I've got some very low level signals at up to 150kHz going down some RG174 cable, which is pretty well shielded (85% coverage or more if I recall the cable spec correctly). It struck me I might be able to reduce noise further by adding a ferrite tube to the cable, which I've often done with unshielded wires. However, when asked to explain this to a colleague, I found I didn't know as much as I thought I did, and couldn't convey how ferrites act as common mode chokes to him.
On thinking about how coax works, with the inner and outer current cancelling each others' fields, I wondered if adding a ferrite might actually *disrupt* the flow of current round the coax and make interference worse rather than better - i.e. by increasing impedance to high frequency currents so that the coax shield was poorer at high frequency. And reading further about ferrite cable shields, they seem to be discussed mainly in terms of reducing emissions rather than reducing incoming interference. Another thing that makes me wonder if mixing ferrites and coax is bad is: articles discussing coax as a solution to interference make no reference to ferrite cores. I am now rather confused on the issue. If anyone has any experience in combining ferrites and coax I'd appreciate advice on this matter.