Topband: Loop-family antenna notes

[Tom Rauch]

Hi Larry,

> That is until about two weeks ago. About that time, the shielded loop that
> has been in place for three years without ever being useful, suddenly
> started working! Turned out that some old local noise sources went away
> and a couple new ones showed up, but very close in direction the remaining
> old source. With the loop set for a groundwave null in that direction, it
> is now providing unprecedented performance. It's always much better than
> the TX antenna now, and even equals the rotatable flag when the DX signal
> is arriving from the same direction as the noise.

As it should! You have removed a local dominant noise source, 
which is all the small loop is good for except.....
 
> Perhaps some of the fame the small loops unjustly enjoy comes from the
> high angle performance they have. The fool thing gives me better than 10
> dB improvement in signals from W6 and W0. As if I need that!

Yes indeed. Small loops  (and even Flags and the family of 
terminated loops) respond quite well to high angle signals 
horizontally polarized broadside to the loop and vertically polarized 
off the ends. Of couse the polarization is "tilted" at points between 
those two extremes.

Another factor that may help re-enforce the wive's tales surrounding 
small loops is the reduction in signal levels. Since AGC systems 
(and S meters) are non-linear, the effect is one of less volume 
compression (and S meter compression) as signal levels are 
reduced. As the AGC falls out of linear range it can have the effect 
of making it seem like the S/N ratio is higher.

Strong noise pulse will also not saturate stages, and that will make 
the pulse duration less, if levels are reduced.

In the old days people used to enjoy this effect with *real* RF gain 
controls on their receivers. IF and AF gain on full, AVC off, and RF 
gain pulled back was the way to dig signals out of noise. The loop, 
if it reduces signal level enough, also would have a similar 
effect....of course so would an attenuator pad.
 
73, Tom W8JI
w8ji@contesting.com


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