> 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 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
