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I have figured out how use the Raspberry Pi to transmit FM Radio, but I also know that this is illegal. All transmitters over 500mW required a licence. What is the output power of the Raspberry Pi (in mW)?

It uses one of his GPIO pins to generate the Radio waves (More specific: The clock generator can generate square waves and now I am using this to 'send' in radio frequencies using some script downloaded on the internet)

Steve Robillard
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Fusseldieb
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2 Answers2

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If we approximate that the dirty square wave coming from the GPIO pin, is a sinewave, and at the same time assume the antenna is directly driven. Well, then as @joan said. The GPIO maximum current draw is 16 mA. So the formula can be approximated to be:

Pavg = Vrms * Imax = (Vpp/Sqrt[2]) * Imax = 3.3 * 0.707 * 0.016 = 0.037 W = 37 mW

But at the end of the day, how much transmitted power you really get on the air (TPO) depends on your antenna and how well the impedance is matched etc. So the fact that this works at all is rather amazing...but then again, almost anything can act like an antenna. Just horribly inefficient, as you can see below.

Here's a picture from a video on the power spectrum from the Pi Wire.

enter image description here

Here you see how it is radiating all over the place, all the way up into GHz. Thus the actual energy transmitted to the exact frequency you want, is extremely small. (Which is why you don't need to listen that much to all those trolls preaching about the FCC rules.)

Community
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not2qubit
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Given that the maximum 3V3 output of the Pi is claimed to be 50 mA that puts an upper limit of 3.3*50 = 165 mW.

Furthermore given that an individual gpio is said be harmed if you source or sink more than 16 mA that suggests an upper limit of 52.8 mW.

In practice I expect it's much less but have no idea how it could be measured.

joan
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