Coaxial (or coax) cable is used to transfer radio frequency radiation from a radio or television transmitter to an antenna. Such cable consists of a metal wire in the centre and an outer metal braid mesh separated by a foam-like insulation material called the "dielectric." Some lengths of coaxial cable can become damaged where the dielectric becomes cracked or pulled apart, allowing the inner wire to come into contact with the outer braid. This causes a short that can damage the transmitter. Cable may be tested for connectivity issues by performing conductivity tests with a multimeter.
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Things you need
- Digital multimeter with continuity selection on dial
Coil the coax cable so the connectors on both ends can be reached easily with the metal probes of the multimeter.
Insert the test probe wire plugs into the multimeter, inserting the black negative (-) wire plug into the multimeter's "COM" jack and the red positive (+) wire plug into the jack marked "+" on the multimeter.
Turn on the multimeter and rotate the measurement-type dial to measure ohms, or resistance. In this area of settings, look for the selection labelled "CONT" or "Continuity" and set the needle of the measurement dial to this setting. Touch the metal probe tips together to create an electrical short and observe that a tone is emitted from the multimeter signalling that the continuity tester of the instrument is working correctly.
Touch the black negative metal probe to the centre pin (if there is a connector) or centre wire (if no connector) on one end of the coaxial cable, without allowing it to touch the outer jacket of the connector or the braided wire on that end. The tip should be making contact only with the centre pin of the connector or the centre wire of the cable.
Touch the red positive metal probe to the centre pin or wire on the other end of the coaxial able, making sure the metal tip is touching only the centre and not making contact with any other metal. If a tone is emitted from the internal speaker of the multimeter, this signals that the centre wire is not broken along the length of the cable and that the centre wire is healthy.
Move the black negative metal probe from the centre pin, or wire, and touch the outer jacket of the connector. If the coaxial cable is being tested without connectors on it, touch the tip of the probe to the braided outer wire of the cable. Keep the tip of the positive probe on the centre wire of the other end. Note whether a tone is emitted from the multimeter. There should be no electrical connection between the centre wire and the outer jacket or braided wire, therefore no tone should be heard. If a tone is heard, this indicates the insulation is broken in the length of coax, allowing the inner and outer wires to touch (short) together. This indicates the coax cable should be replaced with a new length of cable.
Tips and warnings
- If a multimeter doesn't have a continuity-testing selection on the dial, set the dial to measure no more than 5 ohms. When testing both ends of the centre wire, look at the display of the meter, which should read "0.000" or another value where any of the zeros are replaced by numbers. This indicates a solid connection between both ends. If the display shows "1." (one followed by a period) with no following zeros, this indicates a break in the wire, requiring replacement of the cable. Further, when testing for shorts between centre and outer wire on coax, a "1." is desired rather than any other value. There should be no connection between the inner wire and the outer wire, or braid, of good coaxial cable.
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