While the Bird wattmeters are highly respected, consider the following
with respect to any analog wattmeter:
The Bird can be as accurate as 5% of full scale - that means that with a
100 watt slug, a 5 watt potential error at any point on the scale.
Attempting to measure a 20 watt actual signal with a 100 watt slug can
indicate anywhere between 15 watts and 25 watts. Not a lot of precision
in that measurement.
In addition, that 5% figure is only valid right after calibration. How
many hams with Bird slugs have had them recently calibrated - my guess
is that few have done that. So, figure a possible 10 percent error (of
the full scale reading) when evaluating power with a Bird wattmeter.
Yes the Bird can be better than the average amateur wattmeter which are
typically spec'ed for 20% of the full scale reading.
Exceptions to that are several of the digital wattmeters - the Elecraft
W1 and W2 as well as the Telepost LP-100 will give you within 5% of the
actual power reading rather than as a percentage of full scale.
As my wise old college professor said - engineer, know your tools, but
more importantly, know their limitations. In other words, do not depend
on what the instrument says, but consider its potential errors as well.
The oscilloscope method that I described is dependent on the exact
resistance of the dummy load. If you do not have a good 50 ohm
non-reactive dummy load, the result will not be accurate. I have taken
particular pains to have dummy loads that are accurate. Yes, I also
have dummy loads that are based on the Heathkit Cantenna, but I use them
only for dummy loads in the hamshack, those are not used for
measurements at the workbench.
73,
Don W3FPR
On 8/21/2015 11:25 PM, Bill Breeden wrote:
Bob,
I use my Bird 43 terminated with a dummy load when the power level I
want to measure will give a good meter deflection with the slugs I
have available.
Like Don, I find the voltage method handy when working with QRP rigs,
since they barely deflect the Bird with any slug I have. I use an
ME-26D/U Vacuum Tube Voltmeter, the military version of the HP-410B,
to measure the RF voltage across an accurate 50 ohm dummy load and do
the math.
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