> In my own quest for rf proof phone lines, I too went the
CAT 5 route and
> had little
> luck. What I finally did was twist all the unused wires
together at the
> ends of the runs where
> the phones are and then ground the other end at the
service drop.
What you actually did was make a crude low-pass filter.
This illustrates my point perfectly!
Using ONLY a series impedance is not a very wise thing to do
unless the RFI is reasonably low to start with. In order to
be an effective solution, there has to be a low SHUNT
impedance and a high series impedance for RF.
The type of cable or number of twists per inch makes VERY
little if any difference in RF response, because the wires
are so close together for distant sources they already look
like one conductor. The increased twist helps mainly on
pair-to-pair crosstalk, much less on cable to adjacent cable
crosstalk, and does virtually nothing at all more than a few
inches away. It is all really a conductor spacing to source
distance problem of dimensional ratios, so the advice to use
CAT5 for RFI even over close-spaced parallel untwisted lays
is not good advice at all.
Neither beads (even with multiple turns) nor in-line filters
make much sense unless the series RF impedance is extremely
high and very resistive, or you include a shunting
capacitance to earth or the chassis of the protected device
to control common mode impedances.
73 Tom
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