Meeting: 28th October 2013 – software defined radio on Linux

Host: Dave Nelson

This is a copy of the talk I gave to the Rossendale Linux User Group on Monday 28th October 2013, the topic is “Radio on Linux” intending to provide an overview of some of the amateur radio and software define radio technologies I have been playing with during the summer.

Radio on Linux


Hardware required

R820T Digital TV Tuner

A very cheap digital TV tuner that was discovered to allow hackers the ability to read raw radio i/o for software processing.


  • House TV antenna is fine for VHF/UHF.
  • Long-wire antenna for low frequencies under 30MHz, 20 metres of thin wire.
  • Can get quite complicated, lots of information online
Optional and Alternatives

e4000 TV tuners

Earlier DTV tuner chip that is similarly capable as newer R820T


  • 30MHz – 6GHz (wider frequency range)
  • 20MHz sampling bandwidth
  • RX/TX (can also transmit with appropriate licensing)
  • Costs ~£200 when available

Softrock SDR kits[3]

  • DIY electronic kits for self-assembly
  • Various capabilities and prices

“Ham It Up” v1.2 Up-converter[4]

  • Open Source hardware
  • Enables access to 0-30MHz frequencies on other SDR hardware
  • Requires a “pig-tail” lead to link with SDR receiver
  • Costs ~£30

Software Set-up

Blacklist DTV module

Linux may try to load the v4l module for the TV tuner card, this will claim the hardware preventing access by the SDR driver, so we need to disable it

On Ubuntu and derivatives:

$ rmmod dvb_usb_rtl28xxu # unload module if already loaded
$ sudo echo 'blacklist dvb_usb_rtl28xxu' >> /etc/modprobe.d/blacklist.conf # add to blacklist

Install the applications


GQRX is not in Ubuntu repositories, it is also under rapid development so it’s best to compile from source. GNU Radio in Ubuntu 13.10 should be adequate, compiling from source takes a long time!



Fedora has a ham radio special interest group maintaining a lot of packages. Currently GQRX does not seem to be included.

Using the application

  • Once loaded turn it on
  • Beware that frequencies are badly calibrated
  • Zoom into the frequency band by scrolling up on the frequency axis of the top display
  • Use “SQL” to squelch out noise to stop irritating your neighbours, the noise floor changes at different frequencies so may need to be reset
  • Don’t forget to set the right modulation type
    • WFM for commercial FM (Wide-band FM)
    • NFM for ham radio and CB Radio (27MHz), thought some illegal CB is AM
    • Airband is all AM
    • LSB (Lower Sideband) for ham radio < 10MHz
    • USB (Upper Sideband) for ham radio > 10MHz
  • Learn to recognise signals by their sound and pattern on the waterfall
  • Find numbers stations like “The Buzzer” we listened to at 4625KHz

Things to see and do


Try out listening to and decoding radio signals without paying a penny. WebSDR connects remote SDR hardware to the Internet so any user can tune in (without impacting others!).

Go to

My favourites:

University of Twente –

  • Full HF frequency range available
  • Based in Netherlands – Good location to hear all of Europe

Hack Green –

  • Based in Cheshire, similar results to what can be heard locally
  • Covers main HAM radio bands

What to see and do

  • Discuss what you can hear with other listeners
  • Find interesting signals, my favourite is polytones


Used for encoding and decoding digital modes such as Morse or more recent phase-shift-keying and others.

It’s designed for sending and receiving but fine to just receive

Digital communications tend to follow a standard format, don’t expect deep conversations going on (Rag-chews – in ham terminology)

Most common digital modes are:

  • Morse code AKA “CW” or “continuous wave”
  • BPSK31 – phase shift keying, 31Hz wide, very narrow on frequency waterfall
  • RTTY – two tones but wider apart than BPSK31

Keep trying different modes until it starts outputting readable text, you’ll get an ear for what different modes sound like with experience

 Other Applications

  • GNU Radio – build simple and complex radio systems, endless possibilities
  • Sigmira – Decode NATO STANAG 4285 modem signals
  • – See how APRS is translated with Google Maps
  • Dream – Digital Radio Mondiale on shortware frequencies
  • 433MHz home automation

Useful Links

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