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Old September 17th 03, 01:15 PM
Ken
 
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Default Modifying Lafayette SWR & Field Strength meter?

I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Can someone please point me to some documentation for the mod?

Ken KC2JDY

Ken
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Old September 21st 03, 06:35 PM
Fred McKenzie
 
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Default

I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.

You need to provide a power scale for the knob used to set full scale for SWR
readings, to set the full-scale power range. It may be as simple as using a
magic marker to indicate the settings for various full-scale readings such as
10, 100, 1000 watts.

If it has a field strength scale in addition to the SWR scale, that may be used
to read actual power. However, it will probably NOT be linear. In other words
it most likely will read voltage rather than power, so you would need to have a
conversion chart to convert from the scale reading to actual power. I would
expect to find that center scale would indicate one quarter of full-scale
power, but you will need to verify actual calibration.

For very low power levels, maybe one watt or so, you can expect additional
non-linearities due to the forward voltage drop of the detector diodes.

73, Fred, K4DII

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Old September 21st 03, 06:35 PM
Fred McKenzie
 
Posts: n/a
Default

I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.

You need to provide a power scale for the knob used to set full scale for SWR
readings, to set the full-scale power range. It may be as simple as using a
magic marker to indicate the settings for various full-scale readings such as
10, 100, 1000 watts.

If it has a field strength scale in addition to the SWR scale, that may be used
to read actual power. However, it will probably NOT be linear. In other words
it most likely will read voltage rather than power, so you would need to have a
conversion chart to convert from the scale reading to actual power. I would
expect to find that center scale would indicate one quarter of full-scale
power, but you will need to verify actual calibration.

For very low power levels, maybe one watt or so, you can expect additional
non-linearities due to the forward voltage drop of the detector diodes.

73, Fred, K4DII

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Old September 21st 03, 06:51 PM
Ralph Mowery
 
Posts: n/a
Default

I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the

field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.


The ones I have used in the past seem to work as is on 2 meters for SWR.
They may not be too accurate but will tell you when you get to a low SWR on
2 meters and teh 220 Mhz band. Not too sure how much power they will handle
but seem to work up to 30 watts. They will not work on 440 mhz as the line
is too long.

If the sampling lines are cut down to a shorter length they will probably
work on the 440 mhz band. This I have not tried as I now have Bird and
Struthers meters.


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Old September 21st 03, 06:51 PM
Ralph Mowery
 
Posts: n/a
Default

I have been told that there is a well-known, simple mod for converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the

field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.


The ones I have used in the past seem to work as is on 2 meters for SWR.
They may not be too accurate but will tell you when you get to a low SWR on
2 meters and teh 220 Mhz band. Not too sure how much power they will handle
but seem to work up to 30 watts. They will not work on 440 mhz as the line
is too long.

If the sampling lines are cut down to a shorter length they will probably
work on the 440 mhz band. This I have not tried as I now have Bird and
Struthers meters.




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Old September 21st 03, 07:33 PM
Lou
 
Posts: n/a
Default

"Ralph Mowery" wrote in message
...
I have been told that there is a well-known, simple mod for

converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the

field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.


The ones I have used in the past seem to work as is on 2 meters for SWR.
They may not be too accurate but will tell you when you get to a low SWR

on
2 meters and teh 220 Mhz band. Not too sure how much power they will

handle
but seem to work up to 30 watts. They will not work on 440 mhz as the

line
is too long.

If the sampling lines are cut down to a shorter length they will probably
work on the 440 mhz band. This I have not tried as I now have Bird and
Struthers meters.


IF the meter wasn't touted as a "CB" meter, most of the
FS/SWR/Modulation/Wattmeters I've seen were usually good from 3.5 to 150
MHz. 220/440? Hmmm. I don't know. Accuracy would tend to be off a bit more
I'd think. But hey, that is what makes Ham fun, PLAYING. IF it IS listed as
a CB meter, usually - of the ones I've seen - were/seemed pretty much
limited to 25-29 MHz.

IF you have the instructions for that meter or the box, it should tell you
the range(s). Otherwise, going from memory here, it seems I've read where
a/the diode(s) had to/could be swapped out. Seriously, I'd do a check of it
with known conditions first to see if it displays anything near what you
know to be the case. Example: if you know the SWR on a rig's antenna system
is 1.5:1, then the meter in question should show something close at that
frequency. IF you know the power out, again, the meter should measure close.
I've always checked any cheap meters against my bird 43 to see how close
they are before using in the field for field day or prior to selling. IF
that meter won't do what you want, there are usually meters available used
at Hamfests or on E-Bay that will do the job. So long as the meter movement
is ok, the rest is repairable with or without a schematic depending on
experience. You can usually get a meter movement as well, but by the time
you get done screwing around, you can buy a working unit.

Lou



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Old September 21st 03, 07:33 PM
Lou
 
Posts: n/a
Default

"Ralph Mowery" wrote in message
...
I have been told that there is a well-known, simple mod for

converting
an old Lafayette SWR & Field Strength Meter (#99-25835) and similar
meters into 2m/70cm SWR/power meters.

Ken-

I don't think a modification is required per se, other than removing the

field
strength antenna. The instruments I'm familiar with are reflectometers,
similar to the design of many wattmeters such as the Bird 43.


The ones I have used in the past seem to work as is on 2 meters for SWR.
They may not be too accurate but will tell you when you get to a low SWR

on
2 meters and teh 220 Mhz band. Not too sure how much power they will

handle
but seem to work up to 30 watts. They will not work on 440 mhz as the

line
is too long.

If the sampling lines are cut down to a shorter length they will probably
work on the 440 mhz band. This I have not tried as I now have Bird and
Struthers meters.


IF the meter wasn't touted as a "CB" meter, most of the
FS/SWR/Modulation/Wattmeters I've seen were usually good from 3.5 to 150
MHz. 220/440? Hmmm. I don't know. Accuracy would tend to be off a bit more
I'd think. But hey, that is what makes Ham fun, PLAYING. IF it IS listed as
a CB meter, usually - of the ones I've seen - were/seemed pretty much
limited to 25-29 MHz.

IF you have the instructions for that meter or the box, it should tell you
the range(s). Otherwise, going from memory here, it seems I've read where
a/the diode(s) had to/could be swapped out. Seriously, I'd do a check of it
with known conditions first to see if it displays anything near what you
know to be the case. Example: if you know the SWR on a rig's antenna system
is 1.5:1, then the meter in question should show something close at that
frequency. IF you know the power out, again, the meter should measure close.
I've always checked any cheap meters against my bird 43 to see how close
they are before using in the field for field day or prior to selling. IF
that meter won't do what you want, there are usually meters available used
at Hamfests or on E-Bay that will do the job. So long as the meter movement
is ok, the rest is repairable with or without a schematic depending on
experience. You can usually get a meter movement as well, but by the time
you get done screwing around, you can buy a working unit.

Lou





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