[Elecraft] KPA-500 watt meter vs. Palstar

[Don Wilhelm]

Jim,

Being one not to trust any instrument until it has proven its worth (and 
calibration), I would not trust either until calibrated.

OK, that brings up the question of how to do a meaningful calibration.
I connect to a known good (precision) 50 ohm dummy load, and read the RF 
voltage at the dummy load with my oscilloscope equipped with an RF 
Probe.  A coax TEE adapter placed at the dummy load will give you access 
to the center conductor.  Make certain the 'scope and probe are rated 
for greater than 5 times the frequency at which you are doing the 
measurement.

A quick and easy formula is to measure the peak to peak RF voltage, 
square it and divide by 400.
Quicker and more accurate than doing conversion to RMS and then 
calculating - a lot of terms cancel out if you want to attempt to derive 
my formula yourself.

Note that many analog wattmeters are only spec'ed for 20% of the full 
scale reading.  On a 200 watt scale, that means it can be off by 40 
watts at any point on the dial.  Even the much revered Bird wattmeter is 
only to be trusted to 5% of the full scale reading, and that is only 
after lab calibration - so with a 100 watt slug, even the Bird can be in 
error by as much as 5 watts at any point on the scale.  The precision 
dummy load and the 'scope can do 5% or better at the level being 
measured - i.e. at 5 watts, you may have a 0.25 watt error, and at 50 
watts you may have a 2.5 watt error, but that is much better than a 
typical analog wattmeter.

73,
Don W3FPR

On 9/18/2011 8:58 AM, k2zf50 at aol.com wrote:
> Hi,
>
>
>
> My KPA-500 watt meter and my Palstar watt meter read about the same at the higher power readings. But at the 100 watt and lower ranges there is a significant difference. My question is which is the more correct reading at the lower ranges, the Elecraft or Palstar. If I want to calibrate my K3 which one would be more accurate? Being that the Elecraft is a digital display would that one be more accurate?
>
>
> Thanks,
> Jim Douglas  K2ZF
>
>
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