Topband: analyzer filters
Tom Rauch
w8ji at contesting.com
Mon Dec 25 06:56:30 EST 2006
> the shack. BUILD A 50 OHM BANDPASS FILTER, test it, use
> it !!!! Spend the
> $90 on something else......
The typical bandpass or high pass filter will not be usable
with an Antenna Analyzer. They act like transmission lines
of various long lengths, modest to high loss, and with surge
impedances different than 50 ohms near the cutoff
frequencies.
This means they will almost always modify all readings
substantially near the cutoff or stop frequencies. A filter
that rejects the AM BCB will have a major effect on 160
meters.
The MFJ-781 gets around this problem by being a simple
parallel tuned network with a series resonant trap. It looks
almost transparent at the resonant frequency. It has limited
notch depth compared to multiple stage filters but the notch
is adequate for most cases, and it also protects the
sensitive diodes in the analyzer.
73 Tom
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