Asked:
"Why are plate chokes seemingly black magic? (etc.)"
I'll try to dispel some of the mystery and misconceptions. First, the
very principle of operation of 'modern' solenoid, air core plate chokes
is not as simple as discussed here so far. The length to diameter ratio
is made deliberately long and skinny because doing so encourages the
choke to exhibit many, many parallel self resonance modes. Parallel
resonant modes present extremely high effective resistances to the plate
tank circuit. There is first the primary parallel mode, which is the
lowest in frequency of the almost infinite number of desirable parallel
modes. The primary mode can present impedances on order of meg-ohms to
the plate tank. All of my single band amplifiers use plate chokes which
are self resonant to the band of operation. Such a choke is totally
"invisible" to the tank circuit resonance and loading of the tank. You
should be able to dip your tank and then connect or disconnect the choke
and see no change at all. (See the exception below for 160 meter
operation).
So inductance and inductive reactance alone are not the principle of
operation--the calculated Z of most plate chokes will be woefully low
indeed. An exception here is 160 meter operation, where a multi-band
coverage choke is usually made as "small as possible" in order to limit
uncontrolled, dangerous series resonance modes from occurring. As a
result, because of their far lower than ideal inductance, and not
operating with much parallel resonance effect, these small chokes
reflect into the tank circuit on 160 meters "negative reactance" which
has the effect of increasing the tank resonant frequency. Therefore we
usually and unknowingly must add more capacity to C1 in the tank. An
undesirable side effect of doing so, increases circulating currents
within the choke itself, increasing losses and heating the choke. It is
usually tolerated as being a necessary compromise.
Plate choke duty is very demanding service. Solenoid chokes have become
standard because they are very high in performance in this difficult
application. They are also quite critical. The desirable parallel
resonance modes seem to be affected by most everything such as the
enclosure they are mounted in and this should be considered and included
in their design. Same goes for the destructive series resonance modes
which must be avoided at all costs through testing in situ and tracing
the sign of the cross over one's chest before the first QRO testing is
done. Hence reputation for black magic I guess. Ferrite core plate
chokes have historically been considered unfit for QRO duty because they
cannot present the extremely high effective Z loads to the tank that
solenoid air chokes are capable of. But I am open to results from brave
experimenters.
73, Roy K6XK/0
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