Don: I would add a small caution. The maximum safe voltage rating of a 10x probe is a function of frequency.
For example, Tektronix's model P6106A, a 250 MHz 10x probe, is rated at 450V below 1 MHz, but only 60V at 10 MHz and perhaps 50V at 30 MHz (hard to read the graph accurately). All are RMS values. 100 watts is 70.7 V RMS, so the 10 MHz - 30 MHz voltage limits will be exceeded at that power level. (100V RMS probe limit looks to be around 6.5 MHz.) Of course, I'm sure there's a safety factor in the probe specs, but before I measured 100 watts output with a 10X probe I would verify the safe voltage rating in the probe manual. Jack K8ZOA On 12/8/2011 6:09 PM, Don Wilhelm wrote: > Tom, > > OK, it seems now is a good time for me to insert my periodic "wattmeter > accuracy" rant! Sub-titled -- "Technician, know your tools but know > even better their limitations" OR - "No instrument can be relied on > until it has proven it is capable of telling the truth under the > conditions of use". > > If you want quite good accuracy at the 5 watt level, I suggest you use > a 'scope with a 10X probe connected across the dummy load. The peak to > peak RF voltage can be easily measured by the vertical deflection of the > 'scope. To convert that peak to peak voltage reading to power (when the > dummy load is 50 ohms), use your calculator - enter the peak to peak > voltage, square it, and divide by 400. > > If you do not have an open dummy load that makes clipping the 'scope > probe across it easily, use a Tee adapter at the dummy load end so you > can gain access to the center conductor. > > 40 volts peak to peak is 4 watts - you might want to use that instead of > 5 watts since it is even and easy to see on the 'scope face. If you > really need 5 watts, adjust for 44.72 volts peak to peak. > > In fact, you can use the 'scope probe across the dummy load up to any > level permissible by your 'scope and probe. My probes will handle 100 > volts peak easily, and that is the 100 watt level. The accuracy is only > limited by the accuracy of your dummy load (mine is within 1%) and your > ability to interpolate the voltage deflection on the 'scope. I might > suggest that you can come within 5% of the actual power level, which is > much better than analog power meters whose accuracy is expressed as a > percentage of full scale. So a freshly calibrated Bird wattmeter with a > 100 watt slug is spec'ed for 5% of full scale - OK, that is a 5 watt > potential error - and that applies anywhere on the scale - so trying to > read a 5 watt power level with a Bird and a 100 watt slug is an exercise > in futility - use the 'scope, it is a LOT more accurate. > OTOH, many recent digital wattmeters will be spec'ed for accuracy > anywhere within its range rather that as a percentage of full scale used > with the analog wattmeters. > > The (Vp-p)exp2/400 formula (actually [Vp-p]exp2/8R) does not seem to be > well know. I leave it "to the student" to derive it from standard > formulas for power, voltage and resistance. Hint - use SQRT2 rather > than the 1.414 or 0.707 approximations when converting peak to peak > voltage to RMS - it is an exact formula, the squares and square roots > cancel. > > 73, > Don W3FPR > > On 12/8/2011 5:12 PM, TJ Campie wrote: >> I finished building a K3 for the radio club at work and I thought I'd take >> the bull by the horns and attempt to improve the factory setting for the 5w >> calibration on the 100w Bird watt meter I had. I couldn't get it to go up >> the the point that looked like 5w on the meter regardless of how much I >> turned the knob on the K3. The setting ended up at 10 (the minimum) but I >> think it must have been more correct from the factory. I *think* it was up >> at about 37 but I don't remember - is there a way to reset it to the >> factory setting? I don't have access to a 5w accuracy meter. >> >> Otherwise, the build went great and was a blast :) >> >> TJ W0EA >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
