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

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.

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

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

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