RadioBanter - View Single Post

Owen Duffy · June 11th 10, 12:16 PM posted to rec.radio.amateur.antenna

Richard Fry wrote in news:32a66fcc-a742-45c6-bdf1-
:
....

Could you or someone please post the same analysis when the electrical
length of the 200 ohm transmission line is 1/4-wave rather than 1/2-
wave, including the physical locations/components where r-f output
power is dissipated?

Well Richard, since the example discussion implies lossless line, you
know that zero power is dissipated in line losses.

Assuming that you are talking steady state...

If literally, you changed the line length in Roy's example to an
electrical quarter wave, then the transmitter will see a load impedance
of 800+j0, and it will develop exactly the same power in the actual load
(quarter wave 200 ohm line+50+j0 load) as it would in a 800 ohm resistor
connected to its output terminals. Since the line is lossless, 100% of
the actual transmitter output power is absorbed by the load, dissipated
if the load was a resistor. If you were to plot the instantaneous power
at the transmitter output terminals vs time, it would always be positive,
ie there would not be a reactive component.

The transmitter output power is probably different to that developed in a
50+j0 load for several reasons, but there is not enough information to
determine that output power.

Of course, a practical line has line loss, and the heat generated under
standing waves varies along the line (ie W/m is not constant), and the
impedance seen by the transmitter will be a little different (including
being reactive) due to the fact that Zo is not real, the VSWR is not 4,
and effects of line attenuation, so actual transmitter power may be a
little different.

Owen