Tony, All you say is true. and should be heeded by all hams.
I have dummy loads that are known to be flat (within 5%) to 1 GHz, and I have several more that are flat to within 500 MHz. Others will display the same characteristics to 60 MHz. I know which ones are which, and I rely on them to give me precision power measurement capability. I do not depend on anything other than the RF voltage measured across that known good dummy load for true calculation of RF power (although I do have one wattmeter calibrated to NIST standards that I use as a secondary standard to calibrate my other meters). In other words, I have become more and more distrustful of in-line wattmeters which may be in error by a substantial amount (20% of the full scale reading) - just try to measure 5 watts with a wattmeter having a 200 watt scale - the error may be as much as 40 watts anywhere on the scale - so a reading of 5 watts might be anywhere between zero and 45 watts actual power - does that seem ridiculous - certainly, but it is true - some wattmeters can be in error by that much when used at levels considerably less than their full scale reading. OK, this is my wattmeter inaccuracy rant for Feb 2012 - thanks for listening. 73, Don W3FPR On 2/7/2012 10:09 PM, Tony Estep wrote: >> On Tue, Feb 7, 2012 at 3:23 PM,<n...@n5ge.com> wrote: >> >>> Is your dummy load absolutely a flat 1.0:1 from 1.0MHz to 30MHz? > ================ > Suppose that the SWR is 1.2. This means that 0.8% of the power is > reflected (80 milliwatts reflected for 10 forward). If the SWR is 1.1, > the reflected power is 0.2%. A simple coax jumper can sometimes cause > an impedance bump large enough to reflect more power than that. A few > minutes spent on a test bench with even the fanciest precision > equipment will quickly disabuse one of the notion that a dummy load > can be trusted to be "absolutely flat 1.0:1 from 1.0MHz to 30MHz." > > Tony KT0NY > ______________________________________________________________ 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
