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Several years ago, I wired coax cable connections to the main unit of my house and the basement apartment. The cable company ran two cables from an alley pole to the entry point on the house and I took it from there. Inside the house on the other side from where the outside box is to which the alley cables go is a receptacle box in which the inside two cables are connected to what is outside.

Since no one lives in the basement, I didn't get an extra service. The main unit service works fine going through all the same checkpoints (receptacle boxes and entry points). Since I am getting ready to finish the basement, I wanted to test that the basement connection from the inside receptacle still works and that some 30' of cable that runs from the entry point to the basement media box still works, particularly that it hasn't been cut by accident or rodents chewing, before I seal it with drywall.

So I disconnected the main unit connection to the active cable coming in (vs. the one that will be activated once someone actually lives there). However, the modem on the other end of that cable (basement) could not connect. The modem does connect when I take it upstairs and connect it directly to the splitter. When I did this first, several years ago, I performed the same test and it worked. Now it doesn't. It's as though the cable has been cut off somewhere on the way.

Are there alternative ways to test two ends of a coax cable? Like running some current through. I have an organic intolerance for visible wire/cable so I did a lot of remodeling just to hide it, which means my cable is buried behind the wall -- while aesthetically appealing, it makes it a hell of a lot harder to troubleshoot and repair. I am trying to make darn sure the cable is cut before I tear down drywall and reinstall.

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I would disconnect and check it with an ohm meter it should show open for a dc test, if that is good short the end and make sure it shows shorted at the other end. It could be a nail or screw causing the problem or totally cut in half so both tests would give a basic idea on a gross failure.

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  • If you can't identify the problem with a meter, try replacing the connectors at both ends of the cable. It doesn't cost much and there's a reasonable chance it'll fix the problem.
    – mrog
    Dec 4, 2018 at 23:38
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Television type coaxial cable is considered to have a 75 ohm impedance. There are 75 ohm resistors contained within a threaded housing, called a 75 ohm coax terminator.

coax terminator

Picture above courtesy of Show Me Cables.

It may be necessary to add an F-81 barrel connector if you are testing a coax that has a fitting attached. If not, you would need only a bare 75 ohm resistor across the center conductor and ground braid/shield.

Using a multimeter set to resistance reading, you should see a figure from 70-80 ohms. If it is "infinite" (open circuit) there is possibly a break in the cable. If it appears shorted, there may be a cable splitter in the run.

Of course, if it is genuinely shorted, you will know that only if you are certain there are no splitters in the circuit.

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  • A coax cable doesent have an impedance to dc. It will read open with no connection. It also depends on the type of coax used it could have 50 ohms charistic impedance. But most tv use rg6 that has 75 ohms charistic impedance (not resistance)
    – Ed Beal
    Dec 4, 2018 at 20:26
  • television signals are not DC and RG-59 (old) and RG-6 (current) coax is usually used for television signals as you've noted. That's where the impedance reference originates, more or less. Standard in cable tv is the 75 ohm resistor, for reasons provided to me almost 40 years ago, but long forgotten. Splitters are usually (not always!) DC blocking, hence the shorted reading when one is in the line.
    – fred_dot_u
    Dec 5, 2018 at 1:07
  • The op has a single length of cable. The best way to test is with an ohm meter. As an ohm meter is DC you will only measure resistance not impedance. Impedance is the resistive value in an AC circuit.
    – Ed Beal
    Dec 5, 2018 at 13:13
  • Just passing along what 20 years of experience in cable television has imparted to me. It's correct that this is a test of resistance, hence the 75 ohm resistor. It tests continuity and also confirms the correct cable has been selected for the test.
    – fred_dot_u
    Dec 5, 2018 at 23:01
  • The resistor is bunk and a total waste of time it has nothing to do with testing the cable. I have worked on high power RF unless you have a signal analyser and can look at the Smith charts to see any problems a basic resistance test is all that is needed in industry we use 4 point probes that are much more accurate than a standard ohm meter then tune the systems with matching networks under power but the DC test is always done first so a 50k+ power supply is not fried.
    – Ed Beal
    Dec 5, 2018 at 23:11
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As I researched how to check RF signal strength without buying expensive meters, I realized that the cable modem itself is measuring this. Just log into the modem by connecting a laptop directly and, at least on mine (Motorola mb8600) you can see both upstream and downstream channels and the signal strengths.

I went around the house, including right at the street, to discover where I was losing signal. It turned out some of it was due to unnecessarily long cabling which I reduced and gained back 2dB.

So far this seems to be enough to prevent “T3” type connection drops that prompted my investigation. Anyway, besides my neighbors thinking I’m crazy to have my modem and laptop outside doing checks, it was fairly painless and cost nothing.

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