Ham It Up upverter
If you look at the specs, the SDR dongles' tuning range is around 24 to 1850 MHz (approximate on both ends). The good news is that the bottom end is capable (supposedly) of picking up the 10 meter band, plus chicken band (CB), FM broadcast, and on up. The bad news is that if you want anything lower, like 160, 80, 40, 20 and 15 meter amateur bands, AM commercial broadcast, and many world band broadcasts, the dongle just cannot tune low enough to get them. Anything below 30 MHz is called High Frequency (HF), and even though the dongle goes down to 24 MHz, that still leaves most of the HF band untouched.
The way around this is to make or buy an upconverter. This takes an input frequency (say 10 MHz) and converts it up to a frequency that the dongle can hear. It works on similar principle as a superhet radio: you have a local oscillator that adds a steady frequency to the input frequency, and what you get out is the sum of them both. e.g. If you tune to 10 MHz, the local oscillator adds 125 MHz, you get 135 MHz output, which is something the R820 can easily tune. The difference is that on a superhet radio, the local oscillator varies and tracks with the tuned frequency, and you take the difference, so you always get a single frequency out (say, 455 KHz). With this one the LO is fixed and you get a variety out.
KF7LZE.net has a nice roundup of Upverters. The first is the Ham It Up, v1.2, which is what I'm discussing here.
more to come