In article ,
gareth wrote:

One question; why the predominance of a multitude of bandpass
filters on the output of TXs? Why not a tuned circuit, along the
lines of valve PA stages, once the impedance level were to be raised?

Practical answer: designers and manufacturers have probably determined
that this results in transmitters which are smaller, less expensive,
more reliable, and easier to use, than would be the case if tuned
circuits were used.

Fleshing this out a bit:

- In order to meet current FCC emission requirements, most
transmitters require multi-stage low-pass filters (5- and 7-element
seem to be fairly common). With a tunable filter, you'd need a
large number of wide-range adjustable reactances to tune all
stages properly.

- Tunable capacitors and inductors capable of handling 100 watts
or more of RF aren't small, even if you look only at the voltages
and currents present when they're properly tuned. Fixed-value
components (e.g. RF doorknob or chip caps, toroids wound on iron
cores) pack more value into less space, and I'd guess that even
when you need N set of them to handle N bands it's still a win,
space-wise.

- In order to avoid arcing if the transmitter is operated with
the output stage mis-tuned (and high voltages appear in the filter)
you'd need wider vane spacing in the air-variable caps... which
means you need larger (or more) plates and stators to achieve the
required maximum capacitance for operation on the lower-frequency
bands.

- A multi-stage tunable filter requires numerous moving contacts,
which are integral to the components (e.g. ground wipers on
capacitor rotors, rolling wheels on roller inductors). These
typically aren't sealed, get dirty, and can require cleaning.

Bank-switched fixed loss-pass filters usually use relays, which
are sealed and which are easier and less expensive to replace if
the contacts ever fail.

- You can bandswitch in an instant with a relay-switched fixed
filter. Tunable filters with continuously-variable reactances have
to have the caps and inductors "rotated" in some fashion to the
correct value - this takes time. You could build a tunable filter
with banks of switchable reactances in e.g. 1:2:4:8 combinations,
and switch these quickly, but that would involve even more relays
than a "one fixed filter per band" arrangement.

- As somebody else pointed out, it's not easy to build a single
tunable filter which works well down on 80 and 160 meters (needs
physically-large L and C to achieve the necessary values) and up at
6 and 10 meters (where you need small minimum values for these
components, and where wiring parasitics become a big issue). A
naive design for a tunable filter for this wide frequency range may
end up having some nasty parasitic resonances which will can let
the magic smoke out of your transmitter finals.