From Dave G3YMC
It is good that there are now several on the reflector taking a re-look at
loops.
However I feel there is far too much 're-inventing the wheel'. There has been
much
information about loops for some years on my site
(http://www.dsergeant.btinternet.co.uk/loops.htm), GW4ALGs site
(http://www.alg.demon.co.uk/radio/136/intro.htm) and elsewhere. My site gives
a lot
of information including loop calculations, theory and practical advice. It
seems
there may be some on the reflector who have not looked at it.
As Bill Ashlock wrote, the dc resistance of Mal's loop at 1.5 ohms is indeed
far too
high. Similarly the 1 mohm for radiation resistance mentioned by Rik ON7YD is
also
way too high if conventional loop theory is to be believed. They should both
compare
them with the information on my site.
I, along with GW4ALG, use a totally capacitive network for matching - Steve's is
slightly different in that he uses a balanced configuration but also makes the
mistake of having the network in the shack rather than at the feedpoint.
Neither of
us has tried transformer matching, and it remains to be seen if the losses and
potential current handling problems are better or worse than our capacitive
network.
Steve has used his loop at 400W with no problems. But with the values of
capacitor
required (in my case 22nF and 200nF) mica caps are out, and Philips 376
polypropylene
are used
The important things for any successful transmitting loop (which I admit I
haven't
achieved!) are:
Use thick wire to keep the dc (and ac) resistance to as low as value as
possible.
Fractions of an ohm are a ball park figure.
Whatever matching method you use, the network must be at the feedpoint so that
feeder
losses are not in the loop circuit.
Earth losses are immaterial and the loop does NOT need to be kept away from
ground or
other lossy things.
Good luck with your loop Mal.
ps. I note G4CNN mentions working Dick G3LCB yesterday. Not in fact a new
station,
but I haven't heard him for many months - and notice that I have never worked
him.
Was busy on the other computer sorting out Linux at the time so missed him.
73s Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
|
