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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 (to reply via email remove "zz" from address) |
#2
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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 |
#3
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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 |
#4
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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. |
#5
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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. |
#6
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"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 |
#7
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"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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