Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Whatever your Amp or proceedure, Pi-network outputs require maximum
loading. It results in maximum output = maximum efficiency = minimum
harmonics and distortion. Even a minor loading misajustment results in a
big rise in harmonics and distortion. Your Club tuneup routine is an
excellent proceedure since you would be providing sufficient drive at peaks
and If a peak reading meter were handy, very meaningful. CW at the 80-100
watts would also be useful but there is little need to lean on it for more
than a couple of seconds while looking at meters and tweeking a knob.
Always keep an eye on grid currents as they are the most fragile they will
be metered if it matters in your Amp (or protected somehow). Once tuned to
maximum output in this fashion, you can reduce drive, confident that the
final stage is fully loaded. Do this with a 50 ohm Dummy load so that you
will know what this is supposed to look like as it is easier to do a little
tweeking on the air versus tune-up from scratch into an impossible load.


"Straydog" wrote in message
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On Wed, 28 Jun 2006, Roger D Johnson wrote:

Straydog wrote:



because that reflected power will end up being dissipated in the
tube/heatsink.

Another "Old Wives Tale"! Tubes are only "matched" to the point where
the desired output is obtained.

All depends on your definition of matched.

Reflected power will see the amp as a
mismatch and will again be reflected back towards the antenna.

I think that is only partly true. Yes, there will be reflectd power going
back out to the antenna, but there was an article in QST back a number of
years ago. Yes, the mismatch also results in higher plate dissipation,
too. I will correct myself about reflected power being disipated in tube,
but the mismatch will increase the plate dissipation. The other issue is
being off plate resonance. I had my SB-230 trip the overtemp relay when I
moved frequency and did not retune the final.

You also need to allow for loss in the transmission line. Not all of what
gets reflected at the mismatches ends up at the other end of the
transmission line. SWR (i.e. reflected power generated at the
antenna/feedline) would not be a problem if the transmission line
were long and lossy.

73, Roger

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JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Why would reflected power be any more dissipated than forward power?

73, Roger

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Roger D Johnson wrote:
JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not
allow
a proper load.

Why would reflected power be any more dissipated than forward power?

Only because it takes 3 trips through the coax before it reaches the
antenna. Forward power only takes one.

-Chuck

On Fri, 30 Jun 2006, Roger D Johnson wrote:

JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Why would reflected power be any more dissipated than forward power?

I would ask the same question. Or, if you had 100 watts going forward from
the transmiter end of the coax on a 100 feet of 3 db loss/100 foot coax, then there is only 50 watts at the
end of the coax. If half of that gets reflected (i.e. 25 watts coming
back) from ant-coax mismatch, then it suffers another 3 db loss by the
time it gets back to the entry point on the coax (or, 12.5 watts +/-). At
the transmitter end, Roger and I are having substantial discussions by
private email.

73, Roger

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