[Top] [All Lists]

[AMPS] Inductively Coupled VHF Parasitic Suppressor?

To: <>
Subject: [AMPS] Inductively Coupled VHF Parasitic Suppressor?
From: (
Date: Tue, 24 Jul 2001 13:22:53 EDT
 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 .

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>