Bill,
I believe the answer in your example is that the amplifier is only
putting out 900 watts, all of which is being delivered to the antenna
(neglecting losses in the transmission line). One way to understand this
is to compute the INPUT power of the amplifier, which is, as is usual,
the DC plate current times the plate voltage. Then subtract the 900
watts that is delivered to the antenna. Since energy is always
conserved, the difference has to go somewhere, and if we neglect losses
in the transmission line (a reasonable assumption, especially if we
assume open-wire line) then the only thing remaining is the amplifier
itself. It may be helpful also to consider the limiting case of nothing
connected to the amplifier output. Then the power output is zero, the
SWR is infinite, and all the amplifier power is dissipated in the
amplifier itself. Similarly, if we short circuit the amplifier output
connector, so that the amplifier sees zero impedance, the SWR is also
infinite and the same result holds.
A related issue is when you have a antenna tuner on the output of an
amplifier, with a high SWR on the transmission line, which the tuner
matches to the 50 ohm output of the amplifier. Then the amplifier is
seeing a 1:1 SWR, which means that it is delivering its normal output
power to the tuner+transmission line+antenna. Where that power divides
up among the three pieces depends on the details of the setup, but often
much of it is lost in the tuner itself.
73,
Jim W8ZR
73,
Jim
On 5/24/2017 5:12 PM, Bill Turner wrote:
There is something I don't completely understand about the effect of
SWR on an amplifier. Perhaps someone here can clear it up?
Let's say my SWR meter indicates 1000 watts forward and 100 watts
reflected.
1. Does this mean the actual radiated power from the antenna is 900
watts?
2. How much power is the amp actually putting out - 1000 watts or just
900?
2. If the answer to #2 is only 900, why does the SWR meter indicate
1000? Is that a false reading caused by the actual 900 added to the
100 reflected? If so, why does reflected and actual power add to
produce a false reading?
After 60 years in ham radio I suppose I should know this stuff, buy I
must have been absent that day. :-)
All comments appreciated.
73, Bill W6WRT
_______________________________________________
Amps mailing list
Amps@xxxxxxxxxxxxxx
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@xxxxxxxxxxxxxx
http://lists.contesting.com/mailman/listinfo/amps
|