[Amps] SWR and amplifiers

[Jim Garland]

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
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