QMX ETA is still another 3 months away. The company has an order book of 850 radios!

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