K2: Power calibration?
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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??? 73, Randy, KS4L _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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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??? > Elecraft mailing list Post to: [hidden email] 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 |
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Randy,
Don said: > > 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. > Don is very correct in this. I have a WM-2 and found it to be "way off" when adjusted according to the instructions and working into a 50 ohm load. I finally hung a scope on the input to the dummy load and set my K2 to 40m and 5 watts. I adjusted the WM-2 to read the calculated input power at the dummy load based on the pk-pk signal. The K2 and WM-2 were then "close enough" with each other on 40m. Since then, I have relied on the K2 power output. Vy 73 Jay AJ4AY Mobile, AL No virus found in this outgoing message. Checked by AVG - http://www.avg.com Version: 8.0.173 / Virus Database: 270.7.6/1709 - Release Date: 10/5/2008 9:20 AM _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by Don Wilhelm-4
Don,
Thanks for your thorough response! In thinking that through and then going back to read about calibration of the KAT2 I realized what I had done wrong :-( I had not commanded the KAT2 to re-tune into the dummy load! It was still set for whatever it needed to match my antenna system!! After that dawned on me and I re-tuned the KAT2, the WM-1 and the K2 agreed as exactly as I can read the meter at 5 w. For some reason, the WM-2 (even though re-calibrated and double checked) is still off about 20%. But those are close enough readings for my purposes!! Back to setting up the SW-40+! This is my therapy while I anxiously await delivery of my K3, probably near the end of November!! Tnx es 73, Randy, KS4L K2 #337 K3 on order! _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Randy,
If you want to get the WM-2 reading closer than 20%, I suggest the following if you do not have a dummy load that you can trust along with a device to read the RF voltage across that load. 1) Disconnect the KAT2 and drive the best dummy load you can locate with the base K2 - run the power through the WM-2 and adjust the WM-2 to agree with your K2. 2) Put the KAT2 back in your K2 and drive the same setup of WM-2 and dummy load. Adjust the forward power pot on the KAT2 to read the same as the WM-2. You should end up with a WM-2 and a KAT2 that agree within 5% to 10% over the entire scale. And both should be within 5% to 10% of the actual power that would be measured by using better instrumentation. One can use simple tools and still obtain a satisfactory degree of accuracy if you do the steps in an order that provides reliability. It is also good to have an idea how much your instruments vary from absolute accuracy. 73, Don W3FPR Randy Moore wrote: > Don, > > Thanks for your thorough response! In thinking that through and then > going back to read about calibration of the KAT2 I realized what I had > done wrong :-( I had not commanded the KAT2 to re-tune into the dummy > load! It was still set for whatever it needed to match my antenna > system!! After that dawned on me and I re-tuned the KAT2, the WM-1 > and the K2 agreed as exactly as I can read the meter at 5 w. For some > reason, the WM-2 (even though re-calibrated and double checked) is > still off about 20%. But those are close enough readings for my > purposes!! > > Back to setting up the SW-40+! This is my therapy while I anxiously > await delivery of my K3, probably near the end of November!! > > Tnx es 73, > Randy, KS4L > K2 #337 > Elecraft mailing list Post to: [hidden email] 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 |
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In reply to this post by aj4ay
Jay,
Thanks for that comment and your experience. I do not have a WM-2, but I do have a homebrew version of its predecessor, A Simple and Accurate QRP Wattmeter by Roy Lewallen W7EL. It originally appeared in QST Februrary 1990. The calibration method using a DMM to measure the voltages is theoretically correct, but assume that everything in the directional coupler and the diodes is perfect and without variation or loss. I believe it is accurate to within 20% for all cases, but to get closer than that figure, one must include the variations and loss in the coupler in addition. Doing final adjustments by accurately measuring the power output is a good way to include all elements in the calibration. Both Roy's wattmeter and the WM-2 do have a great advantage in that they will be just as accurate at low power levels as it is at higher levels, and that is where many wattmeters fall flat on their face. It is a very good wattmeter for QRP levels. 73, Don W3FPR Jay wrote: > Randy, > > Don said: > >> 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. >> >> > > Don is very correct in this. > > I have a WM-2 and found it to be "way off" when adjusted according to the > instructions and working into a 50 ohm load. I finally hung a scope on the input to > the dummy load and set my K2 to 40m and 5 watts. I adjusted the WM-2 to read the > calculated input power at the dummy load based on the pk-pk signal. The K2 and WM-2 > were then "close enough" with each other on 40m. Since then, I have relied on the K2 > power output. > > Vy 73 > Jay > AJ4AY > Mobile, AL > No virus found in this outgoing message. > Checked by AVG - http://www.avg.com > Version: 8.0.173 / Virus Database: 270.7.6/1709 - Release Date: 10/5/2008 9:20 AM > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > 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 > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - http://www.avg.com > Version: 8.0.173 / Virus Database: 270.7.6/1709 - Release Date: 10/5/2008 9:20 AM > > Elecraft mailing list Post to: [hidden email] 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 |
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