Topband: analyzer filters

[Tom Rauch]

> 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