Effective aperture is not size related.
Effective aperture is exclusively tied to wavelength and gain.
The only time effective aperture means anything for receiving is when
external noise no longer significantly influences noise floor. Other than
that, effective aperture is meaningless for S/N or hearing DX.
The real issue with a small loop is it only has two very sharp nulls through
the axis. It all other directions, even straight up and below, it has
response. This means the only points where it significantly discriminates
against external noise are two points through the axis.
Making it worse, many or most loops have terrible feedpoint designs. I
measured a very popular expensive loop, and the pattern had considerable
skewing because of feedline common mode response. While I had the loop
thousands of feet from noise sources, the pattern told me the feedline was a
major part of the "antenna".
That same antenna, near my house, was nearly dominated by conducted noise
along the feedline. I determined the common mode "noise" sensitivity was
only down 5-10 dB from the loop pickup when a 50 foot feedline was used.
It could only be three things:
1.) The feedpoint or loop design was poor, resulting in local conducted
noise dominating the background
2.) An inherent lack of directivity that comes with the wide peak response
and narrow axis nulls of the loop
3.) The loop efficiency is too low for the noise floor of the preamps, and
internal noise is limiting weak signal response (even a 3 foot loop has
enough theoretical gain for use in a reasonably quiet location)
All of those are reasonably easy to test.
73 Tom
----- Original Message -----
From: "Bill Cromwell" <wrcromwell@gmail.com>
To: <topband@contesting.com>
Sent: Saturday, January 17, 2015 5:27 PM
Subject: Re: Topband: Compact magnetic loop
Hi,
That small mag-loop has a small aperture and so all of the signals
delivered to your radio will be lower levels than from a bigger antenna
(in terms of wavelength. A preamp should improve the signal strength but
not improve the SNR over what you get with the loop and no preamp. The
mag-loop reduces the noise with it's high Q and decreased bandwidth and
those nulls contribute to reduced noise, too. I have excellent luck with
mine but there just isn't any silver bullet. The mag loop I use on 160
meters is four and a half feet in diameter. The receiver I use on 160 is
easily overloaded without some attenuation so the reduced signal levels
from the mag-loop are *ideal*.
73,
Bill KU8H
On 01/17/2015 05:06 PM, Ignacy Misztal wrote:
I homebrewed a magnetic loop for diversity reception with Hi-Z 3 el. in
K3. For perhaps up to 1000 miles the loop provides excellent separation
between signals from Hi-Z. But it hears distant stations poorly, much
worse
than TX inv L. Based on earlier reports I expected the loop to be very
good
for DX.
The loop is made from coax, is tuned and has a transformer. About 50 KHz
2:1 BW. A low-noise preamp does not improve S/N.
Any experiences with the loop here?
Ignacy, NO9E
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