On Mon, 11 Apr 2005 08:03:21 GMT, "Pete KE9OA"
wrote in
:

snip
On a final note...........at 37 ohms, you will have a VSWR of 1.3 to
1.................at 75 ohms, you will have a VSWR of
1.5 to 1. What is the difference here? For a transmitter with a tube
output
and an internal matching network, you wouldn't really see much effect. For
a
typical solid state transmitter, there would be some difference between
the
two antenna impedances, because the broadband solid state transmitter
would
be called upon to deliver more current to the antenna. It probably
wouldn't
have any effect, unless the ALC circuit was aggressive in its operation.
In
this case, power foldback would occur into the 37 ohm load. Would it
happen?
Probably not.


.....uh, what?

Both tubes and transistors use matching networks, so I don't know what
distinction you are trying to make there. Power will be reflected from
an antenna/coax mismatch -regardless- of whether you have a tube or
transistor final. And what does an ALC circuit have to do with
transmission line propogation?

This pertains to solid state amateur transceivers that don't have an
adjustable output matching network.........most of today's units have
fixed-tuned bandpass filters after the output stage. I am not referring to
transmission line propagation; I am referring to the fact that, with a
fixed-tuned output network that expects to see a 50 ohm characteristic
impedance, the ALC can fold back the power. Usually, that doesn't occur
until a VSWR of 2 to 1 is reached. My explanation is for illustrative
purposes only.


I understood that much. My point was that whenever an antenna/coax
mismatch occurs, tuning the output tank (as with a tube final) doesn't
cure the mismatch or the resulting signal loss. All it does is protect
the final from the reflected power. The output tank should be matched
to the characteristic impedance of the coax whether the final is a
tube or a transistor. If the coax shows standing waves, the mismatch
should be fixed at the point of mismatch, not at the radio. I realize
that this is not always practical, but people should know that such a
conjugate match just dissipates that reflected power somewhere else,
usually from the outside of the coax or the radio.

As for ALC, I thought you were referring to a different type of
limiting. My bad.





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