Noel,

One quick test that I didn't see mentioned - does the interference go
away when you disconnect the antenna connector to the radio? If it
does, then the interference is coupled in through an air path to the
radio antenna or antenna coax at RF frequencies. If it doesn't, then
the interference is probably coupled in from one set of power leads to
another. It is also possible to have power lead coupling at RF freqs,
and this is what RF chokes (beads) are intended to address.

Another very effective method of suppressing interference via power
leads is to use a separate power & ground pair from each instrument,
and twist each pair for most of its length. A properly done 'twisted
pair' is quite effective at suppressing 'common-mode' interference at
all frequencies, while a ferrite RF choke is only effective above
100MHz or so. I use a cordless drill to form twisted pairs as this
forms a more uniform twist. A well formed twisted pair will shorten
the normal wire length by a factor of 1.5 to 2. In other words, if
you form a twisted pair from two individual 5' long wires, you will
wind up with a twisted-pair cable about 2.5 - 3.3' long. If possible,
the untwisted portions at each end should be less than 6" (i.e. less
than about 0.1 wavelengths at 150MHz).

Hope this helps,

Frank TA