[Elecraft] K3 Watt-meter question!

[Ron D'Eau Claire]

Jim, you said "supposedly accurate". A Bird wattmeter is accurate only if
it's been properly calibrated and handled carefully - especially the
"slugs". Bumps and bangs can easily ruin the calibration (That's why all
Birds used in critical applications carry calibration stickers showing when
it was last checked - normally no less often than annually - and is
carefully handled and stored in approved containers). 

Let's assume the Bird is recently calibrated and treated kindly: 

1) Its calibration is then dependent upon using the right slug. You probably
know better, but a surprising number of Bird owners think the frequency
range on the slug isn't very important. I believe the proper Bird 43 element
for 6 meters is the type A slug spec'd for 25-60 MHz.

2) Are you allowing for the normal error in even a perfectly-calibrated
Bird? That's +/- 5% of *full scale*. IIRC the Bird 6 meter slugs have full
scale ranges of 25 or 250 watts. If you're measuring 100 watts with the 250
watt slug, the possible error in a properly-calibrated Bird is +/- 5% of 250
watts *anywhere* in the scale. So at 100 watts indicated, the actual power
may be anywhere between 87 and 112 watts.  

That's plenty good enough accuracy for nearly all field use, including for
showing that transmitter power levels comply with FCC regs (although when I
demonstrate a marine transmitter to FCC engineers, it's *their* Bird that is
the final word ;-)

Now, the K3 has simply another wattmeter built in, so it's normal for the
two wattmeters to disagree by the sum of their possible errors. 

You asked, "Which one is more accurate?" One of the Elecraft engineers would
have to answer that, but let's assume the K3's built in wattmeter is as good
as the Bird. In that case a perfectly calibrated Bird on a perfectly
calibrated K3 may disagree by as much as 15 watts at 100 watts giving you an
indication of anywhere between 85 and 115 watts on the Bird when the K3
meter says 100 watts.

And don't forget that assumes everything is in perfect calibration. In the
real world, it's not unusual to see a greater error.  

That's why it's not normally considered important for owners to worry about
the K3's power calibration. Unless you have some laboratory standards to
compare to each other and to the K3, it's almost impossible to improve on
the default factory settings. 

Ron AC7AC



-----Original Message-----

Gentlemen,

How accurate is the built-in watt meter indicator in the K3???

I'm asking because when I've used a supposed;y accurate "Bird 43" while
measuring the K3's various
outputs on the various HF bands there seems to be a slight discrepitancy
between what the K3 is 
showing and what's actually there according to the "BIRD!!!"  I'm
particularly interested in the
output readings as concerns "6 metres!" Which is MORE ACCURATE???

Regards,

Jim/nn6ee
S/N 2406