Rune, Paul --
All of the advice below assumes that the shack is as neat and clean as you
can make it. Balanced feedline for balanced (symmetrical) antennas and good
coax (hardline or good quality, well-shielded flex: RG-214) for unbalanced
(asymmetrical) antennas, good (low-Z at 160) ground. This means short, fat
conductors tied to a driven ground field (size and aftertreatment dependent on
local ground conductivity).
The DSL signal on the phone line is differential mode. You don't mention
how the modem is connected to the computer. Hopefully it is E10/100, also
differential, and not some single ended connection.
The RF is probably getting into the modem common mode. That is, the same
polarity on all of the I/O lines. Importantly, this includes the computer
interface and the power (full voltage or wall wart, either AC or DC).
It is important to treat ALL of the lines going into/out of the modem with
common mode chokes (all wires in each cable going through the same hole in the
core, at least one choke per I/O cable (More than one may be necessary.)
This configuration will have minimal or no effect on the differential mode
signals, so there is no need to have a sharp cutoff between the modem range
(<1.2MHz) and 160.
The chokes should be as close as possible to the modem. It is most important
to use a very high initial permeability material, such as J or 77 (VHF stuff
like 43 is practically useless down here.) Additionally, the modem itself
may not have a well designed PCB and its probably in a plastic case, so it may
pick up RF either inductively (magnetic coupling) or capacitively (electric
coupling). At these frequencies, inductive dominates, so magnetic shielding
is more effective, therefore I would try a steel enclosure, grounded, of
course.
Good luck,
Bill Gilmore WB8FPQ
Electromagnetic Compatibility Engineer
DaimlerChrysler Corporation (Retired)
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