Rich Measures wrote:
> During the recent VHF parasitics debate on the
>rec.radio.amateur.homebrew Newsgroup, myself and the AL-811H's designer
>(Tom Rauch) submitted competing designs of VHF suppressors for evaluation
>by Wes, N7WS, using a Hewlett-Packard Model 4191A RF Impedance Analyzer.
>The VHF Rp of the Ameritron suppressor was about 60% higher than a
>similar suppressor that was made from resistance-wire. Since VHF
>voltage amplification is basically VHF-Mu multiplied by VHF-Rp, more
>VHF-Rp equals more VHF gain.
Sorry, but Rich and his web pages are still getting this wrong - despite
statements to the contrary from several people, including N7WS who made
the measurements (the data are not in dispute, only the interpretation).
To set the record straight, the parasitic suppressor does not appear as
a parallel element in the VHF resonant circuit, so the argument above is
not valid. The suppressor and the wiring inductance are in SERIES, and
that combination is parallel-resonated by the tube's output capacitance.
In that layout, the higher the Rp of the suppressor, the LOWER is the
parallel VHF impedance presented to the tube.
Circuit theory of series<->parallel impedance transformations will
confirm that, but you can apply a simple "reality check" by noting that
the VHF gain is highest when the VHF parasitic suppressor is not used,
ie when the VHF-Rp of the suppressor is zero.
According to N7WS' measurements, the suppressors made of resistance wire
and the conventional L/R (Ameritron) suppressor had very similar
properties at VHF. The significant difference was that the resistance-
wire suppressor had higher losses at HF.
Disclaimer: I have no personal or commercial allegiances here. In
following the whole r.r.a.homebrew discussion in some detail, my only
desire was to work out the real story about VHF parasitics. In all other
respects I have found Rich's web pages very useful and informative, and
learned a lot from them.
Rich's web page has pointers to an archive of the whole discussion; but
to see the whole picture you really do have to read it all, and make
your own judgements.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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