Here's a picture of my SDR receiver setup...
My understanding of a discone antenna is that the downward diagonal elements are supposed to act as an arbitrary, floating ground plane against which the vertical elements resonate.
This setup works as it is but I'm getting problems with pickup on the metal case of the SDR. If I put my hand on the metal case (anodised Aluminium enclosure well connected to the ground plane of the receiver PCB) then the noise floor on my HDSDR software jumps up massively. What was a relatively quiet waterfall becomes a bright red mess as soon as I lay my hand on the case. That said, I cannot hear any difference in the demodulated signal (2 Meter band, AM for instance).
It seems in my mind like this pickup should be prevented because of the connection to a real earth ground (via the PC's mains power supply). There is good connductivity between the diagonal elements of the Discone and earth ground (less than 2 Ohms at DC, according to my multimeter).
The USB is about 2 metres long, the Coax is about 3 metres. All connectors are soundly fastened and have good conductivity.
Edit: The SDR device is entirely USB powered.
It is clear that the PC's USB connection is pulling the metal case down to "Earth ground". Is this detrimental to the use of a discone antenna? Is it bad to have a metal case on an SDR? Is there anything I can do to improve this setup?