On Wed, 28 Nov 2007, Antonio Vernucci wrote:

275 watts may be correct, under two conditions, (1) The Bird Wattmeter
is a peak wattmeter, and the peak reading button is pressed in. (2)
The actual output of a class C modulated final (s) modulated by a
class B modulator is four times the carrier power.

Just a remark on Bird wattmeters:

- it is true that a peak wattmeter will correctly show the peak power of a
100% amplitude-modulated carrier (4 times the unmodulated carrier power)
- conversely, a normal (i.e. non-peak) wattmeter will NOT correctly measure
the average power of a carrier 100% amplitude-modulated by a sinusoidal tone
(that is 1.5 times the carrier power).

Reason is that the (non-peak) wattmeter actually measures the average
voltage of a rectified RF signal sample and displays the measurement result
in terms of RF power by the use of a non-linear (quadratic) meter scale.

The average voltage of the rectified RF signal does not vary when modulation
is applied, as the positive peaks are perfectly compensated for by the
negative peaks.

This is plausible only with a pure, single-tone sine wave audio input
signal. I have looked at my own voice on an AM-modulated carrier on a
scope: it is highly asymmetrical. Even the books sometimes mention this.

Such compensation does not instead occur with regard to RF
power, as the positive-peak power is, as already said, 4 times the
unmodulated carrier power and not just 2 times.

In conclusion the Bird wattmeter (and all other wattmeters working on the
same principle) will show the same RF power, independently of whether the
carrier is modulated or not. And that is clearly wrong.

There are peculiar circumstances when AM results in upward or downward
plate current deflections upon modulation ... all with their own
symtomology and causes.

Only measurement devices that actually measure RF power (e.g. bolometers)
will correctly show the modulated carrier power.

I think a scope looking at RF voltage (with either a digital or
electrostatic waveform "storage" functionality) into a pure resistive
dummy load would be just fine.

73

Tony I0JX