Topband: Loop-family antenna notes

[i4jmy@iol.it]

W8JI:
> Maybe I'm wrong, but I get the impression a large part of our 
> fraternity thinks noise is "electric" and signals are "magnetic". Well-
> intentioned articles then use that misunderstanding to explain the 
> shield makes an antenna "magnetic", and that the shield somehow 
> helps the antenna "filter-out" noise.
> 
> Nothing is further from the truth.

Tom is right. Small vertical loops, infact, perfeclty couple with 
vertically polarized antennas in the nearby.
It's a fact anyway, that medium size resonant small loops are far 
superior if compared with a simple vertical in case of atmospherical 
noises.

> Directivity is the only thing that eliminates noise. 

Not exactly, also polarization or wave angle plays an important role 
with local signals. Non directional antennas with a big response to 
ground wave are sometimes a real pain in the neck with local noises.


> Delta's and other medium size terminated loops act like a pair of 
> phased verticals (with a "feedline" between the vertical or sloping-
> vertical ends that also radiates a bit). Since they are more 
> directional than the small loop you tried, they are quieter when 
> properly oriented.

When the phisical spacing between two antennas is very small it's 
generally not proper to talk about directivity. Phasing and mutual 
coupling are quite critical and to obtain a pattern, a phase lag (and 
energy) should be carefully adjusted all the times the frequency is 
moved. On the other hand, with close spaced antennas it's much easier 
to obtain a notch than a directional pattern, altough this is mostly 
useless when one hasn't the opportunity to set the notch it in the 
proper direction.  
 
> An exception might occur if your noise was within perhaps within 
> 50 feet of the antenna, and if you were very lucky. 
> 
> But the bottom line is small loops are mostly a waste of time, 

It depends by the available alternatives.

> unless you have one groundwave noise source that you can 
> position the loop to null-out. They are worth a try if you have room 
> for nothing else and have local noise, but they are useless for noise 
> that varies in direction.

Althoug critical and not very practical, I had decent results in 
directivity when phasing two small resonant loops with a device that 
allowed me to continuously vary phases and currents between antennas. 

73,
Mauri I4JMY


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