At 09:00 AM 6/15/00 +0100, Peter Chadwick wrote:
>If you are an approved type testing laboratory, your typical conducted power
>measurement to a 95% confidence level is +/-0.75dB. That's 1262 to 1782 watts.
>Larry, you got any buddies at NIST who can calibrate a power meter at the 1500
>watt level to reduce errors in couplers etc?
I don't know anybody at NIST. Not sure I want to. (or need to)
This has been a headache for me for years at my day job. Don't want to get
carried away with it at home. Or, at least didn't want to. Not having a
government inspector peering over my shoulder at home, I would be happy if
I had two pieces of equipment give the same answer and to hell with NIST.
The thing that really bothers me is that three of the instruments are spot
on at 500 watts on 14.1 MHz. One of them is a Drake W-4 purchased new in
1970 and never re-calibrated. One is a Bird 43with a 2500H slug (new) and
the other is a RF Apps VFD. As you crank up to 1500, they start to diverge
every which way. That's why I tried the power meter and directional coupler
approach only to find out my homebrew coupler directivity was too poor to
allow reasonable calibration. I might get acceptable (to me) accuracy if I
had a better coupler.
>You might get better accuracy using a liquid cooled load, measuring inlet and
>outlet temperatures with a constant head apparatus to maintain constant
>coolant
>flow, and then running a substitution with DC. If you keep the ambient
>temperature constant (I guess that's the amp in a different room to the air
>conditioned room in which the load is) I guess you may be able to get to about
>+/-0.4dB - maybe a bit better. Provided the amp is rated for key down
>operation
>for the length of time taken to achieve thermal equilibrium. That'll be +/-
>about 140 watts.
Peter, I believe my ability to accurately measure temperature at home is
even less than my ability to measure RF. I considered this approach and
discarded it because of the work involved. Just too lazy I guess.
>Worrying, this measurement accuracy business, isn't it?
Fortunately, in this instance, it's just a hobby and doesn't matter a bit!
Still, it is annoying.
73,
Larry - W7IUV
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