Six Meter Transmatch

Six Meter Transmatch (Tuner)

I don't have a dedicated 6M antenna (up anyways) and my auto tuner has difficulty tuning my 40-20-10 fan dipole I rolled my own dedicated 6M tuner. Technically it is a Transmatch or Coupler. Antenna tuners are located at the antenna, but the term tuner seems to be widespread for such units now. The typical configuration of most tuners is either an L or T. I selected the Pi configuration because it is simple and the caps I selected (for small size and range) would be grounded.

Yes, there are no knobs. These are air variable trimmers and adjusted with a screwdriver. I use one part of 6M so it is "set and forget". Unlike wide range HF tuners, this one is band specific so the inductor doesn’t need to be adjustable. I just picked a typical value and went with that. It is a classic Pi network with the coil in series and a cap on either side to ground. The caps are not critical, but should be the same values.

(CTA) APC-140
APC style air variable trimmer capacitor.
6.5 - 140 pF @ 650v. Mounts with (2) #6 tapped holes on front. Slot adjust. 13/16" wide x 1-1/4" high x 1-7/8" long. Ceramic insulator.
From: Surplus Sales of Nebraska

Parts List;
BUD Box, 5x4x2        $12
Bud Cover, 5x4          $ 5
C1                              $12
C2                              $12
J1                               $ 7
J2                               $ 7   

Total cost                         ~$55

The coil is 6 1/2 turns of #20 with a ~3/8" ID and 1 1/2" long. A handy form on my bench was the body of an ink pen. The wire springs open a little once wound so it’s a little larger than the 3/8” form. No worries. As wound, this coil is ~0.09µH. I ended up using just four turns of the coil, for a final value of ~0.06µH. You can use larger wire if you wish, it is just harder to work with in tight spaces and small diameters.

**NOTE - I changed the wire I used for the coil to a #12, same ID and same number of turns and same taps. (Finally found the bus wire I was looking for when I started this project.)

You shouldn’t need a larger coil unless you were going to maybe use it on 10M. Some trial and error would be required if you use a different coil. Spreading the turns out lowers the coil value, squeezing them together increases the coil value (with the diameter remaining the same).
As long as you can tune your desired load with the caps not set to Min or Max you have a winner. N connectors are my personal choice for this project (and I despise PL259's in general).

The coil in the photo is one I made out of enameled wire because I didn’t have a piece of #20 buss wire handy at the time. The prototype coil was made from #22, but I wanted something stiffer. The buss wire connecting the caps to the post in the middle is the ground. The caps do not have a direct mechanical ground, so one needed to be added.

It is hand built, it works, costs less than something with MFJ on it (MFJ-903 six meter tuner lists for $69.95) and will outlast me. I hooked it up, took 15 seconds to tune to my antenna using the VSWR indicator on my rig and got a VK4 a few minutes later. Distance of 3,323 miles with 100W and a very non-resonant antenna. The magic band rings true. My closest 6 Meter QSO has been with Guam, 1,534 miles to the west.

CAUTION! Building your own gear puts you at risk of many happy hours of tinkering, drilling, soldering and learning new things!!

1 Mar 2016