Coaxial cable signal loss (dB/100 m) — attenuation is a separate problem from impedance mismatch

The glossary entry on coaxial cable (RG type, impedance) explains why the characteristic impedance of a coax cable (commonly 75 Ω for CATV/antenna work, 50 Ω for RF test and measurement) must match at both ends of a connection, and why a mismatch causes reflections and standing waves. This guide covers a different failure mode that produces a similarly weak or noisy signal at the far end of a run, but has a completely different cause and a completely different fix: plain attenuation, the gradual loss of signal power as it travels along a correctly matched, correctly terminated cable.

Attenuation is a length- and frequency-dependent loss, not a reflection

Even a coax cable with perfectly matched 75 Ω (or 50 Ω) impedance at both ends, correctly terminated with no reflections at all, still loses signal power as it travels down the cable. This loss comes from the resistance of the conductors and the dielectric losses in the insulation between them, and it is normally expressed by cable manufacturers as a dB-per-100-metres (or dB-per-100-feet) figure in the cable's data sheet, generally in line with the characterisation methods in the EN 50117 series for coaxial cables used in information technology and broadband applications. Two properties of this loss matter for a practical installation:

It increases with cable length. A run twice as long loses roughly twice as many dB (attenuation in dB is approximately proportional to length), so a data-sheet figure quoted per 100 m must be scaled to the actual run length before comparing it to a receiver's or camera's sensitivity budget.
It increases with signal frequency. The same physical cable attenuates a higher-frequency signal more per metre than a lower-frequency one, which is why manufacturers publish attenuation figures at several frequency points (for example one value at a UHF terrestrial-broadcast frequency and a different, higher value at a satellite-IF frequency) rather than a single number for the whole cable.

Why this is a "signal budget", not a pass/fail spec

Because attenuation accumulates with both length and frequency, a practical installation is really a budget exercise: the signal level leaving the source, minus the total dB lost in the cable run at the highest frequency of interest, minus losses in any splitters, connectors or taps along the way, must still leave enough signal level at the far end for the receiving equipment (a set-top box, an antenna amplifier, a CCTV DVR input) to work reliably. A cable that is perfectly matched and perfectly terminated can still deliver an unusable picture or a noisy RF measurement simply because the run is longer than the cable's attenuation figure and the source signal level allow — no reflection or mismatch is involved at all.

Distinguishing the two problems on site

Because attenuation and impedance mismatch can both produce a degraded signal, but call for different fixes, it helps to separate the symptoms before troubleshooting:

A mismatch (wrong-impedance cable, a loose or damaged connector, a poorly terminated tap) tends to produce reflections that show up as ghosting, standing-wave patterns on a return-loss or TDR measurement, or a distinctive comb-like ripple in a swept-frequency measurement — and it is often present even on a short run.
Attenuation tends to produce a uniformly weak or noisy signal that gets worse as the run gets longer or as more splitters are added in series, without the reflection-related symptoms, and it is present even when every connector and impedance value is correct.

A cable run can, of course, suffer from both problems at once (for example, an undersized cable run to its attenuation limit that also has one poorly seated connector), which is why a methodical check normally verifies impedance/termination integrity and length-based attenuation budget as two separate items rather than assuming a weak signal is caused by only one or the other.

Practical relevance

When a CATV, CCTV or antenna signal run underperforms, first establish the actual cable length and the dB/100 m attenuation figure from the cable's own data sheet at the relevant frequency, and compare the resulting total loss (plus splitter/tap losses) against the source signal level and the receiving equipment's sensitivity. Only if the budget calculation looks adequate but the symptom persists is it worth investigating impedance mismatch, a damaged connector or a faulty tap as the separate root cause. Relying on cable length alone without checking the frequency-dependent attenuation figure, or relying on impedance/connector inspection alone without checking the length-based signal budget, each addresses only one of the two independent failure modes.

Common mistakes

Assuming a weak signal must be an impedance or connector problem — a correctly matched, correctly terminated cable can still fail simply because the run is too long for its attenuation figure at the frequency in use.
Comparing a manufacturer's dB/100 m figure directly to a longer or shorter run without scaling it — attenuation in dB is approximately proportional to length, so the figure must be scaled to the actual run length before use.
Using a single attenuation figure across all frequencies of interest — attenuation increases with frequency, so a figure quoted at one frequency (for example terrestrial UHF) understates the loss at a higher frequency (for example satellite IF) on the same cable.
Ignoring cumulative splitter and tap losses in the signal budget — even a cable run well within its attenuation limit can still fail if several splitters in series each remove several dB before the signal reaches the far end.