I have the same problem with a 50KW FM station a couple of miles away
affecting my Palstar ZM-30. It is useable on the rig side of an antenna
tuning unit, but most of my antennas are self resonate therefore the FM
broadcast RF rides right into the bridge making it mostly worthless when
directly attached to any antenna such as a dipole, vertical, yagi, etc.
Sometimes I can get a useable reading if I turn the antenna 90 to the
broadcast tower, but that only works with the rotatable antennas.
Experiments with filtering using small value caps, small pi networks, a
series FM trap, or ferrites have been unsuccessful. Any filter I put in
front of the analyzer influences the reading substantially. BTW... this
broadcast station also comes in on my frequency counter too, with no antenna
attached.
Please let the group know if any of you have come up with a "transparent at
ham frequencies" inline filter.
73
Lloyd - N9LB
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com]On Behalf Of Tom
Boucher
Sent: Friday, November 02, 2012 4:21 PM
To: 160 reflector
Subject: Topband: Antenna analysers in close proximity to BC station.
A ham friend asked me to design a matching network for his 160 metre end fed
quarter wave, so I asked him to provide an impedance reading using his
MFJ-259B. I would then use the Berkley site
(http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.
html ) to provide the necessary values for an 'L' network, as I have done
many times at my own station.
The readings he provided were total nonsense and quite erratic, so we
concluded his MFJ-259B was dead. He assured me that he always does a static
discharge before connecting the MFJ.
So I paid him a visit, taking along my Palstar Antenna analyser thing, which
has always performed well at home, and what-do-you-know, the readings on
that were also erratic, total nonsense and it behaved in a way I have never
seen before.
Than someone suggested the problem may be due to a 50Kw BC station on 909
KHz, situated less than 5 miles away, causing both antenna analysers to
misbehave.
We ended up with a good old-fashioned link coupled parallel tuned circuit
with the antenna tapped a few turns up from the ground end. This works fine
but he is power limited due to arcing across the tuning capacitor. So we
would ideally like to revert to the 'L' network plan, but how to use the
antenna analyser in the presence of a high BC station field. Anyone any
ideas?
73
Tom G3OLB
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