Bird accuracy is specified to be +/- 5% of meter full scale ... a 100W element would be a poor choice to measure a 100W output level. Take a look at any Bird's meter scale. It's not linear and can be read with the most accuracy to the left of center scale. A 250W element is much better for measuring 100W.
Wanna chase your tail? Measure the same power source with identical Bird elements...... Don's method is better, but all methods are obviously subject to the accuracy of the final indicating instrument. FWIW ..... 73l Ken Kopp - K0PP On Aug 21, 2015 8:39 PM, "Bob McGraw - K4TAX" <rmcg...@blomand.net> wrote: > A comment here, has anyone thought to use a Bird 43P with a 100 watt HF > slug? Or maybe a TelePost LP-100a? Both are equally good regarding power > measurements but only when the load is 50 ohms resistive. > > 73 > Bob, K4TAX > K3S s/n 10,163 > > On 8/21/2015 8:15 PM, Don Wilhelm wrote: > >> In a slightly related note - for those who are using an oscilloscope to >> measure the RF voltage at the output, instead of converting everything to >> RMS and doing the power calculation, I can give you a shortcut. The >> formula for power by observing the RF voltage across a 50 ohm dummy load is >> Vp-p squared, and then divided by 400. If the load is not 50 ohms, then it >> is Vp-p squared and divided by 8 times R. >> >> The derivation is left to "the student" - Hint, use SQRT 2 in your >> derivation rather than 1.414 or .707 because the radicals will cancel out - >> the numbers will only cause confusion, but will produce a similar result. >> I use this easy formula at the workbench often when determining power >> output, it is especially useful at power levels of 10 watts and below. >> Yes, I do have an oscilloscope probe permanently connected directly across >> my dummy load just for this purpose as well as for looking at relative RF >> voltages during an alignment. >> >> 73, >> Don W3FPR >> >> On 8/21/2015 3:34 PM, Bill Breeden wrote: >> >>> >>> It's important to remember that RF power expressed in watts is always >>> based on RMS values. If RF watts are calculated by measuring the voltage >>> across a known load, the voltage must be expressed as an RMS value to >>> correctly calculate the power in watts. Watts are watts. There is no such >>> thing as "peak to peak" or "RMS watts" when expressing RF power in watts. >>> >>> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:Elecraft@mailman.qth.net >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> Message delivered to rmcg...@blomand.net >> >> >> >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to kengk...@gmail.com > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
