Outside Wiring for Cable Modem

[futureal]

Hi,

Not sure if anybody here has done any outside coax wiring on their homes before, but if you have, you might be able to help me.

We just bought a home and moved in. I'm trying to re-configure the wiring so that I can get my three "inputs" running in harmony: cable modem service, DirecTV and VHF/UHF antenna (for HD local signals).

Right now, the cable modem is wired as such:

Direct line from the community cable box to the local box on the back of the house.
Line hits a standard grounding block, and then follows to a 2-way splitter.
One split line (we'll call this Out-A) goes directly to the cable modem in our "office."
One split line (we'll call this Out-B) goes down to the basement where it is split a further 3 times, each running to a room in the house.

So here' what I want to do:

Run the cable modem line directly to the grounding block, eliminating the splitter.
Run the over-the-air antenna line directly to the interior splitter in the basement (with a second grounding block).

And here's the problem:

The minute I disconnect the Out-B line (see above), the cable modem (on Out-A) stops working.
*OR*
The minute I remove the splitter and connect the cable modem line directly, the cable modem stops working.

As soon as I put the splitter back in, with both Out-A and Out-B connected, the modem picks right up again and I'm back on.

Does anybody know why this would be occuring? Or have any suggestions as to what I need to put on the line to get the modem working again, *without* using the splitter?

Thanks!

 

[ScottMac]

It sounds like the line in is a little hot (too much signal). When you remove the splitter, the signal (at least) doubles.... overloading the cable modem.

It might also be ground loop or other noise related. The only way to tell for sure is to call the cable company and let them come meter it out and re-balance the line.

You can buy attenuators, usually at 3, 6, 9, 12 db increments to bring the signal level down. From what you've posted so far, a 3db attenuator should work Ok, 6db might be better is the signal is so hot.

If it's a grounding or noise issue, there's a number of corrective actions possible. These tend to be fairly specific remedies, depending on the actual causes.

Good Luck

Scott

 

[futureal]

Thanks for the information. I know a little bit about this stuff but this is really the first time I've dabbled in any of it hands on.

The current splitter was marked as 3.5 dB at each output, so I'm guessing that means it is stepping down the signal by that much as it passes through. I'll try a 3 dB attentuator and see if it makes a difference before calling up the cable techs.

 

[PowerEngineer]

I'm thinking along the same lines as ScottMac. A passive splitter essentially splits the incoming signal's energy into two signals that each have half that energy (which is equivalent to 3dB attenuation). If you don't intent to use that splitter somewhere else, you should also be able use a cable terminator on Out-B. The terminator is essentially a resistor that matches the impedance of the coax. It helps eliminate the signal noise generated by open outputs and should give you back that 3 dB. If that doesn't work, then it may be a shield grounding problem...

 

[futureal]

I checked at Radio Shack and all they had were 6 dB attenuators, which I tried, and it didn't do the trick. The cable modem's signal level dropped to -19 dB, where it was previously in the -13 (or so) range when it was working. The modem also gives data on a signal-to-noise ratio, but I'm not certain on how to interpret that accurately.

I didn't think of the terminator, I might try that next if I can find one. I can't think of any other places offhand to by an attenuator, other than ordering one. Radio Shack also has a bidirectional adjustable attenuator, but it says on the package that the minimum return loss at 1000 MHz is 8dB, so I think that's out of the question.

Any ideas on other local stores that might have something like that? A hardware store like Home Depot, perhaps?

 

[PowerEngineer]

I've noticed that some splitters thoughtfully include terminators for any unused outputs; using them reduces signal noise that is produced when the signal running down your 75 ohm coaxial cable crashes into the infinite impedance of the unterminated output. You might find cable terminators in the video section of someplace like Home Depot. If you can't find one, then find a resistor of roughly 75 ohms and connect that from the central hole of the output across to its outer threaded body (that's really what a terminator is). I'm a little surprised that the 6 dB attentuator didn't work...

 

[futureal]

I noted that some of the fancier adjustable attenuators like the one I mentioned above noted that they were bidirectional. Is it possible that the little 6 dB inline one is not?

I will take a look at Home Depot today to see what I can find.

 

[ScottMac]

No, they're just resistors on a connector ..... inherently bidirectional.

I'm not clear on what you did. Was the signal -13 with the splitter in?

Worse case, if you were at -13 with the splitter, the straight line signal should have gone to ~-9 (recovering some losses from the splitter) and gone to -15/16 ish with the 6db attenuator in.

If it was -13 with the splitter out, then adding the splitter should have taken it to ~-16 ish, then with the attenuator ~ -22db.

It might end up being less trouble to cable it the way you wantit, then call the cable company out to balance the line / adjust the drive to you cable to match your cabling. It isn't rocket science, but having the right tools ($$) and the access to the boxes makes a big difference. It'll cost you a service call but, in theory, it'll be done "right."

They fed my BB cable in a little hot (+12), to cover the setup I told them I was going to use (I, um, kinda tricked 'em). I have access to a Field Strength meter, so I do the balance job in my closet.

Good Luck

Scott