[Elecraft] Power output measurement

[Don Wilhelm]

Ken,

Of those instruments, the WM2 is your best choice for general use in 
calibrating your KAT2 wattmeter - since the ranges are set with 1% 
resistors, once calibrated it should track correctly from one range to 
another.  While it is properly calibrated with a DMM, it would be wise 
to verify the calibration using another technique (if only as a 
validation step).

You can check the calibration of the WM2 with your RF Probe and dummy 
load.  Assuming the RF probe is like the one shown in the Elecraft K1 
and K2 manuals with the diode from the capacitor to ground, it will read 
the RMS value of the RF voltage (if the diode is in series with the DMM 
lead like in the DL1, it is a peak reading type).

Use a power level of 4 watts as indicated on your WM2 and measure the RF 
voltage across the dummy load.  If you find 14.142 volts of RF there, 
that is 4 watts and the WM2 is reading correctly.  You can use other 
power levels if you want, th formula is P = V^2/R

Once you have validated the WM2, you can then use it to calibrate the 
wattmeter on your KAT2 - balance the bridge first, then set the forward 
power to agree with the WM2.

Folks having the KPA100 with the higher power wattmeter can use the same 
technique to calibrate the WM2 at 4 watts, and then add an Elecraft 20 
dB coupler (CP1) to reduce the power fed to the WM2 by 20 dB (100 watts 
through the coupler equals 1 watt to the WM2).  Connect the WM2 to the 
forward port on the CP1, terminate the reflected port, a 100 watt dummy 
load to the output port and the KPA100 to the input port, then set the 
KPA100 wattmeter to equal the WM2 reading times 100.

73,
Don W3FPR

Ken Kopp wrote:
> I'd welcome your views on how to measure output power
> of my newly-bilt K2 with resonable accuracy.  The operative word here 
> may be "reasonable". (:-))
> 
> I have a quality 50 ohm dummy load, a Bird with a 50W HF element, the 
> built-in wattmeter in the K2/KAT2, and an OHR
> WM-2.  As one would expect, they all read differently ... too
> differently.
> 
> At first thought one would might think the Bird should be my reference, 
> but it's accuracy is +/- 5% of full scale ... 2.5 watts in this case.  
> I've calibrated the WM-2 with a Fluke DVM,
> but the accuracy of the Fluke can only be assumed.  I don't
> know enough about the sensing and measurement portion of the K2/KAT2 to 
> have a "feel" for it's accuracy.  I also have
> the Elecraft RF probe to use with the DVM, but no reference
> for calibration.  Unfortunately I don't have access to a quality
> 'scope to use as a reference.
> 
> I tend to "trust" the WM-2 as it's accuracy is tied to the DVM
> used to calibrate it.  The DVM is most likely my most accurate 
> measurement device that I have available ... for now, at least.
> 
> 73! Ken Kopp - K0PP
> k0pp at arrl.net