W2PM said,
"A comparison of the Wellbrook product which is a magnetic loop is not a good
one to a AY loop or other low angle directive receive antenna. Mag loops are
not "directive" in the same way they only have a very sharp null to two
opposite directions and omni directional vertically. Nice for nulling strong
local point source noise but that's it. It will enable qsos not possible
otherwise and some DX I suppose but Wellbrook is not a good choice for any
serious DX work. No mag antenna is."
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And so far my testing fully supports the above comments by W2PM, and
calculated RDF values along with takeoff angle really appears to help predict
the S/N performance of one antenna system versus another when dealing with
normal atmospheric noise that's coming in from all directions or at least say
multiple directions (with a higher RDF value and lower takeoff angle generally
being desired for DX which is not a strong point of Mag loops compared with AY
loops, Flags/Pennants, etc.).
For local point source generated noise such as a Plasma TV, the Mag loop really
helps if the offending signal is 90 degrees (or 270 deg) to the desired signal.
Whereas the Pennant/Flag really helps when the offending signal is 180 degrees
to the desired signal.
I have a gut feeling through preliminary testing I've done that a shielded mag
loop has less interaction (less distortion of its pattern) due to nearby
objects when compared with a small rotatable flag, possibly making a well
balanced mag loop usable where the Pennant/Flag might not be (very small lot
size, indoor operation, etc.). Detailed pattern measurements need to be done
in order to quantify this casual observation.
I should also mention that I only obtain an approximate 2 db improvement (on
average) in S/N performance using my half size pennants compared with my 68
foot tall base loaded TX vertical (but in reality I believe this is close to
what I should expect based on calculated RDF values). I also notice that the
S/N difference is much greater between my pennants and TX vertical for stations
close in versus DX which is likely due to differences in elevation patterns
between my pennants and my TX vertical. Contrary to most folks opinion, I set
my preamp gain on my pennants to provide a noise floor identical to that of my
TX vertical for S/N comparison measurements (I find it impossible to quantify
S/N performance between two antenna systems any other way under real life
operating conditions, and I continuously switch rapidly between the two
different antenna systems in order to evaluate the average S/N improvement).
I am by no means an expert on this topic, but I wanted to share my
comments/experiences since I've been doing a lot of testing of shielded mag
loops and small pennants and flags during the past 3 years.
73's
Don (wd8dsb)
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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