Recently I ran across a neat hack. If you buy one of the low priced HDTV tuners that plug into your USB port, there are drivers that can use the internal sampler which runs at above 2.4Ghz to sample RF. Boom, software controlled radio.
The best part is that these only cost on the order of about $20, so you can start scanning the airwaves with a few different software packages:
SDR# – This is a C# program and runs great in windows, but you can run it using Mono in Linux (and perhaps Mac?)
GQRX is powered by gnuradio, and provides a good interface in Linux. Gnuradio is very powerful with routing features, many filters, and decoding possiblities, but if you’re not used to the Linux environment, it can be a little intimidating.
Because the sampler chip only will go down to about 50Mhz, I purchased a $50 upconverter from Nooelec, which provides an IF mode 100Mhz higher than the receive frequency. Simply put, it boosts everything by 100Mhz. So now I can listen to AM radio for example, that was 820Khz, far too low for the dongle to receive at 100,820Khz. Something well within the band of the dongle. This makes CB, Shortwave, and other HF and VHF bands available as well. Shortwave is very interesting. Lots of extreme paranoia emanating from Nashville these days.
The other thing that’s lots of fun is decoding the digital transmissions you’ll find. PSK, FSK, RTTY and even pager transmissions can be decoded.
Since the bandwidth is so huge, I purchased a discone antenna which works well so far. Soon I’m going to combine the dongle and upconvertor into a single box, which makes it easier and may improve performance as well.