[Elecraft] OT-Wattmeter Errors

[Don Wilhelm]

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
>   
>