[ICOM] BC intermod

[Mel Martin]

"The first is to build a high pass filter (as was suggested), but it should
go in the Receive line only.  Otherwise you'll have some unwanted
attenuation of your transmitted signal.  Not much, but who wants to lose
even a fraction of the transmitted power?"

Not always the best solution. I had used a filter in the RX line originally,
it was a tremendous improvement but I still had some residual BC intermod.
It seems allowing the BC signal inside the rig at all can be a problem if
the signal is strong enough. (A nearby 50KW transmitter in my case.) An
external filter between the rig and transmitter (a dunestar HPF-400)
completely eliminated the intermod at the cost of a fractional db insertion
loss.

I should mention the rigs were the IC781, IC751A (and some others) which use
diodes to switch the RX bandpass filters. To get an idea of the magnitude of
the problem, if I hooked my unterminated 160M dipole to my voltmeter, I
measured over 700V AC. Obviously the meter impedance is much higher than the
receiver. however, if I connected a 12V pilot light across the coax, it
lighted to about half brilliance and the brightness throbbed in time with
the local stations modulation!

Mel, VE2DC