Topband: Receive loop observations

[donovanf at starpower.net]

Hi Raoul, 


Congratulations, its difficult to do better than a 40 dB null. 
It usually takes a great deal of care with proper loop balance 
and excellent feed line common isolation to achieve that 
excellent null depth. 


73 
Frank 
W3LPL 

----- Original Message -----

From: "Raoul Coetzee" <raoulcoetzee at yahoo.com> 
To: "Richard (Rick) Karlquist" <richard at karlquist.com>, donovanf at starpower.net, "Top Band Contesting" <topband at contesting.com> 
Sent: Saturday, April 2, 2016 9:57:01 AM 
Subject: Re: Topband: Receive loop observations 



Rick wrote: 
> With a >circumference of 20 to 40 feet on 160 meters, the null is only 

>10 or 15 dB deep. 


I use a medium wave BC station about 10miles away from as a reference to check my 160m loop, (the loop is reasably broadband but no good on 80m) they are about 4 Degrees East off me and with a nice signal of about 40 db and I easily get a a null signal of S 9. 
I have played around a lot with my FT1000 MP MKV's S-meter using a step , attenuator, and the meter is most accurate 
in the range above S9 so I am reasonably happy with my finding of a null of about 40db. 
Come to think of it, I have an old HP Spectrum analyser that I could use to get a decent measurement. 


Regards, 
Raoul Zs1C 










From: Richard (Rick) Karlquist <richard at karlquist.com> 
To: donovanf at starpower.net; Top Band Contesting <topband at contesting.com> 
Sent: Friday, April 1, 2016 11:08 PM 
Subject: Re: Topband: Receive loop observations 




On 4/1/2016 1:25 PM, donovanf at starpower.net wrote: 
> Hi Jim, 
> 
> 
> A properly constructed loop antenna absolutely requires a preamp. 

Yes, but usually the preamp function built into the radio is 
sufficient. 

> If your loop is operating correctly it will be omni-directional for 
> skywave signals and it will have an extremely deep null for an 
> unwanted signal propagated to your antenna from one local 
> vertically polarized interference source. A well constructed loop 
> should have a null depth of 60-80 dB and a null beamwidth of just 
> a few degrees. A very rigid mechanical mount is required to 
> keep the deep null pointed directly at your interference source. 

It is easy to model a loop on NEC, and the results I have seen 
do not predict anything like 60 to 80 dB nulling. With a 
circumference of 20 to 40 feet on 160 meters, the null is only 
10 or 15 dB deep. 

> A properly constructed loop should be transformer matched 
> to keep the loop balanced and both the coaxial cable and power 
> cable must be exceptionally well isolated from the loop. 

Exactly right 

> A low noise figure high gain preamp is essential. 

I've tried that on my loops, and all it does is make 
the S-meter move more. No audible difference. 

> Frank 
> W3LPL 
> 

73 
Rick N6RK 

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