On Thu, 29 Mar 2007 13:10:46 -0700, billcalley wrote:

What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

But then why does the concept of "mismatch loss" exist in
reference to antennas? I have quickly calculated that if a
transmitter outputs 100 watts, and the TX antenna has an impedance
that will cause a VSWR of 10:1 -- using lossless transmission line --
that the mismatch loss in this "lossless" system would be 4.81dB!
(Reflected power 66.9 watts, RL -1.74).

Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

Confused!


It truly wouldn't matter if there were no such thing as resistance
and so on. The whole circuit could be tuned, with the transmission line
a part of it, and all of the power would go out the antenna.

Unfortunately, that's not the way reality works, more's the pity.

Every time those "standing waves" bounce back and forth, they warm
up the transmission line, the connectors, the transmitter tank, the
transmitter itself, etc, etc, and Entropy is conserved. ;-)

Hope This Helps!
Rich