When I was growing up, one of my favourite birthday presents was a Uniden UBC3000XLT radio scanner. I enjoyed finding and listening to various radio transmissions, and I had some success decoding data transmissions and morse code using my PC and Sound Blaster soundcard.
I was interested to read about a technique called Software Defined Radio (SDR), which can sample the entire shortwave spectrum, and use the processing power of a computer to virtually ‘tune’ in to transmissions.
A home-built SDR board operating at the University of Twente in the Netherlands (pictured below) and a receiving element around 5cm x 10cm is capable of receiving a frequency range of 0 – 29.160 MHz.
It uses a Spartan XC3S500E Field-Programmable Gate Array. FPGAs can be programmed using a hardware description language, and arrays of programmable logic blocks which can be virtually inter-wired. The most important advantage over a microcontroller like a Atmel/Arduino, is these operations can happen in parallel, which is critical to creating a software defined radio, as a typical AVR microprocessor simply doesn’t have the necessary clock speed. The sample rate must be at least twice the maximum frequency of the signal (Nyquist theorem).
Taking this a step further, Software Defined Radio can allow multiple users to share the same receiver. At time of writing, 184 users are listening to different frequencies using the receiver at the University.
You can listen for yourself at the Wide-band WebSDR at University of Twente. It’s essentially a free radio scanner anyone can access for free. You might enjoy stumbling upon a variety of transmissions from all over the world, and you may have success using tools like Sorcerer to decode data transmissions, weather fax transmissions, and morse code.
With the ever-continuing advances in data storage capacities and scalable clouds, one day it may be a trivial matter to archive the entire radio frequency spectrum.