Coaxial Cables -- Does the outer sheath HAVE to be connected?

Factual Questions
1

I just got cable internet. The only cable jack that was working was in my kids’ room, which is undesirable because they could mess with it and because the wifi signal strength to the rest of the apartment is marginal. Rather than try to run cable wire all through the apartment, I went outside and figured out where the other wires ran and where the wire with the feed is.

The outside wires were cut straight off in 2 different locations, but in each place there is a wire nearby with F-type connector and a female to female connector or splitter as well. I cut into the coaxial cable and pulled ~1/2 inch of everything off the inner wire and just put the inside wire into the female to female connector/splitter (in both places). In this way I (I think) connecting a cable jack in a different room to the “live feed,” but only by inner wires. But the cable modem still couldn’t connect to the internet.

So my question is, does the outer sheath of the coaxial cable also have to be connected, or is it just for reducing radio frequency noise from the environment? If it does have to be connected, why? I know I could try putting on new F-type connectors, but I’m afraid there’s another problem too. (Is using a splitter a bad idea?)

2

The sheath is rather ineffective at it’s job if it isn’t grounded, it grounds via the F-connector. It may still work without being done properly. Digital signals need good connections, splitters are fine assuming they aren’t knocking the signal strength too low to be functional. The higher rated splitters do have more copper to them so impact signal strength less. If you used one of the older ones that could cause your issue.

Pick up some screw on F-connectors and if needed a newer splitter. it’s a $10 dollar investment and may solve your issue.

3

I believe it also acts as the ground - I’m not sure of the implications on safety, but I know when I used to work swapping cable modems - we’d often come across houses with older rg-59 cable and they would have to be swapped with RG-6 to work correctly.

We had tools (that are cheap) which basically had a thing that cut upto the core and you’d swivel it around and pull the braid down as you would foreskin on a penis. Then you’d crimp this (the wire not the penis) with a new connector (just the outside part)

This in and of itself would often fix connectivity issues. So I’d say even if it isn’t technically required - you should do this.

These tools are cheap and take very little practice to use.

Each splitter takes a certain amount of signal loss (usually printed on the splitter). You also want the splitter to be rated for a high enough frequency range (I don’t remember the number we used, but I’m pretty sure anything sub giga hz we would swap out).

So pick splitters with the lowest db loss and highest frequency range you can find.

Back when they were working on docsis 2.0 I think anything in the range of -15 to + 15 db at the modem was considered acceptable. You can usually find the menu in the cable modem that will show the signal strength. You should also be able to find in the specs the amount of signal strength required.

So if you are getting +12 db at the closest point to the cable provider - you can split several times and get down to -15 (we’ll say -14 to be sure) and you should be good.

But if you are getting -10 - you might only be able to get one split out of it.

Putting on new female to female (we called them barrel connectors), crimping new ends on, and swapping out splitters seemed to fix many, many problems.

No one I worked with would have ever considered not using the sheath - but I can’t tell you the reasons why other than it acts as a ground and provides shielding (both in and out).

I wouldn’t expect it to work properly without it, but it might work in certain situations, but why bother.

4

Yes, it does need to be connected. From one perspective, imagine that you are connecting an electrical circuit. The sheath is the ground, and it needs to be connected in order to receive a signal - just like you can’t power something from a battery by connecting a wire to just the positive terminal.

Separately, the coaxial cable is carrying RF frequencies, not just DC current, and even ensuring DC connectivity is not sufficient. By stripping the shield and connecting just the bare conductors, you have introduced the potential to both generate and receive RF interference from numerous devices in your home (and neighbor’s homes), and likely changed the impedance of the connection, which may interfere with signal propagation across the connector.

Get the proper connectors and the proper tools to install them.

5

To be clear - a cable television / internet system works exactly the same way, using the same principles, as broadcast television, AM/FM radio stations, cell phones, police radio communication, etc. It communicates using radio waves. However, instead of transmitting radio waves through open air with antennas, it uses “shielded” RF transmission lines (like coaxial cable) to transmit those radio waves from the cable company to your modem, TV, or whatever. This allows the cable company to use a broad range of radio frequencies, including essentially every frequency used for radio and wireless communication, without fear of interference, because the RF signal never leaves the coaxial cable, and the coaxial cable is shielded from interference from those outside signals. Coaxial cables and their connectors are specifically constructed to ensure that they do not radiate or receive radio signals: this involves primarily maintaining a precise geometric and electrical separation between the center conductor and the outer shield.

By building your own connector that is not properly constructed or shielded, you have essentially built a tiny transmitting and receiving antenna, and exposed your cable connection to all the radio signals being sent through the air on the same frequencies by cell phone companies, broadcast TV, your city’s public works department, you name it. You are also exposing any device in the immediate vicinity to interference from the cable TV/internet signals that would ordinarily not leave the cable. It would be interesting to hold your cell phone pretty close to that connection and see if you still get a signal.

6

You can sometimes get signal to make its way down coax with breaks in it, simply because there is enough capacitive coupling that you get a little bit of signal jumping the gap. But there will be a significant impedance mis-match at the interface, and most of the signal will be reflected back.

RF running down coax isn’t quite like a simple as at small wavelengths the direction of the current in the core and the shield will be alternating so fast that one part of the core will have current running one way and a bit further along it is running the other. The signal is really embodied in the fields between the core and the shield. But in order to send the power down the cable you need to have both core and shield working together.

Eventually you get to the point where the only thing you need to do is get the EM field to run down the tube that is defined by the shield, and you don’t need the core at all. The these frequencies it gets to be called a waveguide. Engineers that work at these sorts of frequencies regard anything that will actually go down a wire as DC.

7

Well, for others who may want to know the final results, I cut through everything to the bare wire, leaving about 1/2 an inch of bare wire. Then I cut back the outer rubber (with a knife) so that the plastic dielectric would be flush with the inside hole of a crimp connector. I did find the braided wire under the rubber this time, so I pulled it back and cut it off (but left the inner foil-like substance on the plastic dielectric). I then slid on the F-type crimp connector and used pliers to squish it down to the foil and dielectric. I then cut back a little of the bare wire so it stuck out a little past the connector.

I did that on both ends of my cut off coaxial wire, and it worked. The cable modem is getting a decent signal in another room now.

In the end, though I think a screw-on connector might have worked better since I didn’t have the crimping tool. In fact, I one of the crimp connectors split on one side due to being flattened by the pliers. But as long as it works I’ll leave it for now.

Thanks for the help.