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Author Topic: RFI into a CAT5 cable?  (Read 5794 times)

Posts: 129

« on: May 09, 2012, 12:04:25 PM »

OK, this is a new one to me . . .

I hooked up a loop for 80 meters around the perimeter of the back yard a week or so ago. This version of the loop is temporary - I want to put it a little higher up than 10 feet so I might be able to hear a few more stations with a little less noise - so there's a bit of improvisation involved, especially in the feed line to the antenna tuner. Which is to say, there isn't a feed line. I just pulled the ends of the loop into the shack through the open window and hooked them to the tuner inputs. I know, this is Bad Amateur Practice, but like I said, it's temporary while I figure out exactly how I'm going to elevate the loop, at which time I will hook up a length of ladder line ending in a 4:1 balun and run coax into the shack/bedroom.

Anyway, hopefully this sets the table for the story. The other day my wife complained that her computer was beeping at her. It wasn't a normal PC speaker beep, in fact it was like no beep I had ever heard, and it was intermittent. It would beep for a while and then stop.

I eventually put two and two together and figured out that it was only beeping when I was keying the transmitter. Specifically, when I was transmitting a JT65 signal on 20 meters. For those of you not familiar with digital modes, JT65 transmits a continuous set of tones for about 48 seconds at a time. It's sort of like holding the key down in CW.

I think what's going on is, for a couple of inches the improvised "feedline" runs parallel to the CAT5 cable connecting my wife's computer to the router. I think the RF is getting into the CAT5 cable and from there to the network card in her computer, which is intentionally or not detecting this as something it doesn't like and is sounding an alarm.

I had a similar problem when I had to run a 50' CAT5 cable from the router in the bedroom to a network-enabled TiVo in the front room. People were complaining that I was "messing up the TV" when I transmitted on 17 meters with my dipole, but only when the cable was hooked up. My solution, sadly, was to stop transmitting on 17 meters when the cable was attached. (I don't seem to have that problem with the loop.) The interference didn't happen on any other bands, and the beeping only happens when I transmit on 20 meters.

So I appear to be throwing some RFI. Good thing I'm only running 30 watts.

My questions are:

1. Is CAT5 especially susceptible to picking up RF at HF frequencies? It's cheap generic CAT5, not shielded or anything.

2. What's the best way to fix this in the short term? I'm thinking about putting a couple of snap-on ferrite cores on the cables near where they connect to the computer or the TiVo.

3. Has anyone ever heard of net cards beeping like that? Is it usual? Common? I'd never seen it in 30 years in computers but then again I'd never run a working ham radio station in close proximity to a networked computer.

Thanks for any help you can provide.

73, Creede

Posts: 1819

« Reply #1 on: May 11, 2012, 02:08:03 PM »

Cat 5 isn't shielded as you already know.  I have seen one instance where replacing standard Cat 5 with a shielded version remedied a problem with my DSL modem and wireless router connection.  Do you really need an ethernet connection or would a wireless connection suffice?  I think you're looking at this in the wrong way. You aren't "throwing RFI" - you simply have devices that are responding to HF signals when they should not be.  You might want to try snap on ferrites, or wrapping multiple turns of the wires through some of the Radio Shack split core units.  Make sure you treat the device power cables in the same way, as well as any other cables that are connected to the device having the problem.  You may be able to disconnect cables that are not needed and see if that solves the problem.  If so, you'll know that the RF is getting in through those cables. 

73, Floyd - K8AC

Posts: 1208

« Reply #2 on: May 12, 2012, 06:22:34 AM »

From a computer guy, ethernet is RF on twisted pair now. The original ethernet to which all the variants are related started out in RG8 diameter special coax.

Depending upon the number of standards to which an interface card is compatible and the amount of signal that enters the cable, it has varying susceptibility to RFI. The network collision (two cards trying to send at once ) detection is the most vulnerable.

The category of the cable is a standard for loss per foot at certain frequencies and also of the twist rate of the cable pairs. The twist of the pairs is used to "sheild" the signals while minimizing the loss per foot. Ethernet shares the use of a broad swath of HF frequencies with everyone but low power and tight control on the better cabling reduces the interference factor.

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