On Nov 25, 5:39 pm, Roy Lewallen wrote:


How much (estimated) would this common mode impedance be for a 3 foot
diameter loop for instance 80 meter band?

It depends on height above ground, but according to EZNEC it's on the
order of 1000 ohms when 6 feet above ground. You'd probably need a
receiver or circuit with differential input and very good common mode
rejection ratio, and perhaps also an exceptionally good balun, to
prevent the feedline response from reducing the null depth unless using
a "shielded" loop which by its construction provides good common mode
rejection. I'd be interested in hearing how Owen managed to get good
nulls from an unshielded loop.

Roy Lewallen, W7EL

Hummm.. I knew that the shielded loop helped with balance,
but I had never really given much thought to it reducing common
mode currents when feeding with coax. But I guess it should.
I compared using both a plain wire and a shielded coax loop as
the coupling loop on my 16 inch version, and saw little if any
difference in performance, or perceived common mode problems.
But I decided to use the shielded coupling loop just to be
on the safe side and help ensure balance even though the
actual performance of the two were nearly identical.

Maybe I should consider using a shielded coupling loop
on my larger loop just to give it that extra advantage.
I might have already tried one, but I forgot... It's been a
while since I built those.
It does have very deep nulls with just the plain wire
coupling loop though. Both of my loops seem about the same
as far as null depth. I just get more level from the bigger one,
and maybe just a tiny bit better weak signal performance.
I'm usually using an Icom 706mk2g as the receiver, so it's
nothing special thats for sure.
BTW, I just remembered something.. When comparing a
loop fed directly , you often see a skewing of the pattern
slightly off where it's actually pointing.
But I found when using a coupling loop, even if plain wire,
this skewing of the pattern is greatly reduced.
So this leads me to believe using even a plain wire coupling
loop will reduce common mode problems vs feeding directly.
But using the shielded loop for coupling should be a pretty
good "brute force" method.
MK