SWR meter as power meter
 

Owen Duffy Inscribed thus:

If I have followed the thread properlty, you have a VSWR meter and you
have now decided to replace the coupler. The meter movement is
actually intended for another project.

It might not have occurred to you that if the diode voltage drop is
small wrt the RF voltage being rectified, that the 0 to 100 meter
scale could be taken to be rho (the magnitude of the complex
reflection coefficient) in percent. But, there is an if in there,
verification is needed.

The calculator at http://www.vk1od.net/calc/tl/vswrc.php accepts rho
as an input (it is called the voltage reflection coefficient in the
calculator). So, if you were measuring SWR and you had 'set' the fwd
direction for fsd, then read for example 15/100 reflected, rho=0.15
and the calculator will tell you that VSWR=1.35.

Owen

In other words, half scale will be 3:1 !

--
Best Regards:
Baron.

SWR meter as power meter
 

Baron wrote in news:id6a58$sj0$2
@news.eternal-september.org:

In other words, half scale will be 3:1 !

If you think that the entire content is captured in your summary, you
aren't as clever as you think.

Jimmie and other readers might glean more from it than you apparently did.

Owen

SWR meter as power meter
 

Owen Duffy Inscribed thus:

Baron wrote in news:id6a58$sj0$2
@news.eternal-september.org:

In other words, half scale will be 3:1 !

If you think that the entire content is captured in your summary, you
aren't as clever as you think.

Jimmie and other readers might glean more from it than you apparently
did.

Owen

Yes I missed some words ! It wasn't very clear at all.
I should have said that the reverse power would be 3:1 at half scale.

The point being that if you adjust forward power for full scale meter
reading then 3:1 reverse power will be at half scale, 2:1 quarter scale
etc.

--
Best Regards:
Baron.

SWR meter as power meter
 

Baron wrote in news:id6cv7$kto$1
@news.eternal-september.org:

....
The point being that if you adjust forward power for full scale meter
reading then 3:1 reverse power will be at half scale, 2:1 quarter scale

"then 3:1 reverse power will be at half scale" is gobbledegook.

Would you like another guess?

If diode voltage drop is insignificant, quarter scale (rho=0.25) equates to
VSWR=1.67.

Owen

SWR meter as power meter
 

On Dec 1, 2:23*pm, Owen Duffy wrote:
If I have followed the thread properlty, you have a VSWR meter and you have
now decided to replace the coupler. The meter movement is actually intended *
for another project.

It might not have occurred to you that if the diode voltage drop is small
wrt the RF voltage being rectified, that the 0 to 100 meter scale could be
taken to be rho (the magnitude of the complex reflection coefficient) in
percent. But, there is an if in there, verification is needed.

The calculator athttp://www.vk1od.net/calc/tl/vswrc.phpaccepts rho as an
input (it is called the voltage reflection coefficient in the calculator)..
So, if you were measuring SWR and you had 'set' the fwd direction for fsd,
then read for example 15/100 reflected, rho=0.15 and the calculator will
tell you that VSWR=1.35.

Owen

True enough, but checking SWR isnt my main concern I can interpolate
that well enough. I was just wondering if my idea for measuring power
via a calibrated full scale adjust dial would be a valid procedure.

Jimmie

SWR meter as power meter
 

JIMMIE wrote in
:

....
True enough, but checking SWR isnt my main concern I can interpolate
that well enough. I was just wondering if my idea for measuring power
via a calibrated full scale adjust dial would be a valid procedure.

You could do it the way you describe. The question is not whether it can
be done, but rather what the expected error is. Meausuring RF power is a
challenge, and adding the potentiometer setting as a variable element
adds error. That was a weakness with the Oskerblock sheme I mentioned
earlier.

I don't really understand the issue with the meter. I see meter movements
on Futurlec for $10.

An alternative approach is to use a DMM with a lookup graph to
interpolate detector voltage to power. So, you put a jack on the
instrument to pick of the DC from the fwd and rev detectors, and do up a
cal chart relating that voltage to power.

I have added a 1/4" TRS jack to a commercial VSWR / Directional Wattmeter
doing just that, and have created a cal chart in a spreadsheet that
allows cubic spline interpolation for high resolution power measurement.
Whilst improving the resolution doesn't improve absolute accuracy, the
high resolution capability is really handy for some tests.

An article describing the technique is at
http://www.vk1od.net/measurement/irpm.htm , and it includes a link to the
spreadsheet which has the cubic spline interpolation feature.

Owen

SWR meter as power meter
 

Richard Clark wrote:
On Tue, 30 Nov 2010 13:50:35 -0800 (PST), JIMMIE
wrote:

Owen, the meter is just a 1 to 100 linear scale meter on on 100 uA
movement. Ive used it for years just to "guesstamate" VSWR. Not really
as inaccurately as one may think. My thought was to cal a scale for
the "full scale set" knob so that when the meter is adjusted for full
scale the knob position will indicate power. Part of the reason for
doing it this way is that it is a nice large good quality meter and I
dont want to risk damaging it by taking it apart. I may have other
uses for it later on. Im thinking "linear scale capacitance meter
similar to the heathkit model. I intend to use an NE555 osc instead
of vacuum tube osc should I do this .

Jimmie

Hi Jimmie,

On reading this, several thoughts came to mind. You write about
having used a linear scale to guesstimate SWR. Sounds good and it
immediately leads us to an existential question:
"What value is there in knowing the value of SWR?"

The first motivation following a glance at the SWR meter is to LOWER
the SWR, for whatever value it may reveal. In this sense, the value
is a trivial consideration - relative indications are enough to
achieve the goal.

Using the feed from a Bruene style detector pair into a Log-Amp will
give you a power response in a linear scale.

These days, unless you're deliberately trying to retrofit, you'd be
better off with a scratch design using the very nice Analog Devices RF
power detector chips (huge dynamic range, temperature compensated) and a
microcontroller to do the calibration for directivity. A fancy one
might even attempt measure the frequency and do that part of the cal as
well.

Once you're not trying to go directly from detector device to a meter,
you can do a lot of useful stuff. If you still want a meter needle, have
the microcontroller put out an analog signal.

It would probably even be cheaper than trying to make a really flat
coupler with good directivity.