Russ Hines wrote:
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.
Duplexer tuning is not the only thing you'd need to worry about with
temperature extremes you reference. The radio set might have a hard
time with +130 shelter temp. That could easily relate to radio
temperatures exceeding the ability of some equipment manufacturers to
remain stable.
Filters will change with temperature - it's a fact. The amount they
change is, I suppose, what's under question. Most commercially made
units have a temperature rating of -30 to +60 degrees C. This means the
duplexer will remain within its ratings between those extremes. Most of
us engineer our repeater systems to have some amount of isolation
headroom. Headroom is necessary for several reasons, temperature
variations are likely the biggest reason, along with icing of the antenna.
I have a site using a Wacom WP-641 and 250 watt transmitter. This site
sees -25 degree F temperatures in the winter and +85 degree F
temperatures in the summer - outside shelter. The shelter is not
climate controlled. The duplexer loss is 1.5 dB or 29%. This relates
to 72.5 watts going up as heat. If the repeater is in transmit for a
long time, the temperature in the building can approach +100 degrees
ambient. Now, add the heat generated by the loss of the duplexer, and
the duplexer becomes very hot to the touch. Even at these temperature
extremes, the repeater is completely happy with the isolation provided
by the duplexer.
You state that over time the tuning might walk off the reservation.
This seems to allude to the duplexer changing tuning and not coming back
to its settings after it has returned to the temperature it was tuned
at. If this is the case, the duplexer design is faulty - period.
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.
If you were able to move the meter and have differing determined power
readings, something IS/WAS very wrong. But, maybe you aren't using the
instrument correctly? To paraphrase the manual, the reflected power
must be subtracted from the forward power to determine the actual power
delivered to the load. If you move the meter about the line, it is
possible that the power shown on the meter will change, but, if you read
the reflected at the same spot, and determine the power - it should
always subtract to the same determined power reading.
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.
While I'll agree that the Bird doesn't measure power directly, it still
measures power, quite accurately, no matter the impedance of the
connected line. Bird claims the meter to be accurate within 5% of full
scale reading. While it might not be as accurate as a VNA, it doesn't
cost as much either.
Let's revisit what you originally wrote:
/Our in-line power meters, like our trusted Bird 43, do not directly
measure power./
No argument here...
/ They're really voltage meters calibrated in watts at a specific
impedance.
/
When you were reading the manual, you missed something - read on...
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?
Talked yourself right around the circle there. ;-) You didn't say
anything about CURRENT in your first post.
While it reads voltage, it also reads current. These two are combined
to read power.
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.
No - they measure current and voltage and display power in watts, no
matter what the impedance is.
/ 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./
Sorry - I don't agree.
Kevin Custer
