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

On Mon, 7 May 2007 23:03:28 +0200, "Antonio Vernucci"
wrote:

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

Hi Tony,

Non-linear magnetic field.

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.

The difference between silicon and germanium, perhaps.

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

No.

73's
Richard Clark, KB7QHC

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.

"Antonio Vernucci" wrote in
:

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

Have a look at the diode characteristic below the knee, it is a square law
detector (meaning that the DC current is proportional to the square of the
AC voltage, and therefore power).

We are talking about millivolt levels, but the good news is that today,
stable high gain DC amplifiers are a snack (see chopper stabilised Op
Amps).

Owen

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

We are talking about millivolt levels, but the good news is that today,
stable high gain DC amplifiers are a snack (see chopper stabilised Op
Amps).

Yes, but to my knowledge, the Struthers wattmeter is purely passive, no
amplifier whatsoever.

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.

I am not sure whether, confining operation to the square law region, it would be
possible to build a power meter that has no DC amplifier (like the Struthers
wattmeter).

73

Tony I0JX

"Antonio Vernucci" wrote in news:4640a649$0$17940
:

We are talking about millivolt levels, but the good news is that today,
stable high gain DC amplifiers are a snack (see chopper stabilised Op
Amps).

Yes, but to my knowledge, the Struthers wattmeter is purely passive, no
amplifier whatsoever.


You have misunderstood me, my post was in answer to your more general
question "Does anyone know of special diodes resulting in a linear
wattmeter scale?" (which I quoted in my post).

If you weren't interested in the answer to the question, why ask it?

Owen

On Tue, 8 May 2007 18:35:30 +0200, "Antonio Vernucci"
wrote:

I am not sure whether, confining operation to the square law region, it would be
possible to build a power meter that has no DC amplifier (like the Struthers
wattmeter).

Hi Tony,

Most of the expense of the unit is in slugs and the base unit.
Certainly the diodes may be precious (I've rebuilt and calibrated
them), but in today's world you can replace them with garden variety
diodes and make up the difference with analog amplifiers with shaping
to conform to the meter scale. There is more than enough room in the
base unit to do this and the investment would pay off when you would
be tempted to just let it gather dust.

There's no such thing as magic diodes, merely mil-spec hand selected
ones that push cost through the roof.

Look at the meter movement for its full scale deflection current. From
there it is a rather simple matter to use one of several diode i/v
curves to reverse-engineer the solution. Buy pallet full of the
appropriate technology (Si/Ge/Tunnel/Avalanche/what-have-you) and
select. OR take one of those garden variety diodes and build a log
amp (or simply buy a log amp). Add a battery clip with battery and
move on.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Tue, 8 May 2007 18:35:30 +0200, "Antonio Vernucci"
wrote:


I am not sure whether, confining operation to the square law region, it would be
possible to build a power meter that has no DC amplifier (like the Struthers
wattmeter).


Hi Tony,

Most of the expense of the unit is in slugs and the base unit.
Certainly the diodes may be precious (I've rebuilt and calibrated
them), but in today's world you can replace them with garden variety
diodes and make up the difference with analog amplifiers with shaping
to conform to the meter scale. There is more than enough room in the
base unit to do this and the investment would pay off when you would
be tempted to just let it gather dust.

There's no such thing as magic diodes, merely mil-spec hand selected
ones that push cost through the roof.

Look at the meter movement for its full scale deflection current. From
there it is a rather simple matter to use one of several diode i/v
curves to reverse-engineer the solution. Buy pallet full of the
appropriate technology (Si/Ge/Tunnel/Avalanche/what-have-you) and
select. OR take one of those garden variety diodes and build a log
amp (or simply buy a log amp). Add a battery clip with battery and
move on.

or better than that, buy one of the Analog Devices wideband power
detectors with 90dB dynamic range and linearity better than half a dB
for a few bucks... Check out the AD8307 for $6

You could even fork out the $100 for one of the many eval boards, hook
it up to a $20 DVM, and be ready to go.. measuring phase even.