Alex Eban wrote:
> 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
I am being reasonable. In one of my earlier posts I said:
"I can't really think why a ham needs to know RF power very accurately. In which
case, Bird 43's are fine."
I am saying if you want better accuracy, then there are other options. I was
sharing some rather outdated knowledge about what at the time at least
(probably
still is) the most accurate way of measuring power.
I've also suggested some other methods, like a directional coupler, which are
practical for amateur use.
Another method which is quite practical is a high power attenuator. These do
exist, but I've no idea if they are available cheaply. If you do find one, you
need to power at only one end only. I can image a lot on the second hand market
have had some idiot apply power at the wrong end, which will destroy them.
TIP,
If you ever come across an attenuator, here is a quick was to find out if its
working correctly or not.
1) Measure the DC resistance at one end with a DVM. Make sure the resistance
measured at the other end is similar. Call this R1
2) Short one end of the an attenuator with a bit of wire, and measure the
resistance at the other end. It should be lower. Call that R2.
The character impedance Zo is sqrt(R1*R2). If that is about 50, the attenuator
is probably 50 Ohms, and working.
It's also possible to determine the attenuation from simple DC resistance
measurements, though I can't recall the formula. Basically, for attenuators
with
high attenuation, the resistance measured at the other end will not change
much when you put a short across the other end.
Dave
>
> -----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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