> From: "Geurts, Gerard" <ggeurts@amp.com>
> >Not correct. Wattmeters are calibrated for a 50 +/- j0 ohm load. If the
> >load is other than that, the readings are bogus.
>
> That is not the full explanation. The problem is that most of our 'RF
> power meters' (including those Bird Watt meters) are actually RF current
> meters, with a scale calibrated in Watts. This scale is only correct if
> the load is a purely resistive 50 Ohms.
Hi Gerald.
The Bird, Drake, and even the lowly MFJ meter are ALL directional
couplers that respond to current AND voltage.
If the current is half, the voltage is double....and vice versa for a
constant power. Since they are summed in the meter bridge the result
is they read the correct power no matter what the load impedance as
long as you subtract the reflected power from the forward power.
The only precaution is you use the proper reflected power
slug, or consider the scale accuracy of the meter. The rated
accuracy of the Bird is +- 5% of full scale anywhere on the scale.
With a 2500 watt slug, that's 125 watts of error at any power level.
If you see 2000 watts forward and 500 reflected, the real power can
be as high as 2125 forward and as low as 375 reflected for a total
power of 1750 watts.
At the other extreme the power could be 1875 forward and 625
reflected, for a total power of 1250 watts. So the power is between
1250 and 1750 watts.
If you use a low power slug for reflected power, the reading is more
accurate.
> The solution is straightforward. You need a sampling head (directional
> coupler) that samples power, not current or voltage. There is a widely
> published design using a current transformer and a voltage transformer
> that does the trick.
Or a capacitive divider also works. That's what nearly EVERY bridge
made does.
73, Tom W8JI
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