160m Vacuum Variable Capacitor project

While operating on top band at ZD8UW I had quite a few problems getting the inverted “L” to match at 50 ohms. Even after adjusting the element length this is virtually impossible due to the nature of the antenna. To make a suitable match I’ve now built a capacitive loading unit, using a vacuum variable capacitor (10 – 1200pF).

This should solve my matching problems and allow the vertical to be used on 80m as well, as it will match a very broad range of antennas provided the electrical length exceeds the resonant length. While it might seem easier to use inductive loading on a short antenna, but that would be comparatively lossy. I like to have all the efficiency possible.

Vacuum variable capacitors work well at higher potential voltages without arcing, but they are quite expensive. I bought this unit on eBay complete with fittings for about £120 (from Ukraine). I built the housing and connections in a few hours using parts from Farnnell.

Another option is to use L/C circuits (like a ‘T’ match) so that the length of the wire is not so important. Also, a hair-pin match can raise the impedance of a 160m vertical from 25 ohms back to 50 ohms, improving the match. Lastly, a 1:2 un-un can help with the same problem with really great results (and helps with coax feeds, too). I have tended to use this approach more and more recently.

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