Has anyone thought of calling Bird corp and asking this question? If I were to
measure power out with a bird meter, I would accept it as "that's that." I
used to care for my watt meter and the elements with the same care as I would
give a piece of fine crystal. Imo, there was something special in there and I
didn't want it to break. And if an element looked damaged, off it went to
Solon, OH for a checkup. And for all of the elements the were returned from the
checkup, none were found to be "out of spec."
The readings of power out on bird wattmeter have been as accurate as can be
obtained. And that has been over the entire long haul. At the risk of
sounding simple, volts x current draw = watts, power in. Look at the bird,
that will give watts going out to the world along with the efficiency
(inefficiency?) of the transmitter.
I'll wager a buck or two, if one lined up a random number of bird elements
and took the reading a random moment in time, the difference from all the
readings obtained from all of the elements wouldn't make a tinker's dam.
Bird is generally accepted as "the standard for power output measurement."
That's good enough for me. Ah...it's plain to see I'm not much of a
philosopher.
My .02 on this
73 to all,
Gary... wa6fgi
----- Original Message -----
From: G3rzp@aol.com
To: david.kirkby@onetel.net
Cc: amps@contesting.com
Sent: Monday, March 21, 2005 1:03 AM
Subject: Re: [Amps] Bird Element Calibration?
For real fun, how do you measure the power (with what accuracy?) into a load
such as marine antenna at 2MHz? The dummy load is typically 10 ohms in
series with 250pF: a DC calorimetric method might do to calibrate the
resistive
part, but the RF and DC resistances are probably slightly different. How much
loss is there in the capacitor? If you put it on a network analyser, how good
is the answer at this return loss? How accurate is a thermocouple meter? The
books say that you can calibrate a thermocouple with DC, but that ignores
skin
effects, which tend to make it read slightly high at RF. I believe
manufacturers actually compensated for this by putting a slightly higher
figure on the
scale for applications where it mattered - which most of the time, it
doesn't.
I have a very hard time with people who insist on quoting numbers to
emaningless digits - like 2 decimal places of dBs, when most of the time,
they'll be
lucky to have measurements to better than 1 dB anyway. But they wil do it,
because the digital readout gives them great resolution!
73
Peter G3RZP
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