A Plate RF choke always has some resonances. The idea is to keep the
resonances in low Q by having low capacitance between windings, hence the
occasional spacing. "High-Q" resonance at a harmonic of the fundamental
will cause large RF current in the choke coil and burn it up. It will also
cause corona hot spots due to high voltage at harmonic frequencies.
Powdered iron cores have varying degrees of loss and effectiveness depending
on the frequency and the harmonics involved. Choosing a proper core mixture
might be a challenge but I am not very familiar with the stuff. One way to
get away from all this is to series feed the tank. Put the plate tuning
capacitor on insulators (same with the load capacitor) and tie the rotor
stand connector to the plate bypass. Connect the plate directly to the tank
coil and a lesser RF choke from B+ to the 50 ohm end of the tank coil. Use
a much higher value of coupling capacitor from the 50 ohm end of the tank
coil to the RF out connector. And always use a RF choke from RF connector to
ground for static safety grounding. You must then use insulated shaft
couplers to go to the knobs and make sure the knob shaft is grounded.
This is old technologies and a lot of trouble but it is a sure cure for
hotspots on the RF choke.
See
http://wa5bxo.shacknet.nu/HAMPICTURES/series-fed-pi.gif
Even I would be hesitant to do this any more but I have done it.
73, John, WA5BXO
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Tomm Aldridge
Sent: Friday, January 28, 2005 5:42 PM
To: Amps@contesting.com
Subject: [Amps] Plate choke magic?
Why are plate chokes seemingly black magic? Don't you just want a good
decoupling of the PS from the Plate; i.e. lots of impedance from DC to
Light and no resonances? How I get that should not be an issue but all
teh plate chokes I see are long skinny and sometimes segmented single
layer solenoids of questionable wire size. Why wouldn't a really lossy
powdered metal toroid with a few fat turns on it work, assuming the
inductance was high enough?
KD7QAE
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