Post by darklordchris on Apr 20, 2009 22:36:27 GMT -6
i recently bought a small cd/ipod/car radio style, fm transmitter capable of transmitting about 5 feet. i was wondering if their is anyway i can measure the transmitters output power. can anyone give me an idea of how to do this? ;D
It's probably around 10 milliwatts of power. From the FCCs part 15 rules:
Unlicensed broadcasts on the FM broadcast band (88 to 108 MHz) are limited to a field strength of 250 µV/m at a distance of 3 meters from the antenna. This is equivalent to 0.01 microwatts.
I doubt it's even 10 milliwatts though, it may be much less as most of those ipod transmitters run right off of the transmitter chip itself.
It's probably around 10 milliwatts of power. From the FCCs part 15 rules:
Unlicensed broadcasts on the FM broadcast band (88 to 108 MHz) are limited to a field strength of 250 µV/m at a distance of 3 meters from the antenna. This is equivalent to 0.01 microwatts.
I doubt it's even 10 milliwatts though, it may be much less as most of those ipod transmitters run right off of the transmitter chip itself.
0.01 microwatts is 10 nanowatts, not milliwatts.
That's hard to do since even a CMOS clock oscillator is quite capable of putting 50mW into a wire and older TTL oscillators about 200mW.
It's probably around 10 milliwatts of power. From the FCCs part 15 rules:
I doubt it's even 10 milliwatts though, it may be much less as most of those ipod transmitters run right off of the transmitter chip itself.
0.01 microwatts is 10 nanowatts, not milliwatts.
That's hard to do since even a CMOS clock oscillator is quite capable of putting 50mW into a wire and older TTL oscillators about 200mW.
Peace!
Oops I quoted the wrong part of the rules/regulations. My original point still stands though as most legal personal FM transmitters are in the 1-25 milliwatt range. This is not easy to measure unless you had a professional digital watt meter that can measure such flea power at VHF.
Just for everyones future reference here is the worlds most simple field strength meter..
What I do at such low power is take a resistor or two to get around 50 ohms. Connect to transmitter output and wrap the small wire from the field strength meter a few turns around the resistors this way it won't effect the load and still give an idea of power strength. Once it pings the meter I add a sensitivity variable pot to the meter. Once I have enough power output I go for the watt meter then.
I use something like this: www.io.com/~n5fc/rfprobe1.htm It doesn't have great resolution with tiny signals,but it measures anything from a few milliwatts to a few watts fairly accurately. Stick it on your dummy load,and with the help of Ohms Law you can get a fairly close measurement of power. ;D
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