Steve Katz wrote:
> Yep, there is. An accurate power measurement with the model 43 is P(actual)
> = Pf (indicated) - Pr (indicated).
There is no such thing as an accurate power measurement with a Bird 43.
As an 'industrial placement' during an electrical and electronic engineering
degree, I worked in a National Standards Laboratory. I worked in a department
which calibrated tons of Bird 43's. So many failed the +/- 5% of FSD that some
big users would send them in marked saying +/- 10% of FSD was considered
acceptable. Most, but certainly not all the Bird 43's, passed the +/- 10% of
FSD
test.
I had my own 43, bought a brand new slug for it, took it into the lab and found
it was out of spec. Aspen Electronics, who were the distributors in the UK,
adjusted it for me (took them about 5 minutes) so it was accurate at 432 MHz -
the only frequency I cared about. I checked it again, and it was within 5% of
FSD. (I forget how accurate it was at that point).
43's are fine to give you a rough idea of what the power output is, but into a
50 Ohm resistive load, I would not consider them better than +/- 10 or 15% of
FSD.
To my knowledge, which might be outdated now as this was 20 years ago, the most
accurate (but least practical) method of measuring RF power is the water
calorimeter. That was (probably still is) a primary standard.
More practical, and within the realms of amateur budgets, would be calibrated
directional couplers, attenuators and a lab-grade meter with sensor. Mount the
attenuators on the coupler, get it tested as a complete set, then never remove
them.
Thinking about it, if one wanted to, building a water calorimeter using tap
water is probably not that hard for a ham. The only thing is, the measurement
takes ages to stabilise, so unless you have a system to keep the RF power input
constant, it would be a waste of time. I doubt the inpurities in tap water
would
change the specific heat capacity of the water much, but I'm sure information
about that sort of thing can be found on the web.
Another semi-practical method of measuring RF output power of an amp would be
to
measurer the temperature rise of the air exiting the tubes. First run the tubes
with no RF input, so you can know the temperature rise with a specific known
dissipation (say 500 W), then run the amp, and see the temperature rise. If the
DC input power is measured too, it would be possible to approximately calculate
the RF output power. I doubt it would be very accurate, as there are numerous
sources of error, but it should be a lot better than a Bird 43.
I can't really think why a ham needs to know RF power very accurately. In which
case, Bird 43's are fine.
Dave
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