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@yahoo.com>
To: "Richard (Rick) Karlquist" <richard@karlquist.com>, donovanf@starpower.net,
"Top Band Contesting" <topband@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@karlquist.com>
To: donovanf@starpower.net; Top Band Contesting <topband@contesting.com>
Sent: Friday, April 1, 2016 11:08 PM
Subject: Re: Topband: Receive loop observations
On 4/1/2016 1:25 PM, donovanf@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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