Peter Chadwick wrote:
>
>Tom says
>
>>load the detector with 1300 ohms
>
>Thanks for the comment, Tom: I measured it as 1430 ohms, but I'm figuring 1400
>is probably close enough.
>
>Of course, it doesn't alter the fact that readings are only guaranteed to 5% of
>FSD any how.
FWIW, I measured the current through a Bird 43 meter for a series of
indicated power levels (no RF involved, only a DC current source), and
threw it all into Excel.
Got a nice smooth curve, and Excel reported the scale law as:
I = -7.378e-12(W)^6 + 2.547e-9(W)^5 -3.407e-7(W)^4 + 2.248e-5(W)^3
-8.188e-4(W)^2 + 2.692e-2(W) -2.749e-5
where W is the indicated power on the 0-100W scale
I is the fraction of full-scale deflection on the meter
This data can then be converted into fractions of deflection *angle* to
draw up a scale for any meter using a CAD program like AutoSketch.
Actually I wasn't doing this for a Bird 43, but for a Bird 4168F
2000/200W sensor which has built-in detectors (not normal slugs) and
requires a 200uA 520ohm meter.
Like Peter, I'm planning to use an op-amp to fool the sensor into
believing it's loaded with a real 520ohm meter. But never having seen
the special meter for the 4168F, I can only *hope* that it follows the
same scale law as the standard Bird 43 - can anyone confirm this?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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