A couple weeks ago I was skimming ebay for antenna materials for my shortwave radio and ran across a PC card that upconverts shortwave for use in software-defined radio (SDR).
What is SDR, I wondered. After a few hours of reading on yahoo groups and reddit, I decided to give it a try and won an auction for a DVB-T (European television standard) USB dongle. Useless as it is in the USA (we use ATSC), but as SDR it works great. And it receives FM broadcast band, too.
I've had the dongle up and running for about a week now, hooked to the TV antenna with an amplifier, and I'm having a great deal of fun finding frequencies to poke into my old Realistic Pro-2005 scanner. The scanner will hunt for active frequencies just fine, but with SDR there is a graphical interface on your computer and the USB tuner can look at frequencies from 55 mHz to 2.2 gHz in 2 mHz chunks. This means you can record many frequencies within those 2 mHz all at the same time and go back and listen to any of them just by moving the software tuner with your mouse or keyboard.
As far as software goes, after reading all the information mentioned above I settled on SDR Sharp, a freeware program written in C#. I'm a long way from writing my own code but it's an education just to read the source code.
Here you can see the 2 mHz spectrum on top and "waterfall" below. The waterfall has time on the y-axis and signal strength on the x-axis. The bright yellow vertical stripe is the 2-meter amateur band where a couple guys are chatting. The yellow dots on the right are amateur radio data "packets" being relayed by the International Space Station at 145.825 mHz. The yellow horizontal blotch is interference from an FM radio station, haven't figured out how to filter that out yet (or if I really even need to).
The SDR has been most helpful on the aircraft band. Within just a half hour I had a couple dozen very active frequencies to use in one of the scanner's frequency banks.
To use this dongle on the AM broadcast (0.5-1.7 mHz) and HF shortwave (1.8-30 mHz) bands you do need an upconverter. Purchase price is 50 bucks but they appear to be easy to build for about half that much in parts. I think it'll add an interesting dimension, because the 2 mHz scan width of SDR means I could potentially record hundreds of radio broadcasts all at once (the AM band is only 1.2 mHz wide)! Not particularly useful, but still fun.
Note that ONLY the dongles that use the RTL2832U chip will work as SDRs, if you have a USB TV tuner already on your computer, it most likely won't work.
This is called "Poor Man's SDR." You can buy dedicated SDR units for hundreds to thousands of dollars if you're very serious about radio. But for the $20 you'll pay for the DVB-T dongle you'll have just as much fun, I'll bet.
If I had any complaints about SDR so far, it's only that strong signals from nearby FM or TV stations tend to overwhelm the dongle and you get images at many frequencies. Not enough to make it a bad experience, it's been quite a blast.