> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom
> Sent: Friday, November 02, 2012 5:21 PM
> Subject: Topband: Antenna analysers in close proximity to BC station.
>
> 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?
Tom,
The problem with a low pass filter is it acts like a transmission line and
matching network combination. Even if an analyzer allows calibrating out,
results can be compromised. Results are useless if you cannot calibrate out.
The MFJ filter is a parallel tuned network in parallel with a series tuned
trap for the AM band. Because it corrects as an open circuit at the desired
frequency, it has almost no effect on the reading on the setup frequency.
The amount of null depth depends on the characteristics of the antenna
system you have and the spacing of the BC station to the frequency you are
measuring. Typically I'd expect 20 dB. If you change the trap inductor to a
higher Q inductor it might get up to 30 dB.
What won't work is a low pass, high pass, or band pass, unless it appears as
an open with near-zero transmission line effect. This limits the type of
filters that will work to something like the MFJ. The exception is a filter
designed into the detector, or an analyzer and filter that can calibrate out
and tell you what is happening on the other side of the filter.
I've been telling MFJ for years they need to get an analyzer that uses
selective detectors. IMO that would be more important than sweeps and all
that other stuff. Most people probably just want to know the R and j
accurately, and don't care about a computer interface.
73 Tom
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Topband reflector - topband@contesting.com
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