In Class C operation, especially when considering repeater operation,
its way better to run an appropriate sized amplifier full bore than it
is to run a bigger amp at 1/2 or 1/3 its rating. Class C amps run
cleaner when properly (over) driven, as they will get spurious when
under driven. Typically, the smaller amplifier will run more
efficiently (more power for less or same amount of heat) then its big
brother throttled back. The efficiency rating of a power amplifier will
vary to some degree in relation to how hard the amplifier is driven.
Usually the amp will be more efficient when properly driven.
The text below is a generalization of what can happen when commercial
radios are tuned to the ham bands and are not being driven correctly.
My example below is from the Motorola line, but, this can happen with
GE, Motorola, or whatever.
Poor efficiency is what kills power amplifiers. Too many folks believe
its better to run an oversized amp at 1/2 its rating to make it last.
Lets take the Motorola Micor 100 watt UHF mobile for example. The
amplifier in this mobile throttled back to say 50 watts of output will
require about 12 to 15 amps of DC current. A 45 watt model running full
bore (45 to 50 watts out) will require 6 to 8 amps of current. The 45
watt mobile radio will produce about 45 to 60 watts of wasted heat that
is dissipated by the heatsink. The Micor mobile heatsink is rated at
around 40 watts continuous dissipation, so we are close to being able to
run it continuous duty, and can likely stretch that extra 5 to 20 watts
of dissipation if a fan is used. Now lets take the 100 watt mobile that
is throttled back to 50 watts. Its dissipating 117 watts of heat, and
won't matter how many fans you blow on it, it's going to eventually burn up.
What I'm driving at here is *Dissipation* and *Efficiency* are words
that we as repeater builders need to understand.
In my opinion, its better to appropriately size the power amplifier and
save the additional money on the electric bill then to be replacing the
PA every 6 months due to heat failure.
Kevin Custer
georgiaskywarn wrote:
>Good Morning Tom,
>I would have to run the driver just about on full bore though.
>(because of the 30 in 100 out amp...again only running it right now at
>75 watts) I am a little nervous on running it like that. I wonder if
>anyone else is running the driver that hard and getting good results??
>Thanks,
>Robert
>
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