Hi Art,
It sounds like your system is working as it should.
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.
> Also, I have a shielded coax loop as per the http://www.qsl.net/kc2tx/
> site. It is interesting to note that the "Delta" rx antenna shows a lower
> noise level than the shielded loop(quiet conditions no QRN). On a clear
> frequency on 160 the "Delta" shows s0 and the shielded loop shows s3 to
> s6. On rx signals they show about the same signal strength through the
> preamp (Fixed gain through the PT-2), about 10 db down from the short
> vertical tx antenna. Bottom line, the "Delta" presents signals at a
> readable level with little to no background noise, the shielded loop has a
> very noticeable noise level and the short vertical has a noise level that
> is much louder than the rx signal.
Your results are exactly as they should be.
The shield in a "shielded antenna" does nothing at all to filter noise
from signals. The shield simply becomes the actual antenna, and it
can not filter out noise. The only thing a shield does is affect the
balance of the antenna.
Directivity is the only thing that eliminates noise.
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.
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,
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.
73, Tom W8JI
w8ji@contesting.com
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