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Old November 25th 07, 05:35 PM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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
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Old November 25th 07, 10:15 PM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

On Nov 25, 12:46 am, Tony Giacometti wrote:


Ok, I want to build a larger loop to get more gain, if possible deep nulls
for reducing local noise, I would like to try wire instead of coax. I want
to mount it on a PVC pipe frame. Can you give me any ideas as to what shape
and size? Whats a solenoid loop?


A solenoid loop is like this..
http://home.comcast.net/~nm5k/loop5.jpg
A pancake loop is about the same, except
that the wires are stacked in the same plane
like an electric range burner, vs being side by
side like the solenoid loop.
All the PVC loops I make are diamonds,
fed at the bottom corner.
The depth of the nulls is due to balance, not
the size of the loop.
But... In some cases it's easier to get good
balance on a larger loop as it's easier to
ensure a symmetrical loop as related to the
mounting hardware, feed devices, etc..
But in my case, even my smallest 16 inch
loop is very symmetrical and has just as
good of balance as my larger loops.
So size doesn't mean much. Care in symmetrical
construction means much more.
I've never had common mode problems to be a
big deal. I may have a little, but when I tune
the loop, the greatly increased "tuned" level
swamps any small common mode signal I may
have.
So in effect, it's usually not an issue.
I do try to keep the feedlines only as long as
needed to reach the antenna, and I keep
them on the floor.
Another thing that I think helps a bit is to
use a separate coupling loop to feed the
loop. But this may help with balance more
than common mode reduction. But I still
recommend it none the less.
One thing.. make sure there is not metal
around the antenna. This can whack things
out fairly fast. IE: metal frame chairs, file
cabinets, etc...
On my loops, I've use no baluns at all so far,
but as my recordings prove, I have no lack
of nulls, or signal level for that matter.
When I null a single point ground wave signal,
say bye bye.. It's gone. :/
BTW, you will never have good luck at nulling
general "band noise". When you say "band noise",
I will assume you mean all the noise received from
all directions at once. As has been repeated
many times, if the noise source is not from a
single source, the loop will not be able to provide
a decent null. You will never be able to null random
noise. But if say a power line device starts making
noise from a single source, you can null almost
all of it, and still hear most of the skywave signal
you are trying to receive.
It's quite possible you are in a location with multiple
noise sources, and the small loops may not do the
job you want them to. In general, a loop is a loop is
a loop.. If the one you have is working properly, and
you are having trouble nulling the noise sources,
going to a bigger loop may not help much at all.
MK


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Old November 26th 07, 12:39 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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
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Old November 26th 07, 01:26 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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
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Old November 26th 07, 01:37 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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


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Old November 28th 07, 06:03 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

wrote:

On Nov 25, 12:46 am, Tony Giacometti wrote:


Ok, I want to build a larger loop to get more gain, if possible deep
nulls for reducing local noise, I would like to try wire instead of coax.
I want to mount it on a PVC pipe frame. Can you give me any ideas as to
what shape and size? Whats a solenoid loop?


A solenoid loop is like this..
http://home.comcast.net/~nm5k/loop5.jpg

I don't really see the difference between that loop and the one I have now
except for the coupling loop.



A pancake loop is about the same, except
that the wires are stacked in the same plane
like an electric range burner, vs being side by
side like the solenoid loop.
All the PVC loops I make are diamonds,
fed at the bottom corner.
The depth of the nulls is due to balance, not
the size of the loop.
But... In some cases it's easier to get good
balance on a larger loop as it's easier to
ensure a symmetrical loop as related to the
mounting hardware, feed devices, etc..
But in my case, even my smallest 16 inch
loop is very symmetrical and has just as
good of balance as my larger loops.
So size doesn't mean much. Care in symmetrical
construction means much more.
I've never had common mode problems to be a
big deal. I may have a little, but when I tune
the loop, the greatly increased "tuned" level
swamps any small common mode signal I may
have.
So in effect, it's usually not an issue.
I do try to keep the feedlines only as long as
needed to reach the antenna, and I keep
them on the floor.
Another thing that I think helps a bit is to
use a separate coupling loop to feed the
loop. But this may help with balance more
than common mode reduction. But I still
recommend it none the less.
One thing.. make sure there is not metal
around the antenna. This can whack things
out fairly fast. IE: metal frame chairs, file
cabinets, etc...
On my loops, I've use no baluns at all so far,
but as my recordings prove, I have no lack
of nulls, or signal level for that matter.
When I null a single point ground wave signal,
say bye bye.. It's gone. :/
BTW, you will never have good luck at nulling
general "band noise". When you say "band noise",
I will assume you mean all the noise received from
all directions at once.


I rotated the antenna and got at least 3S units less noise
so, that would indicate something other than band noise I think.

As has been repeated
many times, if the noise source is not from a
single source, the loop will not be able to provide
a decent null. You will never be able to null random
noise. But if say a power line device starts making
noise from a single source, you can null almost
all of it, and still hear most of the skywave signal
you are trying to receive.
It's quite possible you are in a location with multiple
noise sources, and the small loops may not do the
job you want them to. In general, a loop is a loop is
a loop.. If the one you have is working properly, and
you are having trouble nulling the noise sources,
going to a bigger loop may not help much at all.
MK



my initial problem is not much gain from the loop.

I have always been able to null the noise.

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Old November 28th 07, 06:08 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony Giacometti wrote in
:

Owen Duffy wrote:

...
You have already been told there is no such thing as a low noise
antenna. The loop has the advantage of being able to null out signals
from two opposite directions, which can help if the interference is
predominantly from one (or both) of those directions. You can improve
the depth of those nulls, but pattern or directivity is unrelated to
the power transfer problem.


did I misunderstand you, I thought the noise was nullable but at 75
meters the signals were mostly omnidirectional? Or is this one of the
unlearned things?


Keep in mind that noise from very close sources can be much stronger than
the aggregated noise from more distant source. If you live in a street
where everyone has the same noisy plasma TV, the noise from your own one
is likely to be much stronger than the aggregate of the others... think
through the propagation mechanism and especially the effects where the
source is in the near field (say within a quarter wave of your antenna).

You can't do much to null the noise from a hundred TVs spread around the
neighborhood, but you can null the noise from a close by dominant source
like your own, or immediate neighbor's. Of course, in nulling the noise
in that direction you also null signals from the same direction.
Fortunately, lots of signals come via an ionospheric path (a higher
angle), and the null of the loop is normally oriented horizontally so it
doesn't knock the ionospheric signals down as much.

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.

Perhaps by solenoid loop you mean a multi turn loop. A multiturn loop is
another way of increasing induced voltage, but it increases the source
impedance and you have to solve the impedance matching issue.


some type of balun maybe?



...
Ok, I want to build a larger loop to get more gain, if possible deep
nulls for reducing local noise, I would like to try wire instead of
coax. I want to mount it on a PVC pipe frame. Can you give me any
ideas as to what shape and size? Whats a solenoid loop?



You started of with a loop that was too insensitive, so you have improved
that with better matching solving that problem. You can increase the gain
by further improvement of the matching... but you don't need more gain
than sufficient to have the band noise swamp the receiver internal noise.


at this point I din't think it does. When I orient the antenna for lowest
noise the S meter never reads above S3, its really quiet.

I guess the question is this, DX signals would be skywave, a loop is omni
directional to skywave signals I believe. Is this correct?


I know you are convinced that matching also degraded the pattern. If the
matching network was entirely inside an effectively shielded box, it
should not affect the nulls. Make sure that you retained the shielded
loop properly.


I didn't use a shielded box, I have a plastic box. The instructions for the
loop I built said either was OK.


Why do you still think that a larger loop has deeper nulls.


If I implied that I didn't intend to do so, what I mean is to "keep" the
deep nulls I now have.


Owen


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Old November 28th 07, 06:18 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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.



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Old November 28th 07, 06:26 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

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.


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Old November 28th 07, 06:53 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
news:aeydnehFvJBYatHanZ2dnUVZ_o2vnZ2d@hawaiiantel. net:

I guess the question is this, DX signals would be skywave, a loop is
omni directional to skywave signals I believe. Is this correct?


In free spacem the nulls are orthogoal to the plane of the loop. If you
make the plane of the loop vertical and mount it near ground, the nulls
will be modified somewhat, but the gain at high elevations (eg
ionospheric propagation from nearby) would be pretty much unaffacted by
loop orientation. That cannot be said for very low angle signals.

....
I didn't use a shielded box, I have a plastic box. The instructions
for the loop I built said either was OK.


Well, the instructions had you tuning the loop in a manner that wasn't
very effective too didn't they. Are they credible?

I am not saying a shielded box is necessary, but I wouldn't use a plastic
box without verifying it works as well as a shielded box... and the way
you do that is to build the shielded box version.

Why do you still think that a larger loop has deeper nulls.


If I implied that I didn't intend to do so, what I mean is to "keep"
the deep nulls I now have.


I pointed out in an earlier post that you might well compromise symmetry
with a larger loop and so compromise the depth of the nulls. You only
need enough gain to swamp receiver internal noise under the best band
conditions and loop orientation.

Of course, when the loop becomes large enough, its directivity does
change, and the nulls are a little broader. Proximity to ground modifies
large loops significantly.

Owen
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