Topband: Receive loop observations

[Raoul Coetzee]

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 accuratein 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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