Keeping in Perfect Tune: A Film-Can Transmatch
|By Bill Jones, KD7S
The ARS Sojourner
|Within hours of my story, "A Thing of Beauty: It All Started with a Tree," appeared in last month's edition of The ARS Sojourner I started getting e-mail.
Dozens of people wrote asking for more information on my little Film-Can Transmatch (Figure 1). Apparently I'm not the only one interested in lightweight, portable gear.
Before I go into detail, perspective builders need to understand that this transmatch has two limitations. First, it is designed to be used with an end fed, half-wave wire antenna working against a quarter wave counterpoise. It will not tune a dipole or quarter wave ground plane antenna.
Second, it is pretty much a single band device. As described, it tunes 20 meters.
The Film-Can Transmatch (Figure 2) consists of little more than a parallel tuned circuit, a couple of binding posts and a short length of coaxial cable.
BP1 and BP2 are 5-way binding posts that connect the antenna and counterpoise to the tuned circuit. A short length of RG-58 coaxial cable is soldered to the 4 turn link on T1. This connects to the transceiver. Figure 3 is a photograph of the transmatch removed from the film can.
Components are soldered to a scrap of perfboard. The perfboard is mounted to the film can with 4-40 screws and some small aluminum spacers. The trimmer capacitor is adjusted via a small hole at the bottom of the film can.
Without some kind of SWR indicator, a transmatch isn't very useful. I have found the LED based bridge designed by Dan Tayloe, N7VE, is a perfect companion to the Film-Can Transmatch. For detailed construction information on this very clever design read Richard Fisher, KI6SN's article, "A Bright Idea for Wrestling With SWR in the Field" in the September 1998 edition of The ARS Sojourner.
For as simple as it is, the Film-Can Transmatch does a great job. I have always been able to achieve a 1:1 SWR regardless of how the antenna and counterpoise are deployed. But don't take my word for it. Build one for yourself. You'll be happy with the results.
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Bill Jones, KD7S, a renowned QRP radio and accessory builder, is a contributing editor to The ARS Sojourner living in Sanger, CA.