I guess I'm not surprised by the 150W fwd power reading, but there
should be a correspondingly large ref power and a high SWR, if the
meter is accurate. Assuming a perfect 50-j442 load and a meter
calibrated perfectly to 50 ohms and perfectly accurate, I'd expect to
see 102.7W going out and 97.7W coming back. But I also do NOT expect
a typical meter to read very accurately at high SWR.
Another thing to remember: the meter will disturb the resonance
significantly, when inserted between the cap and coil. The capacitor
is only about 50pF, and the meter is likely going to look like more
than 5pF at that high impedance point between the inductor and
capacitor. It's easy enough to look at in RFSim99, for example. When
you move the SWR meter, the load on the transmitter does NOT stay the
same! I simulated the meter as 7cm of 50 ohm air-dielectric line, and
the return loss seen at the transmitter (at 7.2MHz) went from very
high (theoretically infinite) without the line/meter inserted, to only
8.14dB (~2.3:1 SWR) with the line/meter between the coil and cap.
It's important to understand the limitations of your test equipment,
and also to realize how that equipment may affect operating conditions
of the circuit.
(Tam: my recommendation is to do the test yourself. It will be a lot
easier to play with "what-ifs" and to check out things that don't at
first make sense if you have direct control of the experiment.)
Cheers,
Tom
"Tarmo Tammaru" wrote in message ...
Cecil,
You changed the load the SWR meter saw. In the first instance it was 50
Ohms. Then you changed it to 50 - j442. I think what you want to calculate
is the phase of the current flowing through the SWR meter relative to the
phase of the voltage. I wonder if anybody on this newsgroup has a contact at
Bird who could shed additional light on this.
Tam/WB2TT
"Cecil Moore" wrote in message
...
Tarmo Tammaru wrote:
I think what you did was to force the current to be in phase with the
voltage and fooled the meter into thinking it was all forward power.
That is
sort of the experiment I was going to do, but you beat me to it. Note
that
in the case where you moved the meter, you actually changed the load,
but
you know that.
I didn't appreciably change the load on the transmitter. All I did was
change the position of the SWR meter in the serial component chain.
This is essentially what Chipman discusses in his book. Now comes the
big question. Does the same thing happen on a line with reflections
when the impedance looking one direction is 100+j100 ohms and the
impedance looking the other direction is 100-j100 ohms? Is there a
localized energy exchange between that +j100 ohms and that -j100 ohms
that affects the SWR meter?
--
73, Cecil http://www.qsl.net/w5dxp
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----