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List:       elecraft
Subject:    RE: [Elecraft] K2 Power Control
From:       <al_lorona () agilent ! com>
Date:       2004-12-30 20:47:40
Message-ID: 03449D1344E4414EB9AAB797FEEADE8102354055 () wcosmb01 ! cos ! agilent ! com
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> Elliott Lawrence wrote:
> 
> > 
> > I have a question regarding the power control. I can adjust 
> the power in 0.1 w increments per the manual.  However for 
> example, if I set the level at 5.0w the resultant output is 
> usually higher. Sometimes by almost a watt. Other times it 
> will be a little lower.  I am using the ATU that is 
> calibrated properly.  From band to band the same power isn't 
> always the same with no adjustmnet to control.  Wonder if RF 
> is getting in the ATU control circuitry or the feedback path 
> from the RF board?
> > 
> 

Hi, Elliott,

The short answer to your questions is... your K2 is just fine. Everything is within \
limits.

The longer answer is... 

1. Measuring power is one of the hardest things to do accurately. The venerable Bird \
wattmeter, which is the de facto amateur radio standard, has an accuracy of +/- 5% of \
full scale, which means that with a 100 W element in a Bird its  reading could be as \
much as 5 W too high or 5 W too low! Therefore, a question that I have is: since your \
wattmeter is unlikely to be any more accurate than this (+/- 5%), both it and the \
"wattmeter" in the K2 seem to be well within limits if they agree to within 1 watt.

2. The next question is... is the K2 or your external wattmeter correct? Without \
doing a lot of reference measurements, or without using a calorimeter as a power \
measurement reference, you don't know! You will just have to either decide to believe \
your wattmeter, especially if it is a Bird-type design, or decide to average the \
measurements to arrive at a figure. Even using an oscilloscope is unreliable because \
you won't know the exact impedance of your line or load, which is necessary to \
calculate power.

3. The next questions is... does the difference you are seeing matter? Well, if you \
set the K2 to 5 W and you actually read 6 W on the external wattmeter, and assuming \
that 6 W is the right answer, your K2 is reading less than 0.8 dB low. As power \
measurements go, this is fantastic accuracy from a simple detector such as is in the \
K2. This instrumentation error is the predominant error at HF frequencies, the next \
highest error coming from mismatches in the cables, connectors, and power detector \
circuitry. Eight-tenths of a dB is a difference far too small to detect on-the-air.

3a. This error varies for different power levels. Thus, you are seeing the K2 \
readings sometimes high and sometimes low. The error will also change at different \
frequencies, especially if there is some mismatch in the system.

4. Now, if you are a true QRPer who enters contests and goes for awards and such at \
the QRP level, then to play fairly you will set the K2 power output so that neither \
the K2 nor the wattmeter reads more than 5 W. This is a direct result of not knowing \
which measurement is correct. And in case you are worried about cheating yourself out \
of every last millliwatt by doing this, see #3 above.

The next time you see a power specification in an advertisement, a product review, or \
a message from one of us on the reflector, remember that it is probably +/- 5% \
accurate. At best.

Sorry for the long message, but this is a subject near and dear to my heart and my \
line of work.

Regards,

Al  W6LX
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