Topband: QST Jun 06 RX Loop

Paul Elliott paab at
Thu May 18 15:00:37 EDT 2006

The subject of transmitting antenna reradiation has been covered in the
past, but some of the newcomers might not be aware of how it can completely
destroy the expected pattern of any nearby receiving antenna.  The effects
(lack of deep nulls, etc) some might consider to be caused by an unbalanced
loop antenna might be caused by the reradiation from a nearby transmitting
antenna.  I check the patterns of a receiving loop by feeding them from the
shack end of the coax feedline with a GDO, then walking around the antenna
at least 50 yards from it with a portable receiver.  The first time I tried
this I detected no null whatsoever.  My transmitting antenna, I guess it
would be called a sloping inverted L, is just the right length that it
matches 50 ohms when the excess inductance is tuned out by a capacitor
(about 170 mmfd) in series with the antenna.  When I opened the connection
between the coax and the capacitor, I had deep nulls where there should be
nulls-even though the loop was less than 20 feet from the drooping end of
the inverted L..  Moral:  if you haven't made sure that any nearby
transmitting antenna has been detuned, there is no way you can know if your
loop is balanced.


There is a quick way to check for reradation if the transmitting antenna
feedpoint is near ground.  Place a portable receiver tuned to 160 within one
foot of the antenna and notice the exact relative positions of the receiver
and the antenna, and the noise level. Then open the feedpoint of the
antenna, put the receiver back where it was, and notice the noise level.  In
my case the drop in noise level was dramatic.  Depending on the type of
antenna, grounding the antenna might be what is needed.  In any case
detuning of the transmitting antenna while receiving is needed if you want
to be sure that any nearby receiving antenna, not just a loop, is effective.
W8JI has some excellent information on his web pages. 



73 Paul W5DM






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