Topband: Slightly OT - amplifier noise

[donovanf at starpower.net]

Hi Rick, 



A noise receiving antenna close to a noise source is used in conjunction 
with a higher performance receiving antenna such as a Beverage or an 
array of short verticals and a passive or active noise canceller. 



A small loop antenna provides a mechanically steerable null off of both 
sides of the loop. Null beamwidth is just a few degrees at the 3 dB 
points, otherwise a small loop is an omni-directional receiving antenna. 
Simply turn the loop to minimize the interfering noise signal strength. 


The smaller the loop, the deeper the null depth but the smaller the signal 
strength of desired signals. A small loop antenna requires a low noise 
high gain pre-amp directly at its feed point for optimum sensitivity. 


A small loop antenna should be close to ground for optimum null depth. 
Horizontally polarized skywave signals penetrate the nulls if a small 
160 meter loop antenna is much higher than than about ten feet above the ground 


On 160 meters a small loop antenna provides: 
- a 30 dB null off each side of a 5 foot diameter loop. 
- a 25 dB null off each side of a 10 foot diameter loop, or 
- a 20 dB null off each side of a 17 foot diameter loop 


73 

Frank 
W3LPL 








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

From: "Richard (Rick) Karlquist" <richard at karlquist.com> 
To: "Dave Cuthbert" <telegrapher9 at gmail.com>, n2ic at arrl.net 
Cc: "Topband" <topband at contesting.com> 
Sent: Saturday, March 14, 2020 10:17:59 PM 
Subject: Re: Topband: Slightly OT - amplifier noise 


I am trying to understand what these noise cancelling 
schemes do that couldn't be done with a simple loop 
(rotating the loop until the noise drops into one of 
the null directions). You can easily prove to yourself 
with a hand held AM BCB receiver equipped with a ferrite 
bar antenna that even the worst power line noise can 
almost always be greatly suppressed by properly rotating 
the receiver. Similarly, I have had good luck with 
small tuned loops (10 feet perimeter) nulling power 
line noise. Smaller loops seem to have deeper nulls. 

It is critical to keep loops away from your other antennas 
and power wiring. Otherwise, they will not exhibit a 
good null. 

Rick N6RK 
_________________ 
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