Hi Kevin:
Regarding temperature, our club has a site, no A/C or heat, where
temperatures inside the shelter can get below +20 deg F in winter, and
well over 130 deg F in the summer heat. I can't imagine filter tuning
not changing under such conditions, Invar or not. I can see over time
where tuning might "walk off the reservation." But I bow to your
greater experience with cavity duplexers.
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
On 8/14/2010 10:11 PM, Kevin Custer wrote:
Russ Hines wrote:
Some related comments, if you don't mind.
Temperature changes seem to be the biggest "detuner" of largely
mechanical devices like cavity duplexers. We often send our
repeaters off to live in less-than-ideal environments, then expect
cavity input/output impedances to remain as we measured them in the
shop? Don't think so.
I largely disagree. Most modern duplexer designs (within the last 25
years or so) use compensating elements to make the duplexer or cavity
temperature stable. Invar is a nickel-steel alloy that exhibits about
1/10 the thermal expansion as a common carbon steel counterpart.
Invar is used to make the tuning rod - many times it's threaded. The
rest of the duplexer or cavity is usually made of similar metals and
generally thermal expansion occurs across these components equally,
resulting in extremely low frequency drift over its rated operating
temperature.
Our in-line power meters, like our trusted Bird 43, do not directly
measure power. They're really voltage meters calibrated in watts at
a specific impedance. That's why they can be fooled into displaying
an erroneous reflected power reading, perhaps lulling us into a sense
of security that the VSWR on the line is acceptable when it may not be.
What? Maybe you would like to have another chance at that one....
Kevin Custer
