J
John Smith
- Jan 1, 1970
- 0
I built a field strength meter using Analog's design which was based on
Paul's requirements. Mine was built for about 73 MHz (the US model aircraft
frequency). I did not have a uA meter, so I just left it out and attached a
digital VOM at those points. I used only one grounded-base RF amplifier
before the meter detector. My range was about 40 feet, but I had not
adjusted the RF amp's coil for maximum indication. I can't find any
reference to the power output specification for my Futaba 8F/U R/C
transmitter, but I think it may be 2 milliwatts.
- Don't count on a known, fixed antenna impedance. It varies widely as the
meter is moved from place to place, according to my MFJ Antenna Analyzer.
Probably best to sit the FSM down somewhere and walk the transmitter around.
In any case, I don't think the FSM should be designed so that it is
dependent on some particular value of antenna terminal impedance. Perhaps a
resistance range from 30 to 100 ohms and perhaps the same for reactance.
- Paul, you are probably underestimating the signal strength your FSM will
be getting. I estimate you will have .01 volts at your meter 100 meters
distant if your transmitter produces 1/2 Watt. Yes, that is with lossless
isotropic radiators in space, no path loss, etc. However, there is some
compensating radiation directivity (which an isotropic radiator would not
have) if you are using a 1/2 wave vertical radiator on earth. So I call it a
wash until more measurements are made.
I had a lot of fun doing this and I think I learned a lot. I thank Paul for
getting this thread started and I thank Analog, Phantom, and all the other
contributors to the threads involving this project.
John
Paul's requirements. Mine was built for about 73 MHz (the US model aircraft
frequency). I did not have a uA meter, so I just left it out and attached a
digital VOM at those points. I used only one grounded-base RF amplifier
before the meter detector. My range was about 40 feet, but I had not
adjusted the RF amp's coil for maximum indication. I can't find any
reference to the power output specification for my Futaba 8F/U R/C
transmitter, but I think it may be 2 milliwatts.
- Don't count on a known, fixed antenna impedance. It varies widely as the
meter is moved from place to place, according to my MFJ Antenna Analyzer.
Probably best to sit the FSM down somewhere and walk the transmitter around.
In any case, I don't think the FSM should be designed so that it is
dependent on some particular value of antenna terminal impedance. Perhaps a
resistance range from 30 to 100 ohms and perhaps the same for reactance.
- Paul, you are probably underestimating the signal strength your FSM will
be getting. I estimate you will have .01 volts at your meter 100 meters
distant if your transmitter produces 1/2 Watt. Yes, that is with lossless
isotropic radiators in space, no path loss, etc. However, there is some
compensating radiation directivity (which an isotropic radiator would not
have) if you are using a 1/2 wave vertical radiator on earth. So I call it a
wash until more measurements are made.
I had a lot of fun doing this and I think I learned a lot. I thank Paul for
getting this thread started and I thank Analog, Phantom, and all the other
contributors to the threads involving this project.
John