Ralph Mowery wrote:
"Cecil Moore" wrote in message
t...
K7ITM wrote:
See earlier posting in this thread.
Thanks Tom, when I said "linear power scale", I meant
e.g. a meter reading where 2000 watts is full scale
and 1000 watts is half scale. I have seen such meters
but not without a digital or analog computer on the
front end.
--
73, Cecil http://www.w5dxp.com
Just jumping in the middle of ths, but look at this watt meter.
http://bama.edebris.com/manuals/miltest/an-urm120/
It has a linear scale. I have one and it has a linear scale. Sort of made
like a Bird meter but much larger elements. It is just a diode and meter.
There is no power needed to run the meter except the sampled power comming
off the transmission line.
There were several versions made. One has a SWR scale on it. I am not sure
how the swr scale is but the wattmeter scale is linear instead of the log
looking scale of the Bird and most other meters.
To have a linear power calibration, that has to be a square-law
detector. On the higher-power ranges, the slug is moved away from the
centre conductor, so that the diode is always operating within its
accurate square-law range.
Thanks very much for posting that link, Ralph. It's interesting to see
the resemblances to the Bird 43, and also the differences.
The Bird meter scale is not logarithmic. It's better described as being
in the transition region between a square-law detector at low meter
currents, and a voltage-detecting rectifier towards the high end of the
meter scale. The compression in the upper half of the power scale is
because the diode is acting as a voltage detector. Power is proportional
to V-squared, so the meter deflection is tending towards the square root
of the power, and that is what compresses the scale towards the high
end.
As with the AN-URM120, the Bird design adjusts the coupling so that all
detector diodes in all the inserts are operating at the same RF levels,
so they can all share the same meter scales. The difference is that
instead of physically moving the same insert inward or outward to adjust
the coupling, Bird do it by selling us more slugs :-)
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek