Randy,
If you are operating the K2 without either the KAT2, KAT100 or KPA100,
then you can trust the K2 indication when operating into a 50 ohm
non-reactive dummy load. If you have any doubts about your dummy load,
check it with an antenna analyzer. Anything other than 50 ohms for the
R component and other than zero for the X component means that your
results may be skewed. But if you do have that 50+j0 load at the
frequency of interest, then the basic K2 reading will be quite accurate.
The WM2 must be calibrated and verified before it can be trusted, and
similarly the wattmeter section of the KAT2, KAT100 and KPA100 must be
calibrated before they can be trusted. After any of these instruments
have been calibrated, the dependency on the precise 50 ohm dummy load
goes away. but some instruments will indicate forward power and reverse
power, and the two must be subtracted for the net power, while others
will do that calculation automatically. As a user, you must know the
characteristics of your measuring instrument before you can correctly
interpret its readings.
My favorite method of calibrating wattmeters is to use a precision 50
ohm dummy load and read the RF voltage produced across that load with
either an RF Probe or an oscilloscope and 10X probe having a frequency
rating at least 3 times (preferable 5 times or more) than the frequency
being measured. The RF Probe is usable at powers up to 8 or 10 watts,
but most 'scopes and probes can be used up to 100 watts or more (the
peak RF voltage must be considered or damage to the probe may occur).
If you can find a LP100 that has been properly calibrated, it is quite
an accurate instrument that you can use for power determination.
Trust only those readings that have earned their accuracy rating, and
re-check the calibration periodically. If you do not have a good 50 ohm
dummy load, you may want to check out some of the offerings at Ridge
Equipment (Google for the URL), or one of the 50 ohm thick film power
resistors mounted on a heat sink will assure you of a dummy load that
can be relied on for measurement and calibration purposes.
If you are not familiar with power measurement techniques, I suggest you
buy, beg, borrow, or otherwise procure a copy of Experimental Methods in
RF Design (an ARRL publication) and read the section on RF Power
measurement, it details several good methods. EMRFD is a very good
addition to any ham's library.
73,
Don W3FPR
Randy Moore wrote:
Today I was trying to check power output on a newly constructed SW-40+
(my second one) and needed to recalibrate my WM-2 wattmeter. After
doing that, I decided to check it against my K2 (#337). Not even
close! The WM-2 shows 2.7 watts when the K2 shows 5. It shows about
7 w. when the K2 is set for 10 w. I double checked the calibration on
the WM-2 and put in a new 9v. battery and got the same results. The
WM-2 is supposed to read within 5% according to OHR.
I seem to remember lots of discussion about power calibration on the
K2 long ago and that there's a 1% resistor in there somewhere that's
involved. I wouldn't worry about it if I was within 10% or so, but
I'm not even close. I don't have another independent way of checking
this. Should I believe the WM-2 or the K2 or neither???
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