Dr. David,
You let the cat out of the bag, so there may be a flurry of arguments
about Bird 43s. We've all been there before. Here is my 2 cents
though: I have stacked several of those wattmeters in series in a
transmission line, and gotten +/-5% (of FS) between them, but not
much better. At work we cannot use them for high power as the
inaccuracy would cause major problems with power balance. I typically
use several parallel methods, calorimetric using water flow, and Tin,
Tout, as well as Plate voltage and plate current (plus drive power
for cathode driven), and directional couplers that are calibrated
with network analyzer, along with a calibrated Hp or Agilent peak or
average power meter. Bird wattmeters are usually not close enough, so
I mainly use them to give rough indications of something wrong (like
half scale on reflected means a problem). I use the Bird Watcher VSWR
alarm products also for interlocks on some equipment. When I am
missing a few hundred kilowatts in calculating efficiency of an
amplifier, there is something to worry about.
73
John
K5PRO
>
>Don't worship the Bird.
>
>The Bird 43's I was testing twenty years ago were rarely within 5% of
>FSD. Most were within 10% of FSD, but still a significant fraction were
>outside of that. (These measurements were made at a government standards
>laboratory.)
>
>Bird 43's are fine for determining if a PA is working, but they are not
>capable of accurate power measurement.
>
>I had a Bird 43 myself and bought a new element that covered 432 MHz. It
>was out of spec the day I got it. I returned it to the UK dealer (Aspen
>Electronics), who adjusted it while I was there to be right at 432 MHz.
>I then checked it again, and it was indeed then within spec at that one
>frequency.
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