Guys:
+/- 10% equals 0.4dB error. I don't know of many general purpose instruments
that are better than +/- 1dB which is about 20%. Don't go too far. The Air
Force and the army use either Birds or Philco-Sierra power meters for
testing military and airborne equipment and it's good enough. When you go
into single digit percent figures, you're talking about tenths or hundredths
of dB less than the thickness of the meter's pointer on an analogue meter.
It's meaningless from a practical standpoint. Be reasonable!
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Dr. David Kirkby
Sent: Wednesday, December 02, 2009 8:22 PM
To: Steve Katz
Cc: AMPS List
Subject: Re: [Amps] Dummy Loads & Wattmeters
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To my knowledge, which might be outdated now as this was 20 years ago, the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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