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Old November 18th 07, 03:08 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

On Nov 17, 7:51 pm, Tony Giacometti wrote:


This link was the guide I used to build the loop, I am only using the 80
meter loop.

http://www.qsl.net/kc2tx/

Without the preamp I do get a noise spike when I tune the cap.
Its very noticeable also.

I would have thought that the signals I have heard would be much louder
especially using the preamp.


They will be down, but the overall s/n ratio is what really counts.


I have 2 different types of preamps and they both behave the same way.

For what its worth, I have never considered just plain wire for the loop.
I do use coax RG-6 - its all I can get, no RG-59 around here.
Another ham mentioned to me that using 75 ohm hardline would be the best.
None of that stuff here either.


Type of coax should matter little if any.. Hardline would be a waste
of time I would think.


I am beginning to think my feedline could be a problem. I can replace that
stuff rather easily.


Not unless you have a common mode/ noise pickup problem.
And changing to another run of coax alone is unlikely to help that.


I like your idea of a separate coupling loop.

Any idea what the loop would need to be electrically and physically?
Do I need to change my tuning cap if I change to a coupling loop?


No. The main loop will still be tuned to the same frequency, no
matter if direct fed, or with a coupling loop. Many people build BC
loops and use the ferrite bar antennas in the portable radios
themselves to couple to the loop. But my radios are not portables,
so I use a coupling loop fed with coax.

MK
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Old November 18th 07, 06:16 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net:

Owen Duffy wrote:

Owen Duffy wrote in
:

....
It is questionable whether the parallel tuned circuit is an efficient
coupling method for a low Z receiver.


I am able to get a noise peak tuning the capacitor and the preamps I
use are supposed to be a match from approx 25 ohms to about 125 ohms.


Tony,

I have written a program to solve for transmission loss from the source
being the emf induced into the electrically small loop through to the 50
ohm receiver load on the output. The program models the transmission line
stub on one side of the loop gap, and the transmission line on the other
side to the tuning capacitor and 50 ohm load.

I need to do some more checking, but the program results support my
proposition that parallel "tuning" is not an efficient coupling.

The only way to ensure a near 50 ohm receiver for test purposes is to put
a 10dB 50ohm attenuator in front of it. If you are able to do such, does
the receiver noise peak with the loop tuning capacitor at minimum C?

Owen
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Old November 18th 07, 08:25 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote in
:

....
Expected ambient noise level from a lossless antenna in 2kHz at 3.6MHz
should be around -82.9+33dBm or -49.9dBm. The 80m loop gain is about -
47dBi, so expected receive level would be -97dBm which is some 40dB
above your receiver noise floor.


That is wrong, the 82.9dBm was for 2kHz bandwidth, and I should not have
added the 33dB bandwidth factor... it is a double count.

So the receive level would be -82.9 -47 or -129dBm which would be just 10dB
above the receiver noise floor... in the ballpark of what you measured
Tony.

Sorry for the bum steer.

I am still working on a more detailed model, and it will drive the gain
figure downwards a bit.

Owen
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Old November 18th 07, 10:14 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

wrote:

On Nov 17, 7:51 pm, Tony Giacometti wrote:


This link was the guide I used to build the loop, I am only using the 80
meter loop.

http://www.qsl.net/kc2tx/

Without the preamp I do get a noise spike when I tune the cap.
Its very noticeable also.

I would have thought that the signals I have heard would be much louder
especially using the preamp.


They will be down, but the overall s/n ratio is what really counts.


I have 2 different types of preamps and they both behave the same way.

For what its worth, I have never considered just plain wire for the loop.
I do use coax RG-6 - its all I can get, no RG-59 around here.
Another ham mentioned to me that using 75 ohm hardline would be the best.
None of that stuff here either.


the hardline would be used for the loop, sorry if I didn't make that clear.


Type of coax should matter little if any.. Hardline would be a waste
of time I would think.


I am beginning to think my feedline could be a problem. I can replace
that stuff rather easily.


Not unless you have a common mode/ noise pickup problem.
And changing to another run of coax alone is unlikely to help that.


I had a defective run of coax running to an antenna.
Not sure if I mistakenly used it for the loop.



I like your idea of a separate coupling loop.

Any idea what the loop would need to be electrically and physically?
Do I need to change my tuning cap if I change to a coupling loop?


No. The main loop will still be tuned to the same frequency, no
matter if direct fed, or with a coupling loop. Many people build BC
loops and use the ferrite bar antennas in the portable radios
themselves to couple to the loop. But my radios are not portables,
so I use a coupling loop fed with coax.

MK


how do I calculate the dimensions of the coupling loop?
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Old November 18th 07, 10:27 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Roy Lewallen wrote:

Tony Giacometti wrote:

Roy, you scared the daylights out of me for a minute,

By the way, any idea why this loop might be under performing?


Well, first of all, I think the problem might be your expectations.


maybe I just don't know what to expect with this loop.
Having never heard one perform I have no idea if this loop is doing what its
supposed to be doing.

A
small loop has a very broad pattern, with a couple of very narrow and
deep nulls. If you have noise coming from a very narrow angular region,
you can use a loop to null it out. But if it's coming from the wiring in
a neighbor's house, is getting on the power lines, or otherwise comes
from a range of angles, the loop won't help. If the noise is getting
into your house via the mains wiring, then the loop will probably make
things worse compared to an outside antenna, since it's closer to at
least one source of the noise.


I powered my receiver and a few other items on a battery backup, switched
all the power off in my home and nothing changed. The noise was still
there.
Funny though, everynite between 8:30 and 9:30pm the noise reduces by half.
I keep watching my neighbors homes to see if I can tell if someone is
turning something off but no clues yet.


And this does seem to be the case. Although you didn't say in so many
words, it sounds like the signal/noise ratio is worse when using the
loop than when using the outside antenna. If so, then the last couple of
sentences in the above paragraph apply.


no, the noise is worse on the transmitting antenna.
the noise on the loop is very low, and if I rotate it I can sometimes make
the noise increase as it points along the plane of the loop at something in
the neighborhood. The loop does reduce the noise but the signals are very
weak and sometimes difficult to copy.


In a recent posting you say the noise level comes up substantially when
you connect the loop, so you can quit worrying about your receiver noise
figure in my opinion -- and with it, the AGC operation, S-meter
calibration, and so forth. It means that external noise is considerably
louder than receiver noise. You can also quit worrying about how many
turns. A preamp, or even an audio amplifier connected to the receiver
output, will make both signals and noise louder, in the same ratio, if
they're not loud enough to hear.

So the only thing which can be wrong with the loop that you can't fix
with a little amplification is that maybe it's poorly balanced so the
nulls aren't what they should be.


The nulls are well defined considering what I am using.
I have the loop about as far away as I can get it from my home and 2 others
right now. when I rotate it where it is now vs. where it was 2 weeks ago its
not very noisy.


The only way I know of to test for
this is to rotate the loop when listening to a distant station or a
small battery powered signal source -- something coming from only one
direction. You should be able to null it out pretty effectively. If you
can't, the problem might be loop construction or it might be proximity
of other conductors warping the pattern. If you can successfully null
out point-source signals, then the loop is performing as it should. And
if that's not good enough, then a loop isn't the solution to your problem.

Roy Lewallen, W7EL


The loop does reduce the noise, but the signals I want to hear are not very
loud even with the preamp. This is what the problem is.



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Old November 18th 07, 10:29 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Owen Duffy wrote in
:

...
Expected ambient noise level from a lossless antenna in 2kHz at 3.6MHz
should be around -82.9+33dBm or -49.9dBm. The 80m loop gain is about -
47dBi, so expected receive level would be -97dBm which is some 40dB
above your receiver noise floor.


That is wrong, the 82.9dBm was for 2kHz bandwidth, and I should not have
added the 33dB bandwidth factor... it is a double count.

So the receive level would be -82.9 -47 or -129dBm which would be just
10dB above the receiver noise floor... in the ballpark of what you
measured Tony.

Sorry for the bum steer.

I am still working on a more detailed model, and it will drive the gain
figure downwards a bit.

Owen



you had me worried.........
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Old November 18th 07, 10:31 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony Giacometti wrote in
news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net:

Owen Duffy wrote:

Owen Duffy wrote in
:

...
It is questionable whether the parallel tuned circuit is an efficient
coupling method for a low Z receiver.


I am able to get a noise peak tuning the capacitor and the preamps I
use are supposed to be a match from approx 25 ohms to about 125 ohms.


Tony,

I have written a program to solve for transmission loss from the source
being the emf induced into the electrically small loop through to the 50
ohm receiver load on the output. The program models the transmission line
stub on one side of the loop gap, and the transmission line on the other
side to the tuning capacitor and 50 ohm load.

I need to do some more checking, but the program results support my
proposition that parallel "tuning" is not an efficient coupling.

The only way to ensure a near 50 ohm receiver for test purposes is to put
a 10dB 50ohm attenuator in front of it. If you are able to do such, does
the receiver noise peak with the loop tuning capacitor at minimum C?

Owen


I dont have access to an attenuator or anything that would work as one.
Right now the loop gets max noise with about 2/3 max capacitance.
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Old November 18th 07, 11:00 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

wrote:

On Nov 17, 7:31 pm, Tony Giacometti wrote:


I am thinking I should take another look at the receiver and make sure it
does'nt have something strange going on.

I do appreciate your input.


You are getting a peak so the loop is tuned.. I doubt the
receiver is the problem, or you wouldn't hear the noise
peak. So it's down to pretty much what Roy just said at this
point. You need to check the nulls. And if the noise
is multiple sources, it can only do one at a time, unless
two are in the same exact direction, or exactly 180 apart.
So it might, or might not do what you want.
The null should be very sharp. Just a slight movement
can cause quite a change in the depth of the null
on a local noise source, so the aim has to be near
perfect for best results.
MK


this is the part that tells me it may be working ok, but the signals even
with the preamp are very low. Some I can't hear at all on the loop but on
the transmitting antenna the signal is there but covered by the S7-9 noise
level.

I have this feeling I don't know what I am supposed to experience from a
working loop.

Anyone have a mp3 file which can show what happens using a loop?

Right now I am hearing very few stations on the loop amd not a one more than
a S5, with an S1 noise level. I hear a couple stations in the SS contest
from the west coast S5 on the loop no noise to be heard, on the
transmitting antenna S6 noise level and the same west coast station S7

I can hear the stations he is working but not well enough to work either on
the loop or the transmitting antenna. The stations he is working are in the
noise and I know they are there.

Does this sound right?


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Old November 18th 07, 11:01 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony Giacometti wrote in
news:sO2dnUZbkeShCaLanZ2dnUVZ_v2pnZ2d@hawaiiantel. net:

I believe this should work, but for some reason not like I thought it
would.


Tony, I have described a simple untuned loop for field strength
measurement. The article is at
http://www.vk1od.net/SmallUntunedSquareLoop/index.htm . The sensitivity
of the loop is sufficient that external noise on 3.6MHz is much greater
than the receiver internal noise, ie S/N of signals on the band will be
about as good as they can be, a higher gain antenna will increase the S
meter reading, but not improve S/N ignoring the effects of noise blankers
and noise reduction. The predicted performance has been confirmed by
comparison to a calibrated EMC measurement loop.

The purpose of tuning a loop is preselection and / or better impedance
matching to improve gain (by reducing loss).

The purpose of shielding a loop is for better balance to achieve deeper
nulls, but shielding isn't the only way, nor the best way necessarily.
Roy mentioned that.

Try a simple untuned loop, the balun is REAL important (for deep nulls),
see how it works then see if you can get the improved version to work. It
is questionable whether the shielded loop construction is a real
improvement, it brings some loss elements (the s/c stub loss, the line
loss in the other half the loop) to the design, losses that be worse than
a balun.

Owen



Its late here, 11:37pm - I will look this over in the morning. Thank You

Just for your info, the loop ends when pointed east-west I get S meter
reading of 3.5
when pointed north-south I get S1 I believe this tell me the noise is
either east or west of the antenna location.
When I had the loop on the other side of the property I got an S5 noise
reading in the same direction.
I believe either one of 3 houses might be the culprit, maybe they will go
away for the holiday and I will be able to make a better determination as
to who the bad guy is.
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Old November 18th 07, 11:02 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
news:NNydnfODM_iOB6LanZ2dnUVZ_r6rnZ2d@hawaiiantel. net:

....
For what its worth, I have never considered just plain wire for the
loop. I do use coax RG-6 - its all I can get, no RG-59 around here.


Did I miss something. I did see you refer us to a web page that described
the loop using RG59... and now you tell us you used RG6. You might not yet
know it, they are different, and the difference is relevant.

Owen
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