
How to Build a Raspberry Pi Radio Transmitter - esbonsa
http://techzei.com/how-to-build-a-raspberry-pi-radio-transmitter/
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edent
This page basically says "Install this file, magic!"

If you want the technical details behind it, a good starting place is
[http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspb...](http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter)

Or the blog of the creator at
[http://www.omattos.com/node/19](http://www.omattos.com/node/19)

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esbonsa
I missed the icrobotics link that was submitted to HN 508 days ago
unfortunately... but at least the link that I posted shows that you don't need
any additional hardware to transmit on FM frequencies.

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unoti
I'm a Ham radio operator, and interested in building my own real 2m/70cm
transceiver that I can put in my car, complete with a little display to
impress my friends and such. So these Pi articles interest me, but I'm more
interested in interfacing a microcontroller to a real quality transmitter that
can operate at, say, 25+ watts.

Ideally I'd like for this project to run Android, and drive my car stereo too,
and do a bunch of other crazy things like GPS.

There's surprisingly little information out there on how to approach such a
project. I've looked for components I could use on Sparkfun, and there's not
really anything that jumps out at me, for either building 2m/70cm
transceivers, or even for doing the stereo amplifier part.

I did find this fascinating article, though:
[http://hackaday.com/2013/02/28/hacking-a-ham-
radio/](http://hackaday.com/2013/02/28/hacking-a-ham-radio/)

~~~
th0ma5
Yeah I've been interested in the links out there around modifying the Baofeng
radios and replacing the CPU in them with an Arduino or an generic computer
interface.

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startrak
Anybody have information regarding the strength of the output signal? I know
the FCC here in the US very touchy about radio broadcast strength...

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synx508
It's definitely above the part 15 limits of 150 μV/m at 3m

I don't know the current drive capabilities of the Broadcom SoC, but let's
assume it can perfectly drive a dipole antenna presenting around 72 ohms. The
supply is 3.3V, I think. 3.3^2/72 = 0.15125W. In reality I doubt it's anywhere
close to that, let's assume 50mW instead (it could be lower, I just don't
know). According to a legit-looking field strength calculator I found online
this works out to just over 0.5V/m at 3m! That sounds a bit high. Various
factors would conspire to making even this much field strength unlikely, but
it'd still exceed the FCC limits easily.

The Pi's output is also a _reasonably_ decent square wave, as pointed out
elsewhere. So there are some harmonics that are, themselves, above the part 15
limits.

In other words, don't do this, unless you have a spectrum analyzer, attenuator
(don't forget to block DC). Actually, someone needs to do this with the right
equipment just to find out how well it really performs.

------
wwwwwwwwww
fun semi-related EE design fact that readers may find interesting:

one of the fastest ways to tell that someone didn't know what they're doing
when they designed a board is that they placed the oscillator anywhere other
than right next to the component it drives. the traces that connect the
oscillator to the component are basically an antenna - the longer they are,
the stronger they broadcast. This becomes an issue when there are multiple
oscillators in close proximity - their broadcasts can interfere with each
other and cause them to synchronize. I forget what this effect is called but
here's a neat video that kinda illustrates it:
[https://www.youtube.com/watch?v=kqFc4wriBvE&feature=kp](https://www.youtube.com/watch?v=kqFc4wriBvE&feature=kp)

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zxcvgm
A related hack: using the Pi to transmit at 433.92 MHz with minimal hardware
[1]. This frequency is commonly used for remote controlled wall outlets and
such.

[1]
[http://www.skagmo.com/page.php?p=projects/22_pihat](http://www.skagmo.com/page.php?p=projects/22_pihat)

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cskau
Does anybody happen to know if it's just as easy to read the pins at 91.25,
94.75, and 95.75 MHz?

Lately I've been trying to find a way to do exactly that - initially using
audio in - but so far without any luck.

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th0ma5
The $20 RTL-SDR sticks can do this.

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Bob_Sheep
I should point out that the pifm code is a bit of a hack and does not properly
reset the DMA controller when you stop running it. It will leave the FM signal
active until you reset the pi.

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lifeisstillgood
Appears to have breached hosting limits (or some other 500)

Any caches? Seemed interesting

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petecooper
[http://webcache.googleusercontent.com/search?q=cache:http://...](http://webcache.googleusercontent.com/search?q=cache:http://techzei.com/how-
to-build-a-raspberry-pi-radio-transmitter/)

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BuffaloBagel
Holy square wave harmonics Batman.

