What are software defined radios ?

I had never heard of SDRs until today. But now that I know it, I can understand why some folks are so excited about it. This is almost like a swiss army knife for the radio hackers.

The HackRF can shift between different frequencies as easily as a computer switches between applications–It can both read and transmit signals from 100 megaherz to 6 gigaherz, including frequencies as low as the range used by FM radio up to the gigaherz frequencies used by Wifi or experimental wireless protocols for cars communicating in traffic. In between those bookends lies everything from police radio to cellular signals from AT&T; and Verizon to garage door openers–all signals that HackRF can instantaneously intercept or reproduce.