Peter Chadwick wrote:
>Rich says;
>>- However, I would be more inclined to put a load C on the output end
>of
>>the L and make it a Pi network tank.
>
>Amen to that. I'd even consider using a Pi-L, bearing in mind that
>according to Part97.307(e) requires all spurious emissions in the band
>30 to 225 MHz to be at least 60dB down. Of course, a low pass filter
>would be the other approach, but it wouldn't be a negligible job
>producing one for these powers.
>
I'd agree with the suggestion to use a pi-output. With link coupling and
a series-tuning C, the loading tends to peak in the middle of the
capacitor's range, so it's always rather uncertain whether you are
increasing the loading or decreasing it. With a pi-output there is no
doubt.
As regards harmonic filtering, most VHF amplifiers never move more than
a few hundred kHz, so it's possible to filter the harmonics using
narrowband notches at 2f, 3f etc. This is much easier than lowpass
filtering. See my web pages for details of some very simple harmonic
notch filters by G4SWX using stubs made from ordinary coax (no cavities,
no machining and hardly any alignment needed).
Coming back to the original question about parasitics, for a VHF/UHF amp
it is important to avoid circuits that leave the input and/or output
unloaded at LOWER frequencies - otherwise you can get LF/HF parasitics
instead!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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