I don't know if you really don't get it or you are just trying to be controversial. I tend to think a little of both.
Either way, I give up. 73 Gary K4FMX _____ From: [email protected] [mailto:[email protected]] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 7:37 PM To: [email protected] Subject: Re: [Repeater-Builder] Coax length, etc. Last round. Hi again, Gary. ;-) On 8/15/2010 7:09 PM, Gary Schafer wrote: Hi again Russ, _____ From: <mailto:[email protected]> [email protected] [mailto:[email protected]] On Behalf Of Russ Hines Sent: Sunday, August 15, 2010 4:54 PM To: [email protected] 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. Yeah, and? The Bird does it better. 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. That's correct. As I said, the 43 isn't a slotted line. Regarding VSWR, all in-line meters make an attempt at this, some have fancy cross-needle indicators where VSWR is represented at the intersection of the needles. How else would you do determine VSWR with such a device? That was a rhetorical question. ;-) 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. Thanks, Gary, that's right. The meter is calibrated at 50 ohms impedance. When the line impedance isn't 50 ohms, you can't just look at the meter, the meter scale is no longer accurate, is it? Subtracting reflected from forward is a given, and never at issue here. Well, impedance does matter. At the characteristic impedance of the meter, line, load, etc., seems a waste of time to subtract nothing, you'll see right away there's no reflected power. ;-) 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. Gary, you seem to be contradicting yourself. A paragraph ago you said "the ratio of voltage and current work out to the power at 50 ohms." Now we don't care what the impedance is? We either do or don't. As for me, I choose to care 'cuz that's the kind of guy I am. ;-) I understand the coupling, both are present, agreed. But if impedance didn't matter, then the meter would indicate power accurately regardless of line impedance. That's simply not so. The Bird manual even says it's not so. It's limited by its own line section. 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. Well, that's convenient, now we really don't care what the VSWR is. Gary, buddy, come on, dude. This is yet another example of why does Bird offer a case of the meter attempting to work with something other than 50 ohm lines, if it doesn't work well with non-50 ohms lines or we don't care? That's nonsensical. That's like, "you could use a hammer as a baseball bat, but it'd be much harder to hit the ball." No kidding? 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. My point exactly. Once again, why does Bird offer this as something you can do with the meter? 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. Anyone can, with a diode and a voltmeter. It's not all black magic. ;-) Bird makes the claims, not I. I've been quoting nearly verbatim from their manual. I think I understand the limitations regarding accuracy. That's what this whole discussion is about (or so I thought). My point is, the accuracy limitations run beyond just a 5% full-scale error. Oh, so much beyond. =-O 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 Aww, you cared earlier, then you didn't, and now I guess you still don't care. Okay, Gary, whatever, man. Thanks for the discussion. 73, Russ WB8ZCC
