The Cranky Sysadmin A world of technology, fun, and ignorant rants.

January 1, 2012

Antennas and a Homebrew T-match

Filed under: ham radio — Cranky Sysadmin @ 10:07 am

In my last post, I mentioned that I was having much better luck with a slinky dipole then a G5RV. It turns out that I was misinterpreting the data. I can match the slinky on 4 HF bands without external tuning, not all of them. I can match the G5RV on 2 bands without external tuning. This of course led me to trying to find an antenna tuner. It turns out that they are expensive if you want one with good range.

I don’t want to spend the coin on a tuner, so I made one. I started with an SPC tuner but I was disappointed by the performance. After I read a detailed page comparing the performance of many matching networks, I converted my SPC tuner into a T-match. The performance is a lot better.

The construction of this T-match is detailed below. I built one of the capacitors and the inductor. I’ll go into a bit of detail on that. I got lazy late in construction and used an air variable capacitor from an old TV tuner. That was probably a mistake since at 80 meters and 100 watts, dielectric failure occurs. Sadly, I didn’t get pictures.

The air variable capacitor is made from a foot of 1/4″ threaded rod, some 1/4″ nuts and flat washers, aluminium flashing, and a plastic cutting board (my thanks to the better half for donating). Below are some pictures during construction:

A CD and a square are used to measure the rotor and stator cutouts

Use a utility knife to cut the flashing

Once there is a nice etch into the flashing, you can just crease it and seperate it.

Use 3 flat washers to seperate the plates. Use end nuts to lock them in place.

The almost finished product. Just add a knob and adjust so the plates mesh without touching.

The inductor is about 20 turnsĀ  of 12 gauge over a 1 and 5/8″ pvc pipe. I disassembled some Romax to get the bare wire. What a pain in the neck that is. All connections inside the match are this same 12 gauge wire. I added a ground pipe on the left side for use by my whole station (all two pieces) since I haven’t built a proper one yet. Below is a picture of the completed transmatch in use.

I didn't have proper knobs, so I made some crude ones.

The performance is pretty good on most bands. There are some problems though. At 80 meters, the little manufactured air variable sparks when I key up. There are also a couple of bands I can’t match because my inductor doesn’t have enough turns. There are also some places where I can’t get down to 1:1 because the capacitors aren’t high enough value. So my list of improvements:

  1. Make an inductor with more turns
  2. Make another air variable capacitor with a wider gap
  3. Add two switches so I have the option of shorting the capacitors and getting a larger range of capacitance (in an L network).
  4. Consider a metal case so I don’t cook my innards or disturb my wife’s TV viewing.

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