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As this project contains components not freely available it is presented for interest only!
My main HF aerial is the classic inverted 'L' and is described elsewhere on this site. It is a great all
round aerial, allowing operation on all bands from 160-10m when tuned with the Picatune ATU. It works flawlessly at the 200
watt level but I have on occasion damaged it when trying to use 400 watts of SSB, particularly on 80m where the aerial
feedpoint impedance is extremely high. The calculated VSWR is in fact 73:1 which is no mean feat to match even at the
200 watt level!
The best way to overcome this would be to alter the geometry of the aerial but I am limited to
what I can do without having to disturb the ground system and feedpoint location.
This led me to think about constructing
a custom, remotley controlled ATU, built just for my aerial and using higher rated components available in my junk
box and those of my friends.
The starting point was the innards of an old commecial 3KW T match ATU.
I decided to use the 'L' match circuit and fortunately
my aerial presents a higher rather than lower impedance than 50 ohms on all bands. This simplified the switching of the L
match components to just a series inductance and parallel capacitance.
The first job was to strip out everything not required
The capacitor drive mechanism is a 148:1 12v hobby motor obtained
from Maplin and an old, but high quality reduction drive.
Next I fashioned a polycarbonate plate to mount the rest of the component
on.
Next up was measurement at the coal face and some 10KV
fixed capacitors donated by Mark G4FPH to help with matching on 160m.
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