> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> Usual problem - we're using the same term and understanding it as
> meaning different things. As Humpty - Dumpty said 'When I use a word,
> it means what I want it to mean'.
Yes, look at the silly thread about common mode current in the
antenna group!
> There's an awful lot of 'Alice in Wonderland' that's applicable to
> radio engineering!
When dealing with terms there is. It's certainly important to ask
what the other guy means, just to be sure.
So when I say "non-dissipative resistance" I mean the real part of an
impedance. If you'd like to know where the power goes, I can tell
you.
Assume we perturb the output of a PA with a reverse generator near,
but not quite on, the TX frequency. I did this with a 30 dB high
power pad, driving one end with a 100-200 watt class A generator and
the device under test at the other.
By tapping the line at multiple points, and looking at the
line voltages with a bridging L-pad (very high Z to the line, and
a proper Z to the measurement device) with a selective level meter we
can measure the voltage change along the line.
If the line is terminated in its surge impedance the voltage will
remain constant along the line, since the line has negligible loss.
What I did was drive the line with the DUT, and tune for maximum
power with fixed drive, but maintaining the drive level at a value
that kept the output device from being damaged. Measuring the reverse
voltage along the line, it was flat for ANY non-class A PA when
efficiency was maximum ! That was true even with a class C PA with
very high efficiency.
Not only can this method tell you if the line is flat for the reverse
generator, it can also tell you what direction the impedance goes
if the exciter is mistuned. My solid state exciters were higher Z
than the line Zo, not lower as claimed in one article.
As a matter of fact all the PA's go up in impedance as the drive is
reduced, until they reach class A. At that point they stop going up.
With a class A stage maximum power output (while remaining class A)
occurred when the reverse generator voltage was flat along the line.
I found the experiment interesting, even if it is useless or
unimportant in real life.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|