[RFI] 160 Meter Interference

[Jim Brown]

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