JGBOYLES wrote:
Hi, I thought I understood this but recent discussions left me wondering.
A Bird Wattmeter provides a voltage that is proportional to the forward power
minus the reflected power. This assumes the output impedance is known and
constant, usually 50+-j0. Assume a Bird has a 0-10vdc meter for indication. I
don't know what it is, but for discussion.
Input 100 watts into a 50 ohm load, and you get 70.7 volts, the Bird scales
this to 1 volt, and you get 10% deflection on your wattmeter, or 100 watts.
Input 1000 watts and you get 223.6 volts which the Bird scales to 3.16 volts,
or 316 watts. 316 does not equal 1000, so the scale on the meter has to have
V**2 relationship to indicate 1000 watts. So 1000 watts is 31.6% of full
scale. I thought all Ham wattmeters did this.

That isn't how any directional wattmeter like the Bird works. There's
information at:
http://www.ifwtech.co.uk/g3sek/in-pr...-of.htm#bruene

This is mostly about the Bruene bridge type (the ones with a toroid).
Those are easiest to understand, because you can see very clearly how
they take separate voltage and current samples from the line.

The original explanation came from the classic 1959 QST article by
Bruene (of the Collins company, who knows how his own bridge works) and
is backed by Walt Maxwell. I repeated it in my magazine column because
today's readers still need it... perhaps now more than ever!

My article goes on to show how exactly the same principles apply to the
Bird. Once you know about taking V and I samples, it becomes much easier
to see how the Bird's pickup loop is doing both at the same time -
taking the I sample by magnetic coupling and the V sample by capacitive
coupling.

It's also very easy to see that the V sample and the I sample are never
multiplied - only added and subtracted at RF, *before* the resulting RF
voltage goes into the detector diode.

Therefore the Bird does not directly sense RF power. As Walt confirmed
very recently, the power indication on the scale is obtained by external
calibration.

Of course it is highly
dependent on the Z the wattmeter sees.

Yes - but the same concepts tell you why the Bird indicates what it
does.

Wattmeters that actually multiply V and
I are another subject, it is hard to keep them in line without causing some
insertion loss, is it not?

And harder still to find one - I've never seen or heard of one that does
it that way for RF.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek