Russ,
Of course the Bird 43 does not measure power directly. But it does sample voltage AND current on the line in amounts that are combined to indicate power. It is a directional coupler. The only time you will have a problem with it deviating from its accuracy is when the directivity becomes too low such as when the line impedance is way off from its design 50 ohms. As I said before it will read power accurately even if the transmission line is no a 50 ohm line. The manual even tells you that you can use it to measure line loss with an open at the far end of the line. Please read chapter 2 "theory of operation" of the Bird manual that you show the reference to. Then read it again! 73 Gary K4FMX Another chance? Which part, erroneous readings, don't directly measure power, or the voltmeter part? Sure, what the heck. ;-) I've had Bird 43's, and calibrated line sections with matched elements for that matter, give erroneous reflected power readings depending upon what was going on with the transmission line. By erroneous, I mean it was usually a reading that was, for example, excessively high versus what we knew was going on, such as a straight piece of rigid line or coax terminated into a known good load. On rare occasion, we found we slipped a bullet or had a bad connector. More often, relocating the instrument somewhere else along the line resolved those "bad" readings. RF calorimeters can measure power directly. But unless they've one hidden in them somewhere, "ThruLine" meters can not. Just because the Commission might accept wattmeter readings, or Bird says so, doesn't make it so. As for the voltmeter part, check out page 6 of the Bird 43 manual (page 18 of the PDF), a copy of which you'll recall is here: http://www.repeater-builder.com/bird/pdf/bird-43-wattmeter-2004.pdf I respectfully submit what is shown is a schematic/diagram of a directional coupler attached to a voltmeter as an indicator. An induced RF voltage sample is rectified, filtered and applied through a dropping resistor to a shunt-connected ammeter. By definition, a voltmeter is the shunt-connected ammeter with series resistor part. But don't take my word for it. Take a peek at Chapter 25 in any recent ARRL Handbook (this works for my 2007 copy anyway). Is it less a voltmeter because the induced voltage tracks current on the line? Want to call it an ammeter or current meter then, after all that's what the actual meter movement is? I submit this particular voltmeter happens to be calibrated to read average power at 50 ohms impedance, and it does this quite well within its limitations. I now await your thrashing. Please be gentle. ;-) Like the manual says, the Bird 43 is "fast, convenient and accurate." I agree it's fast and convenient. I'll agree it's accurate with the caveats expressed. It beats lugging a slotted line around, and it beats every other meter like it, IMHO, including my old Daiwa dual-metered POS wattmeter. ;-) Oh, BTW, the emperor has no clothes either. :-P 73, Russ WB8ZCC
