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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)

  • Are you doing this purely with software?!? Did not know that was possible. Anyway, I have an FM transmitter. If you can tweak things, I think the key is to ensure you have the power to reach what you want, but not beyond that, and to use a frequency that won't interfere with anything else. I don't think the pi will put out 500mW via GPIO. – goldilocks Jan 16 '15 at 18:10
  • Yes, it's only with software. – Fusseldieb Jan 31 '15 at 5:17
  • Huh, neat. And I went and bought one... BTW it is easy to make a pretty effective dipole antenna of whatever size from decent gauge speaker wire. – goldilocks Jan 31 '15 at 6:28
  • But in fact that the RPi can only generate square waves, makes the Transmitter very noisy... – Fusseldieb Feb 1 '15 at 3:18
  • "...but I also know that this is illegal..." just how illegal depends on where you are in the world - in the UK jurisdiction, premises where such devices are operated may be searched under warrant and the equipment seized pending prosecution under the 2006 Wireless Telegraphy Act - under certain circumstances (deliberate interference with "wireless telegraphy", "on conviction on indictment" as per section 68.3.b) operating something like this could (IMHO) lead to up to 2 years in jail! – SlySven Sep 17 '16 at 2:51
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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.

  • 1
    It's technically possible to put a transistor on the output to generate eg. 12V output and sending with a higher range? – Fusseldieb Jan 31 '15 at 5:19
  • Yes, that's what you have to do, also to help protect your GPIO pins. – not2qubit Mar 20 '17 at 2:29
  • 3.3V into a 50 Ohm load is 66 mA, which exceeds the documented max current. A 0V to 3.3V square-wave into a 50 Ohm load will average out equivalent to 33 mA continuous which is also documented as harmful. – hotpaw2 Mar 24 '17 at 23:32
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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 * 0.00016 = 0.37 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.)

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