Testing Coax Cable: What a Meter Proves and What It Does Not

A continuity test is the test most coax installations receive, and it answers a much narrower question than people assume. It confirms the cable is not broken. It says nothing about whether it will carry the service.

Knowing the difference matters, because a cable that passes a continuity check and then fails is common enough to have a pattern.

Quick answer

Test	What it proves
Continuity, end to end	The center conductor is connected
Short check, center to shield	The two conductors are not touching
Insertion loss at the frequency of use	How much signal the cable removes
Return loss	How much signal reflects back because of impedance discontinuities
Time domain reflectometry	The distance to a fault, or the cable’s length

What a multimeter can tell you

Two useful things, and they are worth doing.

Continuity. A resistance reading close to zero between the two ends of the center conductor confirms the conductor is intact and the connectors are fitted. An open reading means a break or a connector that has not captured the conductor.

Shorts. A reading between the center conductor and the shield that is not open means the two are touching somewhere — a stray strand at a connector, a crushed section, or damage where something has been driven through the cable.

Neither test is a guarantee of anything else. A cable with a perfectly continuous center conductor and no short can still have a damaged shield, a corroded braid, an impedance that has been disturbed by a kink, or a connector that opens as soon as the cable is moved.

What a multimeter cannot tell you

Everything that depends on the cable behaving as a transmission line.

Impedance is not a DC property. A 75 ohm cable does not measure 75 ohms on a meter — the meter reads the loop resistance of the center conductor and the shield, which is a small fraction of an ohm per meter. The 75 ohm figure describes behavior at frequency, and it takes a signal at frequency to measure it.

Loss is frequency-dependent, and a DC measurement cannot see it. The attenuation curve is the property that decides whether a run is adequate, and reading it requires a source at the frequencies in use.

Return loss is invisible to a DC test. So is shield integrity along the cable’s length — a shield bonded at both ends and broken in the middle passes a continuity check on the center conductor without difficulty.

The measurements that characterize a run

Three, and they answer the questions a continuity test cannot.

Insertion loss, measured across the frequency band the service uses. This is the number to compare against the loss budget, and it is what a cable manufacturer’s datasheet predicts. Measuring it confirms the installed assembly rather than the reel.

Return loss, which quantifies how much signal reflects back from impedance discontinuities. Every connector, every barrel, every kink and every water-filled section contributes. Poor return loss produces the reflections that cause ghosting on analog video and errors on digital services.

Distance to fault, which is where time domain reflectometry comes in. A TDR sends a short pulse down the cable and times the reflections that return. A reflection at a known time corresponds to a known distance, which locates a fault — a staple through the cable, a crushed section, a badly fitted connector — rather than merely proving one exists.

A TDR also measures the cable’s electrical length, which is useful when the run’s physical length is uncertain.

What a sweep test is for

A swept measurement across the service band, recording loss and return loss against frequency, produces a record of the installed link.

That record is useful in three ways. It confirms the installation meets the budget before the ceiling is closed. It gives a baseline for comparison if the service degrades later. And on a commercial project it is often what the specification asks for.

For a domestic installation, this level of measurement is rarely justified. For installed coax serving a satellite distribution or a broadcast system, it is standard practice.

Practical field testing without instruments

Most faults are found without any of the above, because most faults are connectors.

Three techniques, in order of usefulness.

Substitution. Swap in a known-good patch lead, and move the run to a different port. If the fault follows the swapped part, that part was the fault.

The movement test. Flex the cable gently at each connector while watching the signal. A connector that is not properly made will produce a visible or measurable change. This finds the intermittent faults that a static test misses.

Visual inspection. Look at the connector: is the center conductor the right length, is the shield folded back correctly, is there corrosion or moisture, is the braid captured by the connector rather than pressed against it? Most coax faults are visible at the joint. The connector details are here.

Scenarios

A new domestic installation. Continuity and short check at each end, visual inspection of every connector, and a substitution test against a known-good lead. That is proportionate.

A commercial satellite distribution. Swept insertion loss and return loss recorded before handover, with the results retained.

A cable that passes the meter and fails the service. Suspect the shield, the impedance, or a connector that only fails under movement. Continuity answered the wrong question.

A fault that appears only when the weather changes. Water. No instrument reading taken on a dry day will find it. The diagnostic order is here.

A run of uncertain length. A TDR gives the electrical length and locates any reflection along the way.

FAQ

Does a continuity test prove a coax cable is good? No. It proves the center conductor is intact and there is no short. It says nothing about impedance, loss or shield quality.

Why does my cable pass the meter and still not work? Because the meter tested for continuity, and the service depends on properties a DC test cannot see.

Can I measure coax impedance with a multimeter? No. Impedance is a property at frequency, not a DC resistance.

What is a TDR used for? Locating faults by distance, and measuring electrical length. It sends a pulse and times the reflections.

Do I need a sweep test for a home installation? No. Visual inspection and substitution testing are proportionate for domestic work.

What should I ask a cable supplier for? The published attenuation and return loss figures, so you can check the cable against the loss budget for your run before installing it.