[UK-Contest] Inverted vee etc..... (Peter Hobbs)

[Andy Summers]

> Date: Sat, 22 Feb 2003 01:43:34 -0000
> From: "Peter Hobbs" <peter at tilgate.co.uk>
>
> This is an absolutely wonderful all-band (except WARC) ATU
> designed to feed
> balanced line at any impedance.  It doesn't need a balun, the output coils
> are balanced.  You can also (as I do) feed the antenna off centre, as it
> drives current into the wire at whatever quirky impedance it finds and
> provides a 1:1 match to your expensive and usually fussy PA.

Hi Peter,

Did you realise your feeder should therefore be radiating? See Moxon's book
for the explanation.

The Z-match is an incredibly versatile and simple to use ATU, but you don't
get something for nothing! The bit I don't like about the Z-match is the
step-down transformer action necessary to be able to match impedances lower
than 50R. The net effect is that feeding hi-z loads results in even higher-z
in the resonant tank. They're therefore a bit prone to capacitor arcing,
narrower bandwidth and higher losses. I have an SEM TranZmatch and can vouch
for the arcing! The lowest Q and lowest loss configuration is the L-match,
but it is not as versatile as the Z-match. Current vogue is to place a balun
at the 50R end to deal with balanced loads, avoiding the higher losses
possible with reactive balun loads that Ivan alludes to. However, I'm not
too convinced about how equal the feeder currents would be with such an
arrangement. I'd be interested to know if anyone has actually measured or
simulated the balance with such an arrangement?

I too have used open wire/ribbon feeder in the past. If you can get it into
the shack without it touching trees, roofs, brickwork, etc., you can be sure
that it's a low loss installation, but if you can't there's always a
niggling doubt about whether you have higher losses than you thought. Don't
forget that you can get just as high voltage nodes on the feeder as the
antenna. In my domestic installations I now tend to avoid open wire line
because I can't get an ideal routing, especially when the XYL applies her
veto!

73,
Andy, G4KNO.