On Mon, 01 Aug 2005 22:49:22 GMT, "U-Know-Who"
wrote in :


"Frank Gilliland" wrote in message
.. .
On Sun, 31 Jul 2005 08:36:55 -0700, "John Smith"
wrote in
:

tnom:

Yep, I always worry about terminology...
It will allow you to tune the antenna/coax from a 5:1 to a 1:1 and run
your
transceiver into it--WITHOUT BURNING OUT THE FINALS.


Guess what, John: Most of the finals used in CB radios today (and for
the past 20+ years) can withstand SWR of 30:1 indefinitely. And that
includes the power transistors used in amps, too.


Frank, to be fair, you must now finish this statement. Even at 3 watts, some
of these tiny radios and chassis don't have enough mass to dissipate the
heat that will be created by the miss-match. Sure, you can add a larger sink
and forced air cooling, but stock, they won't take it.


I have yet to see a CB radio that couldn't handle a 3:1 SWR (unless it
was improperly installed, as you mention below). And I know that those
plastic Cobra 19's and those micro-Midlands can be keyed at a normal
duty cycle without -anything- hooked up to the antenna jack. Some of
the older radios are a bit fussy, especially those with the 2SC799.
But those are becoming quite rare -- probably for just that reason.
The 2SC2078 and other modern CB finals are solid transistors that are
pretty darn hard to blow, with or -without- a heatsink (the TO-220
case dissipates quite a bit of heat all by itself).


And it's never a bad
idea to check the heat sink and mica insulator and replace the grease with
Arctic Silver 5 or some comparable superior thermal compound. I lost an
MRF477 due to improper installation by the manufacturer. The screw had been
stripped and was not making proper contact with the heat sink, and that was
with a reasonable SWR.


I have seen a few Unidens with those thick, grey, rubber heat-sink
insulators -- worthless crap like that shouldn't have even been
installed at the factory. Another problem I've seen is amp junkies
(audio and radio) who replace their own transistors and use -WAY- too
much heat sink grease. They were never taught that it's only meant to
fill the air gaps where there's no direct contact -- IOW, just a
little dab'll do ya. But I will qualify my statement to say that the
RF transistors are rated for continuous 30:1 SWR -only- when they have
a sufficient heatsink.








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