On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote:
To me, having a
linear power scale is a big advantage, because then you can reasonably
accurately figure SWR without having to worry about temperature
compensation of the detectors.

Can you define what you mean by linear? Straight line?
Since we can only measure voltage and current, in order
to obtain a linear power scale from a linear meter, it
is necessary to supply some pre-display computing
ability (microcomputer).
--
73, Cecil http://www.w5dxp.com

See earlier posting in this thread. See various Avago ap notes, such
as AN 969. A diode detector run at low input provides an output DC
voltage that's a constant times the square of the input RF voltage.
If the input voltage is, or is assumed to be, at some constant
resistive load impedance, the DC output is linear with RF power
input. The proportionality is temperature dependent, but if two
detectors are constructed the same and run at the same temperature,
and run in the signal level region where that relationship holds, then
the ratio of the output DC voltages is a very good approximation of
the ratio of the input RF power levels, and thus is useful for finding
the SWR if the detectors are attached to the forward and reverse ports
of a good directional coupler. Top end of the useful "linear power"
range using an HSMS-2850 single diode detector is about 10mV DC
output. If you can measure the DC accurately down to 1uV (a bit
tough, given thermal emfs, but possible), that gives you about a
10000:1 power range, or 100:1 RF input voltage range -- or about
1.02:1 SWR. Chances are very good that a home-built coupler won't be
accurately enough matched to 50+j0 ohms to worry about anything that
low anyway, even if you had a reason to care about it.

Cheers,
Tom

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

On Jan 31, 1:46 pm, Cecil Moore wrote:
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

Right, and I'm saying that with a coupler or sampler that picks off,
say, 1e-7 of the power (0.2 milliwatts of RF out for 2000 watts in/
through), a diode detector that shows that power to be full scale on a
linear meter will show 1000 watts at half scale. That assumes that
0.2 milliwatts is low enough to get you into the square law region of
the detector; for an HSMS-2850 diode, that's a bit high. Because the
power at which the square law holds accurately is so low (and the
detector output is so low) you need an electronic way to read that
output; a simple meter movement isn't likely to do the trick. But
there's no need for any fancy processing.

"Cecil Moore" wrote in message
...
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.

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

On Sat, 2 Feb 2008 08:45:27 +0000, Ian White GM3SEK
wrote:

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 :-)

Hi All (Congratulations Ralph),

There are more differences than that. The Bird suffers more from
parts erosion than the URM120 as the URM has bigger elements. With
the bigger elements, the geometries are held to a tighter precision.

It is the coupling link that moves in the URM, not the element. The
link rides on a cam that is stepped for the 4 different power ranges
used. If you remove the knob from the slug, and then the cover plate,
you have access to an adjustment screw that provides the fine control
over the depth of penetration. The smaller PDF at the boat anchors
web page is the more useful of the two offered.

I have calibrated a pile of both the Birds and the URMs, and the URMs
always required less maintenance, and rarely needed adjustment. The
Birds, on the other hand, always arrived out of calibration.

One source of error in the VHF/UHF region was that the rubber gasket
inside the N connector (and for that matter, for any N connector) will
accumulate thread debris. It should be used as a visual correlative
to the amount of similar debris bridging the Teflon insulator. This
debris can whack out these meters and degrade connections.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sat, 2 Feb 2008 08:45:27 +0000, Ian White GM3SEK
wrote:

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 :-)

Hi All (Congratulations Ralph),

There are more differences than that. The Bird suffers more from
parts erosion than the URM120 as the URM has bigger elements. With
the bigger elements, the geometries are held to a tighter precision.


I was luckey enough to find one that was in the origional wrapping paper. I
don't remember what the date was on the wrapper. At a hamfest several years
ago soneone had several new ones. They were in the heavy duty aluminimum
foil , cardboard box, and the hard plastic foam filled case. Never opened
from the day they were calibrated about 10 or 20 years before. It came
with 3 elements that would go from about 3 mhz to 1000 mhz. Think it topped
out at 1 kw up to 30 mhz and 500 watts after that. I bought it for what
some used Birds without elements seem to go for on e-bay now.
One minor thing is the meter movement on those meters are sluggish so it
takes a slow adjustment of amplifiers to get them to the max output.

On Sat, 2 Feb 2008 12:10:10 -0500, "Ralph Mowery"
wrote:

One minor thing is the meter movement on those meters are sluggish so it
takes a slow adjustment of amplifiers to get them to the max output.

Hi Ralph,

The meter needle probably has an aluminum vane in the magnetic field
that is used as a dampener through eddy current induction.

As a general consideration for fine meter movements, and to prevent
their being damaged while being transported; some are equipped with
shorting bars (or switches) across the meter terminals. This is more
commonly encountered with bridge instruments.

This can be simple observed by rotating the meter movement quickly and
noting the deflection of the needle on the scale through inertia. An
undampened meter can be deflected by vigorous movement up to half
scale (possibly more), where a dampened meter would deflect barely a
tenth.

You got a bargain beyond dollars in your purchase.

73's
Richard Clark, KB7QHC

On Feb 2, 9:10 am, "Ralph Mowery" wrote:
"Richard Clark" wrote in message

...

On Sat, 2 Feb 2008 08:45:27 +0000, Ian White GM3SEK
wrote:

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 :-)

Hi All (Congratulations Ralph),

There are more differences than that. The Bird suffers more from
parts erosion than the URM120 as the URM has bigger elements. With
the bigger elements, the geometries are held to a tighter precision.

I was luckey enough to find one that was in the origional wrapping paper. I
don't remember what the date was on the wrapper. At a hamfest several years
ago soneone had several new ones. They were in the heavy duty aluminimum
foil , cardboard box, and the hard plastic foam filled case. Never opened
from the day they were calibrated about 10 or 20 years before. It came
with 3 elements that would go from about 3 mhz to 1000 mhz. Think it topped
out at 1 kw up to 30 mhz and 500 watts after that. I bought it for what
some used Birds without elements seem to go for on e-bay now.
One minor thing is the meter movement on those meters are sluggish so it
takes a slow adjustment of amplifiers to get them to the max output.

I have one of those, or something very similar; a Struthers 1219-D.
The HF slug that came with mine goes up to 5kW, though. The meter
movement measures 24uA full scale, and the coil resistance is 2k ohms,
so the meter is pretty sensitive: about 1.1 microwatts for full scale
deflection. It takes that sort of sensitivity to get down to the
linear detector out versus RF power in region.

Interestingly, if I simulate a single-diode detector using an
HSMS-2850 Schottky diode into a 2000 ohm load, and find the RF voltage
that gives 24uA output, and then re-simulate with voltages in the
right ratios to give 90% of that power, 80%, 70%, etc., down to 10%,
the outputs are not exactly linear with input power. The error is
small, though, no more than about 1.5% of full scale. So then I
checked the calibration on my meter, and put in 50% of full scale
current, and noted that the meter reading is not exactly 50, but just
enough off to be correctly reading the RF power. So though the scale
on my meter appears at first glance to be linear, it's not exactly so,
and I believe that must be intentional. The worst-case "error" to
linear output in the simulation was at 70% of full power, where the
current is predicted to be 71.43 percent of full scale. On my meter,
when I put in 71.43% of full scale, indeed it reads almost dead-on 70%
power. I doubt that's a fluke.

Cheers,
Tom

Richard Clark wrote:
On Sat, 2 Feb 2008 08:45:27 +0000, Ian White GM3SEK
wrote:

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 :-)

Hi All (Congratulations Ralph),

There are more differences than that. The Bird suffers more from
parts erosion than the URM120 as the URM has bigger elements. With
the bigger elements, the geometries are held to a tighter precision.

It is the coupling link that moves in the URM, not the element. The
link rides on a cam that is stepped for the 4 different power ranges
used. If you remove the knob from the slug, and then the cover plate,
you have access to an adjustment screw that provides the fine control
over the depth of penetration. The smaller PDF at the boat anchors
web page is the more useful of the two offered.

I have calibrated a pile of both the Birds and the URMs, and the URMs
always required less maintenance, and rarely needed adjustment. The
Birds, on the other hand, always arrived out of calibration.

One source of error in the VHF/UHF region was that the rubber gasket
inside the N connector (and for that matter, for any N connector) will
accumulate thread debris. It should be used as a visual correlative
to the amount of similar debris bridging the Teflon insulator. This
debris can whack out these meters and degrade connections.


Thanks for the clarification, Richard.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek