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Wednesday, March 20, 2013

Wow! Raspberry Pi as an RF Transmitter


This site shows you how to use a $35 Raspberry Pi Computer as an RF transmitter for the WSPR (Weak Signal Propagation Reporting) System.  All you need is a simple low pass filter and an antenna. (Oh yea, and a ham radio license.)  The site says you can get 10 milliwatts out.  That's enough for WSPR!   Very cool. 

This looks like a real international effort:

Credits goes to Oliver Mattos and Oskar Weigl who implemented PiFM [1]  based on the idea of exploiting RPi DPLL as FM transmitter. Dan MD1CLV combined this effort with WSPR encoding algorithm from F8CHK, resulting  in WsprryPi a WSPR beacon for LF and MF bands. Guido PE1NNZ extended this effort with DMA based PWM modulation of fractional divider that was  part of PiFM, allowing to operate the WSPR beacon also on HF and VHF bands.

For more info on WSPR:  http://soldersmoke.blogspot.com/search/label/WSPR

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  1. My Rasberry Pi runs my DVAP, I can see I will have to buy another one soon. Thanks for posting this up Bill. 73 Nigel Somerset UK

  2. Bill, this looks like an awesome project. One of the other geeks at work here, a non-ham interested in aerial photography and weather balloons and whatnot--he brought in one of those RP's and I couldn't stop visiting his cube to look at it again. The price is right, and I've never WSPR'ed. Thanks for the heads-up. If this works, I'll report.
    As always, best 73s,
    Jonathan KC7FYS

  3. Can someone suggest a suitable circuit for the low-pass filter?

  4. Maybe these designs?

  5. If you are looking for LF/MF designs, some reading and examples;

    - http://www.qrp.pops.net/ndb_more.asp

    - http://www.k5td.com/K5TD/500kHz_Low_Pass_Filter.html

    - http://www.qsl.net/kp4md/lpfilter.htm

  6. I'm not entirely convinced you would even need a ham license to do this. There are always the lowfer, medfer and hifer Part15 bands. You might need to add an attenuator to keep the power down at a legal level though.

    Of course, that would require somebody to be out there to receive the signal.

    I mention this because I think that kind of project on the part 15 bands could be a great gateway to get the younger maker / hacker community interested in ham radio.

  7. The output is a square wave; chock full o' harmonics. The GQRP fiters you mention -are- 7 element and might just do the trick considering they whack down the spurious emissions from Class 'C' amplifiers which in effect are simply choppers. The waveform on a Class C amp. is pretty darned ugly, just a bunch of spikes basically, before it is filtered.

    73.......Steve Smith WB6TNL
    "Snort Rosin"


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