In article ,
highlandham wrote:

Question : Would a PI filter configuration involving an inductor and (a
combination of fixed and ) variable caps be OK for a single band
transmitter with solid state output device(s) ?

This question is posed because of the construction of a low power 3.580
MHz ARDF transmitter with a short vertical wire antenna. The PI filter
would be adjusted for max antenna current at the operating location.

What you seem to need here, is actually two different functions -
impedance matching (the load is a "short monopole", probably
presenting a low radiation resistance and a high capacitive reactance)
and low-pass filtering.

Pi sections like this (or even the simpler L section) are often used
to perform the matching, in cases like this... a series inductor, and
a shunt capacitor on the transmitter side, would be what you'd want
here, I think. Having two variable caps (one on each leg of the pi)
would eliminate the need to have a variable inductor. You can
probably guesstimate the required value of the inductor fairly well
based on the calculated feedpoint impedance of a short vertical of the
length you'll be using.

However: although this is a low-pass network, my guess is that a
single pi section might not provide adequate low-pass filtering of a
typical Class B or Class C transistor final, to meet FCC
harmonic-and-spurious-emission standards. You'll have to ensure that
any spurious emission is at least 43 dB below the power of the
fundamental (current standard for new transmitters at that
frequency).

You can comply with this limit in any number of ways. The brute-force
way is to use multiple low-pass filter sections, sufficient to get
your worse harmonic (whatever it is) down below this limit. You can
use a more sophisticated filter topology which introduces notches at
one or more of the harmonic frequencies. Or, you can use a "cleaner"
transmitter design - e.g. use a clean sine-wave oscillator, and do all
of your amplification using high-bias (e.g. Class A) stages which
don't introduce much harmonic content.

In practice, you could breadboard your initial design, test it very
briefly, and look at the RF output with a spectrum analyzer to see
whether you need additional attenuation.

Probably the most straightforward way to add additional attenuation
would be to add one or more additional series-L and shunt-C stages
prior to your variable pi section... these could probably be
fixed-value components, if you figure that you'll have a fairly
predictable impedance looking into the pi section once it's properly
tuned up for the antenna.