Topband: 160m noise

[Jim Brown]

On Wed, 3 Nov 2004 18:41:53 -0800, Garry Shapiro wrote:

>I found that most poles with transformers--and ground wires to the
>transformer--are radiating, but that they vary greatly in amplitude. 

Simple physics -- you cause RF current to flow in a wire, and it WILL radiate. You may 
CALL it a ground wire, but Mother Nature knows that it is an antenna. 

>Some of
>these sources appear perhaps to emanate from telephone lines (!)---ADSL? But
>the big surprise was to return home and get a huge indication from....my own
>shack. The source was my LInksys 4-port Router/Switch---or the Network
>Interface Cards (NIC's) in the computers. The noise appears to be common
>mode radiation remanating from the CAT 5 cables connecting the computers to
>the router/switch.

>My problem is that I cannot conclusively identify the NIC or Linksys as the
>culprit without wholesale substitution. My inquiries to Linksys have
>resulted in the usual nonsense one receives from "Technical Support" (sic)
>from vendors. In this case, Linksys regurgitated Part 15 back to me and
>suggested a firmware upgrade (!) This is reminiscent of outsourced foreign
>tech support people telling you to reboot Windows. I do not seem able to
>convince them that this is a hardware problem.

>Has anyone on this reflector experienced a similar problem? 

Yes, these beasts are well known as HF noise sources. Linksys is far from the only 
culprit -- there are many. 

>Can anyone offer some insight --specific or general--re signal performance of 
>NIC's or routers in this context?

Pull the power plug on the Linksys and you will notice any of the trash it produces go 
away. What remains is probably coming from your neighbor's router(s). I've identified 
birdies on several HF bands, the most prominent being those around 14.030 and 
21.052 (there are also some very strong ones on the bottom of 10 CW and on 30 
meters). I don't work much SSB, so I haven't cataloged the ones that land in the phone 
bands. 

Yes. What you are experiencing is common-mode radiation from those cables. What 
you need is a bunch of ferrite chokes right up next to each and every device on that 
Ethernet system to raise the series impedance of the line as a common mode antenna. 
I've done this in my own shack, and it works very well. I plan to write a technical article 
for QST on this topic when I have time. 

For 160, a very effective choke can be made by winding approximately 10 turns around 
a 2.5" OD #43 toroid. Fewer turns are needed for higher bands, and the choke that 
works on 160 will be ineffective on higher bands due to stray capacitance between the 
windings. So if you work all HF bands you will need multiple chokes in series. As you 
can see, this can get expensive. :)  But the router is cheap!  

Are they in violation of FCC Part 15?  Probably not -- the most stringent emssions 
regulations apply from 30 MHz up!  

73,