On 25 Nov 2002 at 17:58, Rik Strobbe wrote:
I am about to put up a transmitting loop antenna. Dimensions will be 23m
long and 15m high. Thanks to Dick (PA0SE) I could achieve a wheel of 7mm
litz wire, so wire losses should be minimized (about 0.04 Ohm).
Excellent news! Hope it works out well.
You may like to consider a totally capacitor matching network as
shown on my website. This may be a problem at high powers but nothing
like as bad as some on this reflector prophesy. Philips 376
capacitors are pretty beefy components and can handling pretty big
currents. Doing this will eliminate any loss in the matching
transformer. There may however be some virtue in using a transformer
to provide a balanced feed - see the arrangement Steve GW4ALG used
when he was active on the band
(http://www.alg.demon.co.uk/radio/136/intro.htm) which is a variation
of mine. Steve made the mistake of having his matching network remote
in the shack, so his feeder loss was in series with the loop
resistance - the transformation to 50 ohms MUST be done right at the
feedpoint.
Your hoped for 0.04 ohms looks rather optimistic! If it is really
going to be this low then height above ground may be important. In my
case the effective ground coupling loss relates to around 0.3ohms,
half of my total loss. You may find it hard to reduce this low
enough, even by raising the loop quite high above ground, to avoid
swamping your wire loss.
In my case, the base of the loop is only around 0.5m above ground,
and runs against my garden fence. This was a compromise, as it was in
any case difficult to get it in the clear and putting the base higher
without being able to increase the height of the top would result in
the loop area being reduced. Swings and roundabouts, and probably why
it never really worked that well.
Good luck, keep us informed.
73s Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
|
