Topband: Compact magnetic loop

[Tom W8JI]

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 at gmail.com>
To: <topband at 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
>> _________________
>> Topband Reflector Archives - http://www.contesting.com/_topband
>>
>
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