Low Noise receiving Loop antenna
 

Roy Lewallen wrote:

Tony Giacometti wrote:

it is a shielded loop and no the gap is there, thats why I am wondering
why its not doing what I thought it would do.

What you've done is to build a truly shielded loop. The notion that a
shield somehow blocks the electric field and lets the magnetic field
through is a folk tale -- an intact shield like the one you built blocks
both electric and magnetic fields. (Good thing, too, or else coax cable
wouldn't do its job.) You've just done an experiment that proves it.

The gap in a so-called "shielded loop" provides a path for current
outside the "shield" to get inside. The net result is that the outside
of the "shield" is just an ordinary loop antenna, and the gap acts like
a feedpoint to get the current inside where it can get to the receiver.
It responds to normal electric and magnetic fields exactly like an
unshielded loop. What the "shield" buys you is improved balance, which
helps prevent the feedline from becoming part of the antenna and picking
up local noise which you can null out if balance is good.

Roy Lewallen, W7EL

Roy, I don't fully understand why you made this response to me.
You mentionI built a truly shielded loop. How so? I did exactly what the
directions called for including having a gap.

If these loops are a folk tale, other than using a beverage which I don't
have the room for, how am I going to reduce the noise enough to be able to
woprk 160 and 75 meters effectively?

Low Noise receiving Loop antenna
 

Owen Duffy wrote:

Tony Giacometti wrote in
news:VvSdnQC_g_Sb0qLanZ2dnUVZ_hKdnZ2d@hawaiiantel. net:

So, what to do, I followed the instructions for building the loop from
W2YR and KN4LF and from this link
http://www.hard-core-dx.com/nordicdx.../coaxloop.html

I think you are describing a scenario where, with the loop+preamp, the
receiver system is limited by internal noise rather than external noise.

So, it is a two band design, and that complicates matters somewhat.

Just concentrating on the 160m loop...

My calcs are that the 160m loop alone, tuned for zero series reactance
(should need ~90pF) and loaded with 50 ohms should have equivalent gain
of around -47dBi.

You would have to consider the preamp NF and gain and receiver NF to
evaluate the system noise floor, and you haven't given those details
(though they may be implied... but I am not familiar with the preamp you
are using).

Without using a preamp, the external noise (based on ITU-R P.372-8
Residiential man made noise) with this loop ought be of about the same
magnitude as the internal noise of a good transceiver, give or take. For
a preamp to improve the situation, it would need a NF significantly
better than the transceiver and sufficient gain to overcome the
transceiver noise.

Owen

here are links to the preamps I use

http://www.iceradioproducts.com/reconly.html

http://www.isp.ca/ve3nh/kd9sv.htm

Low Noise receiving Loop antenna
 

Tony Giacometti wrote:

Richard Clark wrote:

On Sat, 17 Nov 2007 07:36:28 -1000, Tony Giacometti
wrote:

Anyone familiar enough with receiving loops to be able to assist me in
figuring out whats wrong here?

Hi Tony,

There is no such thing as a Low Noise antenna (loop or otherwise).

Given that what you build is deaf, that should be a reality check of
this presumed quality of the antenna.

Well, actually, you built it wrong. However, building it right stands
only a partial chance of lowering noise, and not because the antenna
has some remarkable quality that is not otherwise found in the plug
ordinary dipole. In that sense, the reduction of noise would only
follow turning the dipole (or loop, same thing) until the source of
that noise fell into a null. This is the conventional method of
employing a "Low Noise" antenna.

Let me guess. It is a shielded loop. You forgot to leave a gap in
the shield.

73's
Richard Clark, KB7QHC

it is a shielded loop and no, the gap is there, thats why I am wondering
why its not doing what I thought it would do.


I have corrected my statement! Sorry!

Low Noise receiving Loop antenna
 

On Sat, 17 Nov 2007 12:02:35 -1000, Tony Giacometti
wrote:

I did do a search for loops of this type and basically they are all the
same.

Hi Tony,

If the link you supplied is an indication of sameness, they are
equally complex and problem magnets.

No, loops are not all the same. There is quite a variety (you
supplied one poor variant to a simple turn of wire in the sky).

I have been told Wellbrook makes good loops, but I don't like the idea of
spending $300-$500 for one at this point.

More than $20 spent is only buying custom designed furniture.

73's
Richard Clark, KB7QHC

Low Noise receiving Loop antenna
 

On Sat, 17 Nov 2007 12:15:11 -1000, Tony Giacometti
wrote:

Tony Giacometti wrote:
it is a shielded loop and no, the gap is there, thats why I am wondering
why its not doing what I thought it would do.


I have corrected my statement! Sorry!

Hi Tony,

No, as Owen pointed out, a poor reading on my part. Sorry.

73's
Richard Clark, KB7QHC

Low Noise receiving Loop antenna
 

Tony Giacometti wrote in news:CpKdnabH6fWF-
:

here are links to the preamps I use

http://www.iceradioproducts.com/reconly.html

NF=1.8dB and Gain=18dB


http://www.isp.ca/ve3nh/kd9sv.htm

Ok, I can't work with specs like "Noise figure is quite low and the
design appears to have no vices." I am not saying it is bad, it just
doesn't say anything meaninful... well to me anyway.

Still no info on the transceiver, but lets guess it has a NF=8dB.

Transceiver + preamp has a NF of 2dB, which suggests an equivalent noise
floor of about -139dBm (2kHz bandwidth).... provided that the preamp
doesn't generate internal noise due to IMD and which would not be
captured in the 1.8dB spec NF.

ITU-R P.372-8 suggests ambient noise on 1.8MHz in Residential locality
should be about -75dBm+AvgAntGain or about -122dBm, so you should get
noticeably more noise (~17dB) from the loop than from a dummy load. That
would mean the configuration should deliver almost as good a S/N ratio as
possible... even though the S meter reading might be shy.

Again, I am talking about the 160m loop alone. You can't diagnose this
easily with the two loops in parallel.

Owen

Low Noise receiving Loop antenna
 

Richard Clark wrote:

On Sat, 17 Nov 2007 12:02:35 -1000, Tony Giacometti
wrote:

I did do a search for loops of this type and basically they are all the
same.

Hi Tony,

If the link you supplied is an indication of sameness, they are
equally complex and problem magnets.

No, loops are not all the same. There is quite a variety (you
supplied one poor variant to a simple turn of wire in the sky).

I have been told Wellbrook makes good loops, but I don't like the idea of
spending $300-$500 for one at this point.

More than $20 spent is only buying custom designed furniture.

73's
Richard Clark, KB7QHC


OK, other than leveling all existing structures within 2,500 ft of my house
and burying the power lines, how do I solve this problem?

also, if these loops don't work then why do these designs exist?

Low Noise receiving Loop antenna
 

Owen Duffy wrote:

Tony Giacometti wrote in news:CpKdnabH6fWF-
:

here are links to the preamps I use

http://www.iceradioproducts.com/reconly.html

NF=1.8dB and Gain=18dB


http://www.isp.ca/ve3nh/kd9sv.htm

Ok, I can't work with specs like "Noise figure is quite low and the
design appears to have no vices." I am not saying it is bad, it just
doesn't say anything meaninful... well to me anyway.

Still no info on the transceiver, but lets guess it has a NF=8dB.

Transceiver + preamp has a NF of 2dB, which suggests an equivalent noise
floor of about -139dBm (2kHz bandwidth).... provided that the preamp
doesn't generate internal noise due to IMD and which would not be
captured in the 1.8dB spec NF.

ITU-R P.372-8 suggests ambient noise on 1.8MHz in Residential locality
should be about -75dBm+AvgAntGain or about -122dBm, so you should get
noticeably more noise (~17dB) from the loop than from a dummy load. That
would mean the configuration should deliver almost as good a S/N ratio as
possible... even though the S meter reading might be shy.

Again, I am talking about the 160m loop alone. You can't diagnose this
easily with the two loops in parallel.

Owen


I am using a heavily modified Drake R-4c receiver. Sherwood mods.
And I at this point I only built the 80 meter loop as I wanted to make sure
it worked on 80 before I went and built one for 160.

I don't think ITU-R P.372-8 had my neighborhood in mind when it was
composed.

S-9 on this receiver is about the minimum on 160 and S-7 is the minimum on
75.

Much of the time its higher and a royal pain in the arse.

Low Noise receiving Loop antenna
 

Tony Giacometti wrote in
news:suednRqSYJd58aLanZ2dnUVZ_uDinZ2d@hawaiiantel. net:

also, if these loops don't work then why do these designs exist?

Tony,

A few questions:

1. With a 50 load on the preamp input, when tuned to a quiet spot in the
band, RF gain full, preamps on, telephony bandwidth, does the S meter
deflect at all?

2. If no s meter deflection, note the receiver audio output voltage.

3. With the loop connected on the preamp input, when tuned to a quiet
spot in the band, RF gain full, preamps on, telephony bandwidth, does the
S meter deflect at all?

4. If no s meter deflection, note the receiver audio output voltage.

5. What is the ratio of the voltage at 4 to the voltage at 3? If it is
more than about 3:1, you have achieved nearly as good a S/N ratio as is
possible.

Owen

Low Noise receiving Loop antenna
 

On Sat, 17 Nov 2007 12:38:11 -1000, Tony Giacometti
wrote:

OK, other than leveling all existing structures within 2,500 ft of my house
and burying the power lines, how do I solve this problem?

also, if these loops don't work then why do these designs exist?

-sigh-

Tony, if "loops don't work" is the only message you've gotten to this
point, it is the wrong message.

Certainly your loop doesn't work - or at least I have to take your
word for it. That is a far different issue.

Loops do work if you follow standard practices.

The real question is:
"Is a loop my solution?"
and with that comes the $64,000 prize.

In other correspondence you describe noise levels pushing the S-Meter
to S9. If that noise is coming from one direction, a dipole or loop
can solve it. If it is general (meaning that your neighbor's fish
tank heater is bathing you in noise), then you don't stand a chance.

You don't need an antenna, yet. You need to find the source of noise.

73's
Richard Clark, KB7QHC