I think the important issue is how the capacitor reacts when passing RF
current. A capacitor that works fine in one application may not stay
within tolerance when you turn up the heat from within.
Most folks using trimmers in RF power amplifiers use good quality mica
compression trimmers or small air variable capacitors. Good quality will
likely be interpreted to mean construction on a ceramic base and real
mica, no plastic, between the metal plates. Though ceramic trimmers,
those with ceramic disks with deposited metal for capacitor plates, may
work well in a receiver, they don't handle RF current very well so
ceramic doesn't necessarily mean it is a good capacitor for RF power
amps. Disk ceramic capacitors are a poor choice where RF currents will
cause them to heat up. If we look at the capacitors used by M and GE in
their PAs we find lots of small stacks of mica between metal plates
where high RF currents are found. Low impedance, low inductance, high
current handling capability... they're all things we think about.
But back to the topic... Trimmers for RF amps really come in only two
good forms: mica compression trimmers and air variables. For other
types of RF service, there are lots of choices including vacuum
variables where we need high voltage and high current handling ability.
Most capacitors (except maybe electrolytics) will handle some RF at some
level at some frequency with no ill effects.
73, Tony W4ZT
skipp025 wrote:
>
>
> How do you know the trimmers are rated for
> RF Service?
> skipp
>
>
>>Q <[EMAIL PROTECTED]> wrote:
>>I have a huge inventory of quality NOS
>>trimmer caps of all kinds,as
>>well as feed thrus and ferrite beads
>>and cores...
>>
>>
>>>skipp025 wrote:
>>>Spectrum doesn't put rf rated caps in the
>>>pa section of the transmitter. The [EMAIL PROTECTED]&
>>>low dollar trimmer caps they use don't handle
>>>RF well.
>>>
>>>I replaced all the rf section caps with rf
>>>rated units and the unit has never farted
>>>since. Spectrum is not the only company that
>>>makes this mistake.
>>>
<snip>
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