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20m and Up Shut Down Cable Modem

Aug 28th 2011, 00:09


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I have had slight issues with RFI in the shack ever since I built it - RF in computer speakers.

Recently, I noticed that when I transmitted on 40m, I could not transmit on 2m. This doesn't happen often, but if I'm transmitting JT65 on 40m, I have opportunity to talk on the local 2m repeater too. Currently, the 2m antenna is a 300ohm twin-lead j-pole in the attic. I figured the RFI getting into 2m was due to the j-pole (it still may be). However, it got me investigating.

I checked the resistance between AC neutral and my ground bus bar in the shack. AC neutral and ground should be tied together at the meter box, so I was expecting a small value. I got infinity. I decided that was a bad sign so I went to the meter base and investigated the ground. I pulled it and it came out of the ground. "Hmm... this cannot be good," I thought.

I dug down today and found where the ground clamp had corroded to an oxidized mass of "blob." I cleaned off the ground rod and the ground wire and connect the wire to the rod with a copper/bronze clamp.

Well, 40m still interferes with 2m. I'm suspecting the j-pole at this point. The birthday is coming up, so maybe someone will get me a nice dual-band antenna.

All of that said, I now have a NEW problem (the topic of this post). When I transmit on 20m and above, my cable modem shuts down. I took the F-type connector off and crimped a ring lug on a ground wire and connected it to ground - no change. I verifed that the case and the negative lead on the power source were at 0ohms. I disconnected the cable input to the modem and tried again, the modem did NOT shut down. So, the problem is fixed as long as the modem is not connected to the cable, but this does not solve the problem of getting on the Internet. I did find that the lightning protection outside on the cable had infinity on it between the shield and ground. I cleaned both of those connections up and and reconnected. Resistance went to 0ohms.

Anyone have any other ideas what to try? Oh, I forgot to mention, this happens on either my Mystery Antenna or my Hexbeam. I'm fairly convinced my cable is radiating, but how do I fix it?

Joel - W4JBB
Aug 28th 2011, 00:39


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Ok. I have been doing a little more reading. Seems if I put type 43 ferrite beads on the cable modem input, this may reduce or eliminate the RFI going into the modem. Is this correct?

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