Topband: Bird Meter

[Tom Rauch]

> even if the impedance is other than 50 Ohms and reactive.
The same
> doesn'apply to bird wattmeter, which doesn't measure power
directly and
> whose accuracy is affected by loads different from 50
Ohms, and reactive.

As for power, the Bird samples a vector sum of line current
and line voltage with its directional coupler.
A properly calibrated Bird 43 meter, when used with a slug
within frequency range, is rated at + - 5% of full scale
anywhere on the scale. That means with a 2500 watt slug
reading is + - 125 watts anywhere on the scale.

All you need to do is subtract reflected reading from
forward reading and the result is (within limits of
calibration) the true power.

In other words if my meter shows a reactive or resistive
load that shows 1200 watts forward and 350 watts reflected,
it is ~850 watts true power. We should always use a slug as
close to full scale as possible for both directions, since
accuracy is + or  - 5% of full scale anywhere on the scale.

Most thermocouple meters are not so good.

73 Tom