yes it is illegal listening to any private conservation is illegal,,, you said cordless phones not me,,,,,,I asked a honest question is there anything that will go that high what are there names? The R2600 is not $25,000 the ones Ive seen are around 15k
5.8Ghz cordless phones are also digital/spread spectrum. What else would you be listening to up there? 6Ghz inputs to C-band satellites (they downlink on 3.7-4.2Ghz). There's also the new public service agency WiMax type system at 4.9Ghz, TV news crews' microwave from the trucks at 7Ghz (digital MPEG pictures), etc
You can also use a heterodyne converter and a normal receiver as an IF. Here's the problem - anything at 6 GHz will NOT be analog FM, AM, SSB or CW. It will be multiplexed digital in one form or another. You won't be able to demodulate anything using a conventional (or even $25,000) receiver.