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FM Transmitter Watt Measurement

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Actually tcm, in the US licenses are easy to get now.
 
Hello. Could anyone please tell me how I can test the watt output on a unit I bought? I bought a 30 watt FM transmitter and I would like to know if it is transmitting 30 watts. Thank you.


If you can't find someone with a meter to help measure this for you and must consider doing it yourself, then consider this approach. First, you need a 50 ohm resistor to act as a load for your transmitter. Get a Non-Inductive power resistor, like this one:
**broken link removed**

and figure out how to attach this resistor directly to the RF output connector of your transmitter with the shortest possible connection wires. Connection wires must be shorter than 2 inches at most. You should also bolt it to a chunk of metal to act as a heatsink, or blow air over the resistor with a small computer fan. It will get hot with 30 watts going into it and you don't want it to burn out.

Next, you need an RF voltmeter to measure the voltage across the resistor. This can be very simple since you hope to measure quite a large voltage. First connect a 5.6 pF ceramic capacitor (100V rating) to the "hot" side of the load resistor (the side of the resistor that is not connected to ground). To the other end of this capacitor, attach the anode of a 1N5711 (or similar) schottky RF diode and then at the cathode end of the diode, attach a 1000 pF ceramic capacitor (100V rating) from the diode cathode to the ground side of the 50 ohm load resistor. All wire connections must be kept very short, like less than 1 inch. When you have put this together, you have created an AM detector that generates a DC voltage that is approximately the same as the peak RF voltage coming out of your transmitter. In order to read the DC voltage, next you must place a DC voltmeter (any typical DVM) across that 1000 pF capacitor set to read DC volts up to about 40 volts or so.

Once you have this all hooked up, you can turn on the transmitter and read the DC voltage. To calculate the power output from this, multiply the DC voltage by 0.707 to get the approximate RMS AC voltage, and then calculate the power knowing that power equals voltage squared divided by the load resistance. Expect a result of about 39 volts approximately.

This tells you the output power from your transmitter, but unfortunately it does not tell you how much power is radiating from your antenna. To measure that will require the use of a VSWR meter and someone who knows how to use it.
 
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