Hi, George!
Addendum: Yes, I have helped several people who had squealing in their
com radios at several transmit frequencies. In two of the cases, using
the clamp-on ferrite toroid-halfs with 1/4" ID and about 1" length,
placed over the coax as close as possible to the antenna took care of
it. It would even suppress the antenna currents as long as it was
between the antenna and where the radiation was getting into the mic.
lead. In several of the other cases, close proximity of leaky RG58
with the microphone and or PTT cable introduced the cross-talk. By the
way, what we call the PTT, Push-To-Talk, the Brits, cheeky buggers that
they are, refer to it as PTS, Press-To-Speak!
I'll soon have a web-site; my daughter is setting one up for me. If
there is an interest, I'll show the triangular dipole antenna
dimensions and graphs of VSWR vs. frequency. They can be scaled for
other frequencies! The dimensions were not all that critical; trimming
of the length mainly was to get the VSWR symmetrical about the band
center. Even at extreme frequencies the VSWR was quite low!
The antenna I made for the transponder initially made me fail my
transponder test by pulling the transmit frequency too far off. It was
a slot-fed dipole as illustrated in the Rad Lab Series, Vol 12,
Microwave Antenna Theory and Design, Silver, 1949, p.246, Fig. 8.5. The
King KT -76A has its output coming from a tuned stage, and is very
sensitive to frequency pulling from the reactive components of standing
waves; they prefer a tuned transmission line. What I did was insert
various lengths of BNC adapters - barrels, M-F, right angle, etc. until
I got the reflection's reactive component to the correct value to get
the frequency correct! Paul