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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