OT-Wattmeter Errors

[alsopb]

Interesting review of five commercial wattmeters in March QST.
Power levels from 5 to 1000 watts and frequencies from 2 to 50 MHz.

For CW:
Most are within +/-10% at 100 watts.
Many are way off at 5 watts (as much as 70%) and 1000 watts (up to 24%)

Bottom line: Hard to measure watts accurately over a wide
frequency/power range-- even with a pure resistive dummy load attached.

Do you believe yours?

Article notes +/-5% is typical for lab grade wattmeters.

73 de Brian/K3KO
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[Don Wilhelm-4]

Brian,

I believe those numbers are 'typical'.  Wattmeters are difficult to
trust.  Another "gotcha" is that the specifications are often stated as
a percentage of full scale.  What that means is for a 100 watt full
scale meter rated at 5% of FS, the power reading can be off by as much
as 5 watts *anywhere on the scale* - not good if one is trying to
measure a 5 watt power level.
Even the much respected Bird wattmeter is only guaranteed to 5% of full
scale immediately after calibration, and low power slugs are hard to find.

The difficulties encountered with wattmeters has driven me to measuring
the RF voltage across a precision 50 ohm dummy load with a calibrated
'scope and a 10X probe - the power is then calculated, and can trusted
to 2% or better.
The section in Experimental Methods of RF Design has several methods of
maesuring RF power, and all will be more accurate than most wattmeters
if the dummy load is known to be non-reactive at the frequency of use
and the exact resistance is known.  If you are looking for a dummy load
that meets those requirements, check Ridge Equipment (Google for URL),
they have very accurate dummy loads at a value price, and they will even
run a plot over frequency for you if you pay a small extra fee.  I am
only a satisfied customer.

Brian Alsop wrote:

> Interesting review of five commercial wattmeters in March QST.
> Power levels from 5 to 1000 watts and frequencies from 2 to 50 MHz.
>
> For CW:
> Most are within +/-10% at 100 watts.
> Many are way off at 5 watts (as much as 70%) and 1000 watts (up to 24%)
>
> Bottom line: Hard to measure watts accurately over a wide
> frequency/power range-- even with a pure resistive dummy load attached.
>
> Do you believe yours?
>
> Article notes +/-5% is typical for lab grade wattmeters.
>
> 73 de Brian/K3KO
>  
>

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[P.B. Christensen]

> Do you believe yours? Article notes +/-5% is typical for lab grade
> wattmeters.

Even some of the pseudo-lab-grade wattmeters can vary from published
specifications.   One very popular wattmeter with a perfect review rating on
eHam, suffers from very poor indicated VSWR accuracy below 5-watts.  Based
on what I could measure here, it was symptomatic of the low-power accuracy
problems described by Roy Lewallen, W7EL, in the February 1990 issue of QST.
Although the manufacturer claims calibration to NIST traceability, it is
likely calibrated at moderate power levels.  This wattmeter is frequency
compensated -- but not temperature compensated.

OTOH, I own two competing wattmeters that are truly temperature compensated
with NIST-traceable calibration, and VSWR agree with each other down to 100
mW.

Paul, W9AC







 

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[AC7AC]

CONTENTS DELETED

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[Alan Bloom]

On Fri, 2009-02-13 at 08:00, Don Wilhelm wrote:
...
> The section in Experimental Methods of RF Design has several methods of
> maesuring RF power, and all will be more accurate than most wattmeters
> if the dummy load is known to be non-reactive at the frequency of use
> and the exact resistance is known.
...

In many inexpensive dummy loads the resistance changes significantly as
it heats up, a significant source of error.

Al N1AL


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