Imagine the following setup:
Amplifier - SWR meter #1 - antenna tuner - SWR meter #2 - feedline - antenna.
Suppose all of the inductors, capacitors, feedlines and antennas have no
ohmic losses (i.e. they don't heat up).
If the antenna has an SWR of 3:1, it reflects 50% of the power reaching it.
Adjust the antenna tuner until SWR meter #1 says the amplifier sees a perfect
match, and adjust drive to 1500 watts on meter 1. This means meter 1 will
show 1500 watts forward, and 0 watts reflected.
Then SWR meter #2 will show 3000 watts forward and 1500 watts reflected.
Net power radiated from the antenna is 1500 watts, same as the amplifier
produces. The voltage on the feedline will also vary along its length, and
in places by 3 times what they would be with a perfect match (as a result
high power and high SWR can sometimes cause insulation failure in small cable).
As we step back into the real world where components have resistance and thus
dissapate part of the current flow as heat, power will be lost, and because
the feedline has more power (trying) to run through it, losses will
be higher than with a perfectly matched antenna. SWR meter #2 will also show
less than a 3:1 SWR because not all of the power makes it back to the meter.
If the feedline has 1 dB of loss, meter #2 will show 2193 watts forward,
693 reverse (for an SWR indication of 1.8), 871 watts radiated (and 871 watts
reflected at the antenna) and 629 watts lost as heat (more if there are losses
in the tuner). In this case the radiated signal is 1.4 dB worse than from
a perfectly matched antenna.
With a perfectly matched antenna, SWR meter #2 would show 1500 watts forward
and 0 reverse (SWR is 1.0), and the antenna would radiate 1191 watts and
309 watts would be lost in the feedline.
With 3 dB loss in the feedline, a 3:1 SWR antenna would show 1700 watts forward
at meter 2, 200 watts reflected, radiate 425 watts and lose 1075 watts as heat,
while a matched antenna would radiate 750 watts and lose 750 watts as heat, or
2.5 dB better than the high SWR antenna.
As feedline loss rises, the 3:1 SWR antenna approaches 3 dB worse than a
good match as most of the reflected power "disappears" in the feedline rather
than circulating (and the SWR measured near the tuner "improves").
At an SWR of 1.5, only 4% of the power is reflected, and there is less than
0.2 dB difference from a perfect match even in the worst case.
At an SWR of 2:1, about 25% of the power is reflected, would is 1.2 dB worse
in bad feed line, 0.5 dB worse in a 1 dB loss feedline.
The problem area is very high antenna SWR with lossy feedline where the power
reflected from the antenna is lost as heat instead of being re-reflected
from the tuner back to the antenna.
Using the output network of the linear amp instead of a tuner does not change
the physics of the reflected power, per se, but it may make it more difficult
to meter actual circumstances. Since the output impedance of the tube is
much higher than 50 ohms, most of the power reflected from the antenna
will still be re-reflected to the antenna, but the high SWR can result in
higher voltages in the output network.
Darrel AK6I
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