How to Test a Coax Cable

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Data centers in your house serviced by a cable company, such as your cable TV or Internet router, are connected to the service by coaxial cables, which are the standard cables for conducting radio frequency signals. You can often trace reception problems to a coax cable that's in poor condition. Testing your cable is a simple procedure that requires an ohmmeter or continuity tester, and if you discover the cable to be damaged, it's easy to replace.

Coax Cable Anatomy

If you unscrew your coax cable from the back of your television, you can readily see the main signal conductor, which runs through the center of the cable and extends a short distance from each end. Surrounding this conductor is a layer of dielectric material, and wrapped around that is a copper braid, which is fused to or crimped by the connectors. Finally, the entire cable is coated with plastic sheathing.

Signal Loss

The wire mesh grounds the cable, and if it gets damaged -- perhaps by a kink in the wire or an impact -- signal quality suffers or the signal may disappear altogether. You'll also lose the signal if the center wire gets damaged or if a breach in the barrier between the conductor and the ground braid allows contact between them. You can often identify a damaged cable by a visual inspection, but the most definitive test is to pass a small electric current through both conductors and measure the resistance. This is called a continuity test.

Testing Procedure

Step 1

Unscrew both ends of the cable from their receptacles. You cannot conduct a continuity test while the cable is connected to power.

Step 2

Conduct a visual inspection of the entire cable. If you see nicks or slices through the sheathing, it probably means the copper braid has been damaged. Look inside the connector to ensure the braid and pin are properly soldered. If you see small bits of copper wire protruding inside the connector, it probably means the braid is no longer properly connected.

Step 3

Set your multimeter to measure resistance in the ohms X100 scale. You can also use a continuity tester with a light that illuminates or a tone that sounds to signal continuity. The test is easier to conduct if the leads on the meter or tester have alligator clips, but you can do the test without it.

Step 4

Check the central conducting wire by touching or clipping one lead to one end and the other lead to the other. The light or tone should sound to signify continuity. If you're using a meter, the resistance reading should be close to zero.

Step 5

Test the braiding by touching or clipping a lead onto the connector on one end of the cable and the other lead to the connector on the other end. You should get continuity.

Step 6

Leave one clip in contact with the connector on one end of the cable and touch the other lead to the central wire inside the connector. You should not get continuity -- if you have a meter, it should display infinite resistance. If the meter shows a small resistance, or you get a signal from the continuity tester, it means the insulation between the two conductors has been breached and the cable needs to be replaced.


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