Software-defined radio, part II

A couple of months ago, I wrote about how a cheap USB dongle designed to pick up digital television broadcasts could be hacked to act as a software-defined radio, or SDR. With this dongle and a TV antenna, I could decode an impressive range of analog and digital radio transmissions from about 50 megahertz up to a little over 2 gigahertz [see "A $40 Software-Defined Radio," IEEE Spectrum, July]. But I wanted more. I wanted to get below 50 MHz, into the shortwave bands where signals can be sent vast distances around the globe by commercial, government, and amateur broadcasters.