Hi again Russ,
_____ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 4:54 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Coax length, etc. I see some folks are heading for the Advil. My apologies. Thanks, Gary, for admitting the 43 doesn't measure power directly. One myth down. Of course, it is a directional coupler, no argument. That makes it a reflectometer, it enables the instrument to isolate forward/reflected samples to some degree of reliability. What's the rest of the circuit? ;-) IMHO, what makes the 43 better than most (if not all) meters of its type, is the directional coupler is a true transmission line coupler, not a ferrite transformer, directly connected capacitor, etc. But it works the same way. As far as rereading the manual, I have been. Bird's explanation requires the reader to suspend a "standing wave" viewpoint of transmission line theory, and buy into their "traveling wave" viewpoint. Uh, okay. But that kind of thing sends up red flags for me. I shouldn't have to suspend accepted transmission line theory to understand how their meter works. There are no standing waves that you can measure directly with the Bird meter. In order to truly measure standing waves you need to have a line length greater than a half wave length and measure where the nulls are along the line. Swr is calculated from forward and reflected power at one point on the line with a Bird type of meter. As it turns out, I don't. When line impedances get away from 50 ohms, accuracy falls and the meter behaves like you'd expect. It tracks whatever current is on the line at that (the meter's) point in the line without regard for impedance. Since it's just not calibrated for whatever that impedance might be, how can it be accurate? The Bird is set up so that the ratio of voltage and current that are detected work out to the power at 50 ohms. When the line is not 50 ohms that voltage/current ratio change that the meter detects. So you can no longer simply look at the scale on the meter and directly read power. For ANY reflected power reading you must subtract the reflected power shown from the forward power shown to find the true power delivered to the load. This holds true no matter what the impedance of the line is. If the meter did as you suggest, then it would show what the voltage and current are at any point in the line, and therefore be able to tell you what the impedance is at that point, all with some level of accuracy. It simply can't do all that. With the Bird meter you don't care what the impedance is because it measures voltage (by way of capacitive coupling) and current (by way of inductive coupling). Both create voltages that add together in the proper ratio to give the meter reading that represents power level for that combination of voltage and current. Yes, Bird describes what happens when using 70 ohm lines with the meter under less-than-perfect conditions. IMHO, it's really messy. It can't tell the difference between a 1:1 VSWR and a 2:1 VSWR (both will calculate out to 1.4:1) on a 70 ohm line. That's not accuracy, that's nearly useless. Yes it gets a little tricky to find VSWR with a non 50 ohm line. But most of the time we really don't care what it is. I say we don't care because it is rare that the 50 ohm Bird meter gets used in a non 50 ohm transmission line. With a 50 ohm line things work out nicely to find power and VSWR no matter what kind of reflection the load presents. BTW, my POS Daiwa can show me a 100% reflected condition, just like the Bird. And just like the Bird, it doesn't indicate if that's an open or a short. So what? If you need to know that then you are using the wrong instrument. I believe Bird wants us to believe that their meter is faster and more convenient (it is) yet as accurate as a slotted line and calorimeter (sorry, nope). It's a calibrated voltmeter, not a network analyzer. Try doing the same thing with a voltmeter. :>) No one claims it to be anything other than a simple wattmeter. It is not a super accurate at measuring power either. It is claimed to be 5% of full scale reading. That means with a 100 watt slug the best accuracy that you can depend on is +- 5 watts anywhere on the scale. So at 25 watts on the meter scale it could be as low as 20 watts or as much as 30 watts. But for what it is it works very well. For most everyday, mundane RF chores, it's just dandy as we don't really need high accuracy. And as long as line impedances stay reasonably close to 50 ohms, it turns out accuracy is pretty good, too. Again, line impedance doesn't matter for power measurement. 73 Gary K4FMX Certainly not bad for a portable instrument, and that's the point. If we remember what its limitations are, we should be good to go. That's why I own one and want more. Okay, I'm done picking nits. It's the next yahoo's turn. ;-) 73, Russ WB8ZCC On 8/15/2010 2:08 PM, Gary Schafer wrote: 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
