On May 7, 4:04 pm, Jim Lux wrote:
Antonio Vernucci wrote:
My question concerns the well known Struthers RF Wattmeter
TS-1285/URM-120 which has three big plug-ins (2-30 MHz 1000 Watts,
25-250 MHz 500 Watts, 200-1000 MHz 500 Watts).

Differently from all common wattmeters, the Struthers meter has a
perfectly linear scale. How could they achieve that?

On QST magazine (May 1996, pag. 77) K3BRS states that replacing the
diode with a different type would cause the initial part of the scale to
become very inaccurate.

Does anyone know of special diodes resulting in a linear wattmeter scale?

73

Tony I0JX

If you operate a diode detector in the "square law" region, the voltage
output is proportional to the incident power. You don't get a huge
dynamic range where this is true (10-20 dB, perhaps?), but on the other
hand, people have been building diode detector based power meters for
decades. Different diodes have different curves, so changing diode type
would affect the calibration.

Welllll...I think a bit more than 20dB. The HP zero-bias detector
diode I used quite a few years ago in a field strength meter has
usable output down to about -55dBm input, and as I recall the square
law response holds up to a bit above 0dBm. Admittedly you need a
pretty good amplifier to actually use the low end to that low a level.

On the other hand, I'm not convinced that my Struthers meter is all
that accurate. I should compare it with a good RF power meter
sometime.

Cheers,
Tom