I have a Homebrewed SWR meter I was thinking of calibrating it to read
power by appling a dial for the "full scale adjust" potentiometer.
Meter would be initially calibrated against a know accurate wattmeter.
Variable HIgh Power RF source would be an Amateur band SSB TX
modulated by a single tone audio source. Power varied by adjusting the
level of the tone. Power would be read from the dial pointer. One
thought is to use a difference meter reference for each ham band ....
10M full scale 15M a little less. Any thought s on this method would
be appreciated.


Jimmie

In message
,
JIMMIE writes
I have a Homebrewed SWR meter I was thinking of calibrating it to read
power by appling a dial for the "full scale adjust" potentiometer.
Meter would be initially calibrated against a know accurate wattmeter.
Variable HIgh Power RF source would be an Amateur band SSB TX
modulated by a single tone audio source. Power varied by adjusting the
level of the tone. Power would be read from the dial pointer. One
thought is to use a difference meter reference for each ham band ....
10M full scale 15M a little less. Any thought s on this method would
be appreciated.

Provided you calibrate it, the meter should be able to 'dual-task' as a
power meter. An RF power meter is really only an SWR meter which is
calibrated accurately in power.

The variation of the SWR meter's sensitivity from band to band depends
on which type of circuit it uses.

If it's the 'old-fashioned' type, which uses a short length of
air-spaced coax with an additional wire inserted as a coupling loop, the
sensitivity increases significantly with increasing frequency.

If it is of the ferrite transformer type, the sensitivity should be
pretty constant on all bands, and that is the best type to use for power
measurements.
--
Ian

"JIMMIE" skrev i en meddelelse
...
I have a Homebrewed SWR meter I was thinking of calibrating it to read
power by appling a dial for the "full scale adjust" potentiometer.
Meter would be initially calibrated against a know accurate wattmeter.
Variable HIgh Power RF source would be an Amateur band SSB TX
modulated by a single tone audio source. Power varied by adjusting the
level of the tone. Power would be read from the dial pointer. One
thought is to use a difference meter reference for each ham band ....
10M full scale 15M a little less. Any thought s on this method would
be appreciated.


Jimmie

dr Jimmie,

From your last remark I guss that your SWR meter bridge principle consists
of some kind of coupling pick-up loop where the RF output versus frequency
is not flat but follows a +6 dB/octave characteristic, i.e. more sensitive
as the RF frequency rises.

In that case you have you have two options: Calibrate the meter individually
for each amateur band or pass the coupled RF through a -6 dB/octave low pass
filter beforde detection.

It might be easier to add to your SWR meter a separate RF detector to sample
the RF at the main line. This is easily made (mostly) frequency independent
and it can be calibrated separatey from the SWR part. You will of course
need to add a switch in order to change from SWR to power readings.

vy 73
OZ7S Sven
Technical Editor Emeritus
OZ Magazine

On Nov 27, 6:50*pm, Ian Jackson
wrote:
In message
,
JIMMIE writesI have a Homebrewed SWR meter I was thinking of calibrating it to read
power by appling a dial for the *"full scale adjust" potentiometer.
Meter would be initially calibrated against a know accurate wattmeter.
Variable HIgh Power RF source would be an *Amateur band SSB TX
modulated by a single tone audio source. Power varied by adjusting the
level of the tone. Power would be read from the dial pointer. One
thought is to use a difference meter reference for each ham band ....
10M full scale 15M a little less. Any thought s on this method would
be appreciated.

Provided you calibrate it, the meter should be able to 'dual-task' as a
power meter. An RF power meter is really only an SWR meter which is
calibrated accurately in power.

The variation of the SWR meter's sensitivity from band to band depends
on which type of circuit it uses.

If it's the 'old-fashioned' type, which uses a short length of
air-spaced coax with an additional wire inserted as a coupling loop, the
sensitivity increases significantly with increasing frequency.

If it is of the ferrite transformer type, the sensitivity should be
pretty constant on all bands, and that is the best type to use for power
measurements.
--
Ian

Yes Ian it does use a piece of coax for the pickup. It should be a
simple matter to modify it to the ferrite transformer type.

Thanks

If it is the type where calibration varied with frequency, you could copy
what was done with some commercial instruments of the seventies, eg the
Oskerblok.

1. Calibrate the sensitivity knob with a logging scale.

2. Add a power scale to the meter face.

3. Measure a set of points of sensitivity setting vs frequency where the
meter scale is correctly calibrated for power.

4. Create an interpolated smooth graph as a lookup, and attach it to the
instrument.

Be prepared for significant linearity error at the low frequency end if
you calibrated the meter for low fsd power.

If it is a Bruene type sensor, the response should be reasonably flat
within a frequency range which you must establish.

The interesting thing with either type of sensor (and both are used in
Bird elements, though the first type is frequency compensated), is that
although the Pfwd and Pref power readings are of each of no value by
themselves, the power in the line is Pfwd-Pref irrespective of the load
impedance or line Zo.

Owen

On Nov 29, 2:51*pm, Owen Duffy wrote:
If it is the type where calibration varied with frequency, you could copy
what was done with some commercial instruments of the seventies, eg the
Oskerblok.

1. Calibrate the sensitivity knob with a logging scale.

2. Add a power scale to the meter face.

3. Measure a set of points of sensitivity setting vs frequency where the
meter scale is correctly calibrated for power.

4. Create an interpolated smooth graph as a lookup, and attach it to the
instrument.

Be prepared for significant linearity error at the low frequency end if
you calibrated the meter for low fsd power.

If it is a Bruene type sensor, the response should be reasonably flat
within a frequency range which you must establish.

The interesting thing with either type of sensor (and both are used in
Bird elements, though the first type is frequency compensated), is that
although the Pfwd and Pref power readings are of each of no value by
themselves, the power in the line is Pfwd-Pref irrespective of the load
impedance or line Zo.

Owen

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

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. If you want to know the
SWR with accuracy, you can throw the reverse switch for the second
power reading and compute, or you can throw the reverse switch and
start twisting nobs to make the power indication go away.
consider:
http://rfdesign.com/mag/503rfd33.pdf
Says it all in one page - with pictures, schematics, and math too.

I don't quite catch the drift of the "capacitance meter" paired up
with NE555 osc. I presume you mean to construct a DC-Freq converter?

73's
Richard Clark, KB7QHC

On Nov 30, 9:23*pm, 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. *If you want to know the
SWR with accuracy, you can throw the reverse switch for the second
power reading and compute, or you can throw the reverse switch and
start twisting nobs to make the power indication go away.
consider:http://rfdesign.com/mag/503rfd33.pdf
Says it all in one page - with pictures, schematics, and math too.

I don't quite catch the drift of the "capacitance meter" paired up
with NE555 osc. *I presume you mean to construct a DC-Freq converter?

73's
Richard Clark, KB7QHC

Heathkit made a linear scale capacitance meter years ago dont remember
the model #. CM? I think. Circuit use a tube in a multivibrator
configuration the NE555 is just a more modern approach to th same
circuit. Cap to be tested is in series with the osc and meter. Current
through the circuit is a linear function of capacitance or for that
matter frequency. I never got around and may never still build the
circuit because I can always bread board a little circuit to use with
my function generator when I want to measure a capacitor. I cal the
circuit using some precision caps before use. I agree with your view
of usefulness of knowing SWR. I use it mostly as a relative health
indicator of my antenna system. . A high SWR on my 40M dipole may
indicate something is not well and I should look out the window while
an unrealistically low reading on 2M probably means I have water in my
coax.

BTW the reason I bought the little home brew SWR meter was to source
the meter and box for this project. My lack of real need for it has
kept the little hamfest treasure an SWR bridge.

Jimmie

JIMMIE Inscribed thus:

On Nov 30, 9:23Â*pm, 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 Â*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 Â*vacuum tube osc should I do this .

Jimmie

Hi Jimmie,

On reading this, several thoughts came to mind. Â*You write about
having used a linear scale to guesstimate SWR. Â*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. Â*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. Â*If you want to know the
SWR with accuracy, you can throw the reverse switch for the second
power reading and compute, or you can throw the reverse switch and
start twisting nobs to make the power indication go away.
consider:http://rfdesign.com/mag/503rfd33.pdf
Says it all in one page - with pictures, schematics, and math too.

I don't quite catch the drift of the "capacitance meter" paired up
with NE555 osc. Â*I presume you mean to construct a DC-Freq converter?

73's
Richard Clark, KB7QHC

Heathkit made a linear scale capacitance meter years ago dont remember
the model #. CM? I think. Circuit use a tube in a multivibrator
configuration the NE555 is just a more modern approach to th same
circuit. Cap to be tested is in series with the osc and meter. Current
through the circuit is a linear function of capacitance or for that
matter frequency. I never got around and may never still build the
circuit because I can always bread board a little circuit to use with
my function generator when I want to measure a capacitor. I cal the
circuit using some precision caps before use. I agree with your view
of usefulness of knowing SWR. I use it mostly as a relative health
indicator of my antenna system. . A high SWR on my 40M dipole may
indicate something is not well and I should look out the window while
an unrealistically low reading on 2M probably means I have water in my
coax.

BTW the reason I bought the little home brew SWR meter was to source
the meter and box for this project. My lack of real need for it has
kept the little hamfest treasure an SWR bridge.

Jimmie

Jimmie.
Have you seen this ?
http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
I've just built this version.
http://panteltje.com/panteltje/pic/lc_pic

--
Best Regards:
Baron.

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