"Roy Lewallen" wrote in message
...
dykesc wrote:
The manual on the MFJ 259b warns that strong local broadcast band rf
can cause problems with accuracy of the unit. The warning mentions
strong AM broadcast signals. A new tall tower near my house is
emitting a strong FM broadcast signal. When I hooked up my G5RV to the
frequency counter input on my 259b, it immediately locked on at
100.500 MHz. The signal is so strong that I can hear the station at
multiple frequencies when I tune across the FM broadcast band on a
small FM radio. Could this be interferring with my attempts at getting
reliable R-X SWR readings on my G5RV with the 259b?

Absolutely. Ironically, among the many uses for an "antenna analyzer", the
one they're least suited for is analyzing actual antennas -- at least in a
lot of urban settings. What I've done is constructed a number of "half
wave filters" which effectively filter out TV and FM signals without
affecting the measured impedance appreciably on the band they're designed
for. You do need one for each band, however.

A "half wave filter" has a unity voltage, current, and impedance
transformation, and a 180 degree delay, at the design frequency, like a
half wave transmission line, but provides a lowpass filtering function.
They're broad enough that you can use one over a whole band without too
much impedance error. It consists of a shunt C - series L - shunt C -
series L - shunt C, with the reactance of every element except the middle
C equal to 50 ohms (to mimic a 50 ohm transmission line). The reactance of
the middle C is half the value of the others -- you can conveniently use
two capacitors in parallel, each having the same reactance as the end
capacitors. I generally use mica or NPO ceramic capacitors, and inductors
wound on type 6 or type 2 powdered iron cores, for HF. You should test the
filter by measuring several known impedances with it in series with the
impedances, and with it removed.

You can make a highpass version, too, by exchanging the Ls and Cs. In your
case, though, you need lowpass ones.

Roy Lewallen, W7EL

I tried a 2.5 MHz HPF with an MFJ analyzer to measure a 75 m antenna.
Unfortunately there is too much phase shift. Really not usable below about 5
MHz. The MFJ works fine during daytime hours before all the signals come up.

Tam/WB2TT