[Lowfer] Isolation Transformers

[Lyle Koehler]

I'm not sure if isolation transformers alone will do a lot of good when you
still have a DC path to the antenna via the power and tuning lines. With a
good ground system at the antenna site, it's likely that a combination of
isolation transformers on the coax, and chokes in both the power and ground
lines would be effective in reducing the conducted noise from the shack. The
chokes could be something like 1 mH on MF; larger on LF if you can tolerate
the DC resistance. It's important to stay below the self-resonant frequency.
There should also be isolation in the tuning line. Since the tuning circuit
is usually high impedance, you can put a big resistor like 100k in series. I
haven't actually tried this kind of isolation, but theoretically it should
work. Maybe others on the reflector have some practical experience with
isolating remote-tuned preamps that are powered from the shack.

Lyle, K0LR