At a given operating point, there is always some load Z that will minimize
amplifier dissipation (and probably another that minimizes IMD). The problem
is, we don't normally know what it is, so guessing about it is pointless. Hence
specs like X watts into Y SWR (nominal).
Andy provided three data points. Changing the sign of the reactance will leave
the same SWR, but more than likely different amplifier response. In fact, 50
+j0 might not be optimum. Who knows, maybe some point on the 1.5:1 circle on
the Smith chart is better :-)
Adding to this uncertainty is the fact that with the typical measurement
circuitry in our PAs and tuners, we don't know the real SWR either.
I, and others, discussed this in an earlier thread
:http://elecraft.365791.n2.nabble.com/K3-SWR-Numerical-Indication-td7643839.html
Wes N7WS
On 4/10/2019 6:59 AM, Andy Durbin wrote:
As a counter to those who are asserting that SWR 1.5:1 is a good enough match
for an antenna system I offer measured data for the KPA500.
At 14.005 MHz my KPA500, when connected to a 1.44:1 SWR load (R37, X9), had a
PA dissipation of over 500 watts when producing 400 watts RF output. When the
load was changed to 1.30:1 (R39, X3) the PA dissipation was about 410 watts.
When the load was changed to SWR 1.07:1 (R49, X3) the PA dissipation was
reduced to under 400 watts. (The plots are far more informative than a text
description but I can't post them here).
If you don't mind your KPA500 running hot, and are prepared to accept it
shutting down with PA DISS fault, then 1.5:1 SWR may be good enough for you.
I'd rather know the complex impedance of the antenna system load and avoid
loads that result in high PA dissipation.
73,
Andy, k3wyc
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