Jeff Liebermann wrote in
:

....
In the real world, the power that a transmitter delivers to a non
ideal load is not so simply predicted, and it is entirely possible
that it delivers more power to the mismatched load.

Owen

I beg to differ somewhat. In order for the reflected power to
contribute to the incident power, the reflected power would first be
attenuated by the coax loss. It would then require a substantial

Are you proposing vector addition of power?

mismatch at the transmitter, which is unlikely. However, assuming
there is a mismatch at the source, some of the reflected power will be
sent back to the load (antenna), after getting attenuated by the coax
for a 2nd time. There may be some contribution, but it will very very
very very small.

Let's try some more or less real numbers. I kinda prefer doing
everything in dBm but hams have this thing about using watts...

Of course it doesn't matter, which unit system you use, but if you start
adding 'forward' and 'reflected' power in dBm because it is real
convenient, you have peformed a vector addition of power. Is that valid?


Start with a 50 watt xmitter and 20 meters of LMR-240 coax at
0.09dB/meter for an attenuation of 1.8dB.

The power delivered to the antenna is:
50 watts / ((1.8/10)^10) = 50 / 1.5 = 33 watts
The 1.5:1 VSWR reflects 4% of 33 watts for 1.3 watts reflected.

The 1.3 watts is again attenuated by the 1.8dB coax loss resulting in:
1.3 watts / (1.8/10)^10) = 1.3 / 1.5 = 0.87 watts

You start with a limited view of the mismatch, VSWR conveys only one
dimension of a two dimensional mismatch.

Your treatment of the forward wave and reflected waves as independently
attenuated is an approximation that will lead to significant errors in
some cases.

For example, what percentage of the power at the source end of the line
is lost as heat in 1m of LMR400 at 1MHz with a) a 5+j0 ohm load, and b) a
500+j0 ohm load. The VSWR is approximatly the same in both cases but the
answers are very different, one is almost 100 times the other.

Doesn't it stand to reason that as the length of the transmission line
approaches zero, that the power lost transmission in this type of line in
the high voltage low current load scenario is lower than the low voltage
high current load scenario.

Another issue is that the V/I characteristics of a transmitter output
stage is not necessarily (or usually for most ham transmitters) a
straight line, in other words it does not exibit a constant Thevenin
equivalent source impedance with varying loads and the application of
some linear circuit analysis techniques to the output stage are
inappropriate.

Owen