I recently switched from isdn to dsl, which of course meant a new round of
rf filtering for the new dsl modem/router. Ferrites on the power and phone
line did pretty good, but not quite enough for the low 80m V and low 40m
beam, both of which kind of aim through the phone line and aren't that far
from the shack. While picking up some capacitors to build my own line
filter at, of all places, radio shack, the clerk actually came up with an
interesting product. They have a 3-30mhz rf line filter for 2 line phones.
(279-151). Its basically a balanced LC lowpass filter. At normal home dsl
speeds it worked fine right out of the bubble pack, I only know this because
for about 3 weeks my isp was fighting with verizon trying to get me up to
the 3mb business line speed. When they finally got the speed up to where it
belonged the line loss from the filter was too high. For those who don't
know about dsl it uses frequencies up to about 500khz. So I adjusted the
filter a bit by removing the 4 inductors for the 2nd line and putting them
in parallel with the 4 for the line I was using, this should effectively
reduce the inductance and shift the passband up a bit higher... this seems
to be working, it still has a bit more loss than I would like, but does
effectively block 80m and 40m.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
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