[Elecraft] KPA1500 amplifier with built-in ATU, separate power supply

[David Woolley]

Real PA designs are very far from correctly reverse terminating the 
transmission line.  The maximum power transfer theorem is a complete red 
herring in this sort of situation.  For example, on the top of page 12 
of the third edition of the Art of Electronics, immediately after 
setting the maximum transfer theorem as an exercise, they point out that 
ordinary output stages are operated far from that condition.

The only case in which you might get reverse termination, is if there 
was a (typically ferrite) isolator.  In that case, the PA wouldn't care 
about the mismatch (at least not for an ideal isolator), but the 
terminating resistor in the isolator might glow red or white if you 
shorted or opened the output.  In practice they are normally microwave 
devices, but I suppose they may be used in professional HF systems.

In real systems, most of the reflected power bounces straight back out 
again when it hits the PA, to get another chance to be lost in the 
feeder, or even radiated.  Of course this may cause the PA to run out of 
volts, or gain, and clip or distort, or exceed the safe operating area, 
and melt bond wires, or destroy the chip itself.

-- 
David Woolley Owner K2 06123

On 22/04/17 05:47, John Perlick wrote:
>
> Well, it might incrementally improve the loss in the coax because the reflected wave from a high SWR antenna would not be the-reflected at the amp.  It would be fully absorbed into the amp which is well matched.
>