FCC compliance for small FM transmitter

Jan 04, 2005 6 Replies

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jondoeengineer 21 years ago

I know this question probably comes up a lot, as I've noticed from the posts....

I want to build a very short range transmitter for operation in my living room. Basically, I want to take the output of my subwoofer channel from my home theater reciever and convert it to a digital signal for broadcast to another part of the room. I'd like to be able to hide the subwoofer and relocate it as needed without having to reroute wires.

I think I can find enough info for the various circuits. However, I'm pretty ignorant on FCC regs. I'm aware that the FCC requires that the signal level be less than 250 microvolts at 13 meters from the antenna. But, I have no idea how to test this. I understand this is typically done in a lab, so of course, I wouldn't be able to test this. I also understand that there is a formula for approximate conversion from the transmitter power to signal strength: (P*G)/(4*pi*D^2) = (E^2)/(120*pi) P=Transmitter Power (Watts) G=Antenna Gain D=Distance (m) E=Field Strength (V/m)

Ok, to make a long question even longer, I guess I'm asking if there's an easy way to make sure I'm not violating FCC regs. I'd like to know that by experimenting, I'm not going to get sued, fined or go to jail. And also, I'd like to respect proper use of the spectrum.

I apologize for my ignorance. I'm a power engineer by education and am trying to revive my long lost interest in elecronics. I can't imagine a more useful thing to know.

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mc 21 years ago

An extremely simple way to be practically sure of compliance is that if in an FM radio that is known to be in good working condition, you can't get a solid signal from more than about 100 feet away, you're fine. That is, a radio makes a good field-strength meter.

That is, make it no more powerful than it needs to be. (Reducing the antenna is the easiest way to cut back.)

The old specification was to have a 0.1-watt final amplifier. That's a good starting point.

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gwhite 21 years ago

Just don't do anything really stupid and you don't have to worry about regulators if you are building a one-off "low" powered unit.

You don't even mention band.

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Mac 21 years ago

Maybe you should just use infrared. This will keep you outside the purview of the FCC and help you maintain good karma vis-a-vis the RF spectrum.

Barring that, if you are willing to stick to the bands used by cordless phones and WiFi devices (~900 MHz and ~2.5 GHz) you might be able to buy some kind of cordless phone chipset that would take care of most of the work for you. I think this would be technically legal, but even if it is not, I don't think it would cause anyone any problems.

Good luck!

--Mac

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Jim 21 years ago

Less than 100 mW DC to the final amplifier and less than 2 Ft antenna

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It was something of that order, which when considering final efficiency and the small radiating element would make the device meet the radiated power spec for unliscenced operation.

In any case, a setup like that, particularly indoors, would be no problem as far as the FCC is concerned. The RF signal isn't going to get very far.

Jim Pennell

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JeffM 21 years ago

Great--until someone walks across the room.

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Mac 21 years ago

If you use enough power, line of sight is not even remotely necessary. For example, my remote control will operate my dvd player even when I am behind the dvd player.

--Mac

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