Tom,
Actually, a ferrite core can be used if it's of the correct type of
material. The material is determined by the frequency that the coil will
operate at. There a couple of ferrite and iron powder types that would
work. The reason most are air coils I would think is they are cheaper to
make. An insulated form is all that's really needed. The air coil formula
is then used to determine the number of turns for the amount of inductance
wanted. The higher the frequency, the lesser amount of inductance is
needed to block the RF, so the choke needs to be designed around the
lesser frequency that will be encountered. Then you need to make sure the
choke is not self-resonant at any frequency you wish to operate it on.
This is done by using a grid dip meter and shorting the coils leads
together. Any dip at any desired frequency means that the inductance will
have to be changed slightly to move the resonance point to where it wont
be encountered. Most of the time this is done by simply adding or
shortening a few turns of wire. Those staggered windings on some chokes
are done to stop self-resonance at a particular operating frequency, and
are really several inductors being connected in series where Ltotal = L1
+ L2 + L3, etc.. Hope this helps as an explanation.
Will
On Fri, 28 Jan 2005 23:42:10 +0000, Tomm Aldridge <KD7QAE@ARRL.NET> wrote:
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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