Cecil Moore wrote:
Ian White GM3SEK wrote:

And here you've made that big leap again. Where did "power" come from?
Nothing in what you or I have said above explains how come the meter can
read "Watts".

It is calibrated in watts assuming that:

Thank you, Cecil: those key words "calibrated" and "assuming" are
exactly what I wanted to bring out.

Vfor^2/50 = Ifor^2*50 = Vfor*Ifor = forward power

Vref^2/50 = Iref^2*50 = Vref*Iref = reflected power.

This is in accordance with the laws of physics embodied in the wave
reflection model. '50' in the above equations is critical. In any other
environment besides 50 ohms, the assumptions are not valid and the
calibration will be in error.

So the printing on the Bird scale depends on a calibration using a
different kind of wattmeter - one that measures real RF power delivered
into a 50 ohm load. But beyond that, the calibration for a mismatched
load depends on a large additional helping of theory.

Those are the reasons for the point I'm always wanting to make about the
Bird: it cannot be called in evidence to "prove" anything about forward
and reflected power, because it is entirely dependent on the theories
under debate.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek