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.
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.
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?
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.
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.
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.
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.
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.
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
