Re: Strong Local FM Broadcast / MFJ 259b


"Roy Lewallen" <w7el@xxxxxxxxx> wrote in message news:13v0j5rib6orp36@xxxxxxxxxxxxxxxxxxxxx
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

.

Relevant Pages

  • Re: Norton noiseless feedback amp calculation
    ... You may want to add an emitter follower. ... It's not a good idea to use the output impedance of an emitter ... to be added to the filter insertion loss. ... and large signals. ...
    (sci.electronics.design)
  • Re: Strong Local FM Broadcast / MFJ 259b
    ... strong AM broadcast signals. ... 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. ...
    (rec.radio.amateur.antenna)
  • Re: Strong Local FM Broadcast / MFJ 259b
    ... strong AM broadcast signals. ... A "half wave filter" has a unity voltage, current, and impedance ...
    (rec.radio.amateur.antenna)
  • Re: SI and EMI training
    ... The basic postulate is that the decoupling should provide a total ... effective impedance of 1/10 of the effective impedance of a known ... If the differential signals are properly ... designed to be very close to the power plane they won't radiate. ...
    (comp.arch.embedded)
  • Re: Neural netss (was Re: death of the mind.)
    ... its 30+ visual areas and 1100+ feedforward and feedback pathways, ... This whole thing makes sense, regards the cortex, if you think about ... feedback signals from the other 29 areas, ... they filter and perform the basic ...
    (sci.cognitive)