Receiving 24 GHz with the RTL-SDR
Over on YouTube Adam Alicajic has uploaded two videos which show an experiment where he successfully receives a test 24 GHz carrier signal with an RTL-SDR dongle.
In the first video he uses mixer setup to convert the 24 GHz signal down to 432 MHz, which is within the tunable range of the RTL-SDR. In the second video he uses a salvaged downconverter unit from some sort of communications device to do the same.
RTL2832 24GHz RX
24GHz downconverter mod