On Wed, 6 Oct 2004 03:21:31 -0500, Dale J. wrote:
>I had a signal on 75 meters which I traced to my eithernet cable
Yes. 10BaseT traffic on an ethernet cable will radiate strong carriers
throughout the
spectrum. Some I have identified are around 14,030, 21,052, and 28,014. There
are
also carriers on the low end of 80 and in the 30m band, but I've managed to
eliminate all
of mine so I can't find them any more. As I recall, those on 80 were around
3,506 or
3,510. On 30 meters, 10,114 and 10,122 sticks in my mind, but that may not be
right.
I'm sure there are other carriers associated with Ethernet, but these are the
ones I have
identified (I mostly work CW).
These will be present on 100BaseT systems if there are 10BaseT adapters. Most
Internet modems (for cable and DSL) use 10BaseT to connect to your Ethernet
system,
and older computers may also have a 10BaseT adapter.
The carrier frequencies will be different for different modems or adapters. I
have two 20
meter dipoles, one behind my house and one over the front. Different carriers
around
14,030 are audible (or are louder), depending on which antenna I am using. I
strongly
suspect at least one of them is from a neighbor's cable modem.
The mechanism for this radiation appears to be mostly (if not all) common mode
so this
stuff can be choked rather effectively by winding the Ethernet cable several
turns around
a ferrite toroid very close to the adapters at each end of the cable. The lower
the
frequency of the carrier, the more turns you will need. If you have too many
turns, the
stray capacitance around the choke can render it much less effective at higher
frequencies. I have multiple chokes on each cable that is a source of
interference.
These lines radiate trash well into VHF -- you'll hear lots of hash if you
bring your 2m
talkie close to an Ethernet cable. I hear stuff on 6 meters (especially between
50.1 and
50.125) that I strongly suspect is Ethernet.
The big (2.5") ferrite toroids sold by Amidon and others are mostly Fair Rite
#43
material. 6 turns around one of these is pretty effective on 80 meters, and a
few more
turns will make it more effective on 160. Many of these carriers used to be
20-30 dB
over the noise level. Now that I've choked themm many of them are gone
completely,
and the rest are down within 5-10 of the noise. I suspect most of what I still
hear is from
my neighbors. That's my next challenge.
Jim Brown K9YC
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