I would like to run this by the AMPS group. Has anyone ever experimented
with a VHF parasitic suppressor like the ones used in large-cavity enclosure
High-Power HF amplifiers where the size of the enclosure could be
self-resonant in the VHF range and the cure was to position a resistive loop
in the enclosure to dampen the VHF resonance? My understanding of a VHF
parasitic suppressor is that it is basically acting as a low-Q broadband
"wave-trap" in the Anode lead of the amplifier tube. A wave-trap can be
coupled inductively into a circuit as well as wired directly in series with
the circuit as is commonly done in most HF linear amplifiers. As Rich
described in his amplifier web page a grid-dip meter can be easily coupled
into the anode circuit to check for a VHF resonant condition, in fact it can
be easily "overcoupled" if placed too close to the anode circuit. My proposal
would be to experiment with placing a resisitive loop physically near the
tube or the anode wiring to inductively couple into the VHF resonant circuit.
I would guess the resistive loop could be made of nichrome ribbon and its
size adjusted for maximum damping at the VHF resonant frequency. Several
nichrome ribbon loops of slightly different sizes could be placed next to the
tube for "stagger-tuning" for more broadband coverage. I guess someone would
just have to experiment with placement of the loops to determine if they were
effective in the circuit. There is no reason to believe that they would not
be effective. Placing them near the tube would promote strong inductive
coupling and the resistive material in the loop would provide loading at the
VHF frequency. One could check the effectiveness of the resistive loop by
again placing a grid-dip meter near the circuit and checking for strength of
dip at the VHF resonant frequency.This might revolutionize the way builders
approach the problem of suppressing VHF parasitics while preserving top
efficiency in the HF tank circuit and not introduce a lossy series-resistance
directly into the high-RF current carrying anode lead in the amplifier output
circuit. 73 Todd Roberts WD4NGG .
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