Owen Duffy wrote:
. . .
If you try something beside a shielded loop, make sure you use an
effective means of isolating the feed line from the loop so that pickup
on the feedline does not feed the receiver.
. . .

In my limited experience, it's extremely difficult or impossible to do a
good job of isolating the feedline from a small loop. The common mode
impedance is just too high.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

Owen Duffy wrote:
. . .
If you try something beside a shielded loop, make sure you use an
effective means of isolating the feed line from the loop so that
pickup on the feedline does not feed the receiver.
. . .

In my limited experience, it's extremely difficult or impossible to do
a good job of isolating the feedline from a small loop. The common
mode impedance is just too high.

Roy Lewallen, W7EL


Hello Roy,

I have a small loop with inexpesive voltage balun, and it gives a quite
deep null (though I can't put dB figures on it off hand), and very sharp
null.

I wonder if the reason that Tony perceives that the nulls are shallow is
that he is assessing it on band noise. If so Tony, you should assess the
depth of the nulls on a local (ie low elevation) point source that is
much stronger than band noise. The test signal should dominate the
receiver.

If an antenna with deep nulls doesn't reduce band noise much, it
suggests that the noise is not mainly from a single direction... as
discussed earlier in this thread.

Owen

Owen Duffy wrote:

Roy Lewallen wrote in
:

Owen Duffy wrote:
. . .
If you try something beside a shielded loop, make sure you use an
effective means of isolating the feed line from the loop so that
pickup on the feedline does not feed the receiver.
. . .

In my limited experience, it's extremely difficult or impossible to do
a good job of isolating the feedline from a small loop. The common
mode impedance is just too high.

Roy Lewallen, W7EL


Hello Roy,

I have a small loop with inexpesive voltage balun, and it gives a quite
deep null (though I can't put dB figures on it off hand), and very sharp
null.

I wonder if the reason that Tony perceives that the nulls are shallow is
that he is assessing it on band noise. If so Tony, you should assess the
depth of the nulls on a local (ie low elevation) point source that is
much stronger than band noise. The test signal should dominate the
receiver.

If an antenna with deep nulls doesn't reduce band noise much, it
suggests that the noise is not mainly from a single direction... as
discussed earlier in this thread.

Owen

the band has been devoid of strong noise the past few days. I think I know
who isn't home in my neighborhood and where the noise may be coming from.
All I need do is have the noise return when she gets home from her trip.

The loop points at her home and the one behind me when the noise is up.
Maybe we got the culprit.

I had recorded an hour and a half of a pileup awhile back and in lstening to
the recording again there was almost no noise. this was 3-4 months ago.
As I had posted previously the noise showed up a little over a month ago.

Oh, one more thing, I will be replacing the coax on the loop over the next
day or so. My gut tells me to try this.

Just because you purchase a new run of coax it doesn't mean it doesn't have
a defect. Its the only thing I haven't looked at.

On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote:

Owen Duffy wrote:
. . .
If you try something beside a shielded loop, make sure you use an
effective means of isolating the feed line from the loop so that pickup
on the feedline does not feed the receiver.
. . .

In my limited experience, it's extremely difficult or impossible to do a
good job of isolating the feedline from a small loop. The common mode
impedance is just too high.

Roy Lewallen, W7EL

Hello

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

Mario

mario wrote:
On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote:
In my limited experience, it's extremely difficult or impossible to do a
good job of isolating the feedline from a small loop. The common mode
impedance is just too high.

Roy Lewallen, W7EL

Hello

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

Roy Lewallen wrote in
:

....
mode rejection. I'd be interested in hearing how Owen managed to get
good nulls from an unshielded loop.

Roy,

Perhaps some of this goes to the meaning of "good nulls".

I have just performed a simple test with the antenna described at
http://www.vk1od.net/SmallUntunedSquareLoop/index.htm .

I have inserted a step attenuator between the antenna and receiver, have
10m of coax draped over the ground, and measured the depth of null on a
nearby broadcast station on 1.2MHz. The depth of the nulls are between
30dB and 40dB, and the two nulls are within a couple of degrees of 180°
apart and of similar depth as you would expect from a loop with little
feed line contribution.

The nulls from this antenna are certainly very sharp for the purposes of
direction finding, they are orthogonal to the plane of the loop, and
symmetrical, the nulls or of similar depth. In my experience, the
smallest contribution of feedline pickup is shown as unequal nulls.

Owen

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

Roy Lewallen wrote:

mario wrote:
On Sat, 24 Nov 2007 23:41:19 -0800, Roy Lewallen wrote:
In my limited experience, it's extremely difficult or impossible to do a
good job of isolating the feedline from a small loop. The common mode
impedance is just too high.

Roy Lewallen, W7EL

Hello

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


I ran across a small loop with some interesting add ons to solve the common
mode problem while looking for info on the loop here on the web. I will try
and find it.