[Amps] Re: [Amps] Re: [Amps] Re: [Amps] Bird® 43 Manual

[Steve Thompson]

On Thursday 28 March 2002 12:35, Paul Christensen wrote:
> > It is incorrect folklore that all reflected power is absorbed in the PA
> > stage, even with a solid state PA. Under some conditions
> > dissipation increases with SWR, under some conditions it can
> > decrease. Of course it is also true that dissipation might not
> > change at all with high SWR.
>
> Thanks for jumping in Tom.  Whether all the power is absorbed in the PA
> does not matter to me as long as some of it is.  My point is that my
> transceiver which is drawing 20-amps DC at 13.8 VDC is dissipating power
> somewhere in the radio.  If I take my Omni Six and attach no transmission
> line whatsoever or if I attach an essentially lossless length of
> unterminated line, where is the power being dissipated?
If there's no termination, there's no power - for power, you have to have a 
dissipating resistance.

It can help to think of it as two different things:

Whatever is on the far end of the transmission line determines the impedance 
presented to your tx output stage. When you drive up the output stage, it 
will draw some level of current depending on the load it's seeing. That 
current can vary wildly depending on the load, and it can be destructively 
high at some phases even though there's no power being delivered to the 
outside world.

The output stage will generate some level of rf voltage at its output and 
it's this voltage that the Bird meter measures and indicates as an equivalent 
power. If there's no resistance on the far end of the (lossless) transmission 
line, the transmitter doesn't deliver any power into it at all - it just 
produces the rf voltage.

Hope that helps.

Steve