[Amps] Plate choke magic?

[wc6w at juno.com]

>-- Tomm Aldridge <KD7QAE at 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
amps at contesting.com

Hi Todd,
  No magic involved.

  A compromise was reached long ago that most ham amplifiers have employed ever since.  By allowing the plate choke to carry some reactive current at its lowest frequency of operation, the value may be reduced such that only a single series resonance occurs within the 3-30 MHz region, which may then be conveniently placed at a "safe" frequency.

  The long form factor provides the best L/C(stray) ratio. 

  This topic is well covered in: "R.F. Chokes for High Power Parallel Feed",  by Vernon Chambers, QST, May 1954, pp30-33.

  A toroid would not be the best choice for a ferrite loaded plate choke due to the large DC component which would tend to saturate a closed magnetic circuit.

  A reference on this approach is: "A Unique RF Plate Choke", by Bill Deane, 73, Sept. 1969 p. 147.

73 & Good afternoon,
  Marv WC6W

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