> If there's no termination, there's no power - for power, you have to have a
> dissipating resistance.
Correct in that the unterminated end of a line presents no load. Without a
load on the of a transmission line, no power is
dissipated.
However, power is being generated and dissipated somewhere. Why? If I take my
fixed output impedance transmitter and connect to an
unterminated transmission line, my Bird 43 reads 100-watts of forward power and
100-watts of reflected power. The difference
between the two is the amount of power delivered to the load. 100-100 = 0
watts into the load. There is no load, therefore no
power is being dissipated here. No surprise. But power is being dissipated
somewhere. If we exclude the relatively loss-less
transmission line, where can this power be dissipated? It must be in the
transmitter. The directional coupler is still sensing
the presence of that power, even if it is not accurately displayed. For those
who do not believe the transmitter is incapable of
producing full output power into such an extreme case....fine. For argument's
sake, call it 1 watt...call it 1 mW of RF power being
generated. My point all along is that *any power* generated at the source, no
matter how minute, has no place to dissipate in a
lossless, or near lossless, unterminated line...other than in the source itself.
My real world example is the Omni Six with a displayed power reading of
100-watts forward on the Bird 43, 100-watts forward on the
Omni Six, and 100-watts reflected on Bird 43 into an unterminated length of
LMR-400. I can make this test on any band and the
results are constant. The Omni is generating power. Is it exactly 100-watts?
I don't know and quite honestly, I don't care. But
it IS generating power. I have evidence: under this condition, the radio is
drawing 20 DC amps of current at 13.8 VDC. P still
equals I X E. Power is being generated. Is all of it RF? Perhaps not.
Perhaps under this extreme loading condition, the
efficiency of the PA drops to a fraction of what is normal and some of the
total power is dissipated as DC collector current. Fine.
Nevertheless, the PA cannot be totally inefficient. *Some* RF power is being
generated and dissipated and it must be dissipated in
the transmitter's PA. Want more proof? Keep the Omni keyed at full power
under this condition. After a few minutes, the
transceiver is ready to melt down from extreme internal heat. The same test
into a matched 50-ohm load does not produce this level
of heat in the transceiver. All generated power, whether in the RF domain or
not is being dissipated in the transmitter.
-Paul, W9AC
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