> I want something that I can use behind a 160m MOSFET
amplifier, which
> haven't much filtering in the output.
> The amp has to be runned in a multi-multi station.......
You won't be able to use stubs. They make terrible filters.
The Q is very low, the losses high, and the space
requirements are bad for HF bands. They have poor to no
filtering on higher harmonics.
The only thing stubs have going for them is they are cheap
to build. For example I had a harmonic of my 80 meter signal
that fell on 40 meters. It was many dB down, but with an
antenna 50 meters in the air a few people could hear it. I
added a 1/4 wl shorted stub from good RG-213 coax, but that
stub looks like a 2K ohm resistor on 80m and a 1-2 ohm
resistor on 40. That's a lot of space for something that
works so poorly.
The only good solution is to build a simple multiple section
low pass filter using transmitting components. Don't try to
use one of the narrow extra-deep notch resonant filters that
have second harmonic traps and the like. In the first place
you probably won't need it, in the second place voltages and
currents will be too high, and in the third place you might
not terminate the PA stage with the proper load impedance on
harmonics to prevent high voltages from damaging the FET's.
If you have a small remaining problem you can clean it up
with a few stubs (if you can tolerate the shortfalls), but
the primary filter should be a standard low pass minimum
ripple design with multiple sections and must generally be
located very close to the FETs and have a very low input
impedance on harmonics. The primary filter after a stage
having a conduction angle of less than 360 degrees has an
entirely different design goal than something to clean up a
little extra very low level junk bothering the next band.
73 Tom
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