A single coil Z-Match ATU
This
single coil Z match ATU project, reproduced here with the kind
permission of the editor of Practical Wireless magazine, is the
easiest and cheapest to build and the most effective of all the
ATUs I have built. It tunes my W3DZZ trapped dipole perfectly
from the top of the 10m band to the bottom of the 80m band albeit
that like all Z matches its resonance points are very sharp and
reduction drives are therefore essential. You need a single
500pF air variable capacitor, a twin ganged 500pF/500pF air variable
capacitor, a T130-2 torr
oid (available from
several mail order retailers for a couple of pounds), a couple
of reduction drives, some solid core 22swg wire either PVC covered
or enamelled, and the sockets and a metal case to put them all
in of course. First wind the torroid as shown with 11 turns
to the first tap, 5 turns to the next tap and then 11 turns to
the end. The whole 27 turns should occupy only about 3/4
of the torroid. Then wind on 7 turns from the earthy end
as shown in green. Since I was feeding my antenna with coaxial
cable, I could twist the ends of the output winding and the 'earthy'
end of the 11 turns together and ground them both as shown in
the circuit diagram. However, if you want to feed a balanced
output, just leave the ends of
the output winding free and take them straight to connectors insulated
from the case. Once you have wound the torroid there is
nothing more to do than connect everything as per the circuit
diagram. Don't forget though that the loading capacitor
is live on both sides, so you must insulate it from the case and
use an insulated shaft to your control knob. Talking
of control knobs, I was lucky enough to pick up a pair of lovely
old calibra
ted dials with built-in
reduction drives at a rally, and I used these for the fianl version,
(right). Calibrated dials or turn counter knobs are
necessary because the average reduction drive will rotate 3 complete
turns to move the capacitors from fully unmeshed to fully meshed. If
you can't lay your hands on calibrated dials or turn counter knobs
you will have to use your ingenuity to devise a method of marking
the front panel scales to show the capacitor settings. This
is important because you will want to make up a table of the capacitor
settings for the centre of each band so that you don't have to
go through the tuning process from scratch every time to find
your exact tuning point.
A T-match ATU
There is no
t
really much to say about a T-match ATU because it has only 3 off-the-shelf
components. The hardest part is laying your hands on a decent
inductor. You can easily make one up of course and use switched
taps - 25 turns of 2mm diameter wire spaced at about 6tpi on a
50mm former and tapped at 2, 5, 8, 12, 16 and 20 turns from the
non-earthed end should do it - but if you can get hold of a roller
coaster so much the better. Having got your inductor the
only other things you need are a couple of 200 - 250pF variable
capacitors and a couple of wafer switches, all suitably rated
for your intended power output of course, and a box to put them
all in. Connect the switch wipers to the output sockets
with coaxial cable, join their braids together at the switch and
th
at's about all there is to it.
I didn't have 
a suitable metal enclosure
to put my T match in but I did have a nice ABS plastic enclosure
on the shelf so I used that and lined it with kitchen foil to
screen it. The dial labels were printed out on the computer and
stuck on and the whole thing looks nice and works very well indeed. By
the way, if your roller coaster has not got a counter you can
make one by robbing the mechanical counter from an old cassette
deck and drive it with a simple elastic band round the roller
coaster drive shaft. That's all that I did here and it works brilliantly.
The antenna switching system I incorporated works nicely too and
it's probably worth showing it, so here it is.....................................