Coax Cable Signal Loss: The Two Variables and the Hidden Third
Quick answer / TL;DR: Short answer: loss rises with frequency and with length, so the same cable loses more at higher channel frequencies. The variable people miss is connector and termination quality, which can dominate the total. Compare measured performance against the loss budget rather than against the datasheet figure alone.
Headroom is built: separator, 23 AWG copper, foil and drain.Coaxial cable loss is usually described as a number per hundred feet, which is true and incomplete. The figure applies at a stated frequency, and loss rises steeply as frequency rises.
That means a cable that is comfortable for one service can be marginal for another on the same run, because the higher frequencies lose more.
This article covers the two variables that set loss, the third that people miss, and the diagnostic method that separates a cable problem from a connector problem.
A coaxial cable is a transmission line. The dielectric absorbs energy, the conductor resistance adds more, and both effects scale with frequency.
The consequence is that a length figure quoted without a frequency is not usable. A hundred feet is comfortable at low frequency and marginal at several hundred megahertz.
Where the Loss Comes From
| Contribution | Rises with | Notes |
|---|
| Dielectric loss | Frequency | Set by the dielectric material |
| Conductor loss | Frequency and current | Set by conductor size and plating |
| Length | Directly proportional | Always specify per unit length |
| Connector loss | Termination quality | Small when correct, large when not |
| Impedance mismatch | Reflections | Raises effective loss without appearing in the datasheet |
The last row is why a measured loss sometimes exceeds the calculated figure. A reflection at a poor termination does not appear as loss until it is measured at the far end.
Diagnosing a Loss Problem
- Compare measured signal level at both ends rather than assuming the datasheet value
- Check connectors first, because they are the cheapest element to replace and the most common fault
- Measure at the highest frequency the service uses, not at a convenient test frequency
- Look for a single dropout rather than a gradual decline, which indicates a termination rather than a cable
- Record the baseline so a later measurement has a reference
Real-World Applications
A cable modem installation shows low downstream level on the highest channels. The drop is old and the loss is genuinely frequency-dependent, which is why the lowest channels look acceptable.
An analogue video run shows a soft image rather than a lost signal. The loss is gradual and frequency-dependent, which attenuates the higher video frequencies first.
A satellite installation fails only on certain transponders. The loss is at the top of the band, and the cable is adequate at the lower frequencies the other transponders use.
A distribution system shows loss exceeding the calculated value. Two connectors were crimped with the wrong die, and the reflections account for the difference.
Standards and Verification
Coaxial cable performance is specified by the cable standards applicable to its construction, and attenuation is normally published per unit length at stated frequencies. Connector performance comes from the component manufacturers, and system requirements from the service operator.
Verification should record the measurement frequency alongside the result. A loss figure without a frequency cannot be compared with a specification.
Where a measurement is worse than expected, check the terminations before replacing the cable. The pattern of the shortfall usually says which it is.
Building a Loss Budget
A loss budget is the arithmetic that decides whether a design will work before anything is installed. It has four lines: source level, cable loss at the highest frequency, splitter loss, and the minimum level the receiver accepts.
The useful discipline is to compute it at the top of the band, because that is where the cable is worst and where the margin disappears first.
Where the result is negative, the answer is a shorter run, a different cable, fewer splits or an amplifier, in that order of preference.
Why Amplifiers Come Last
An amplifier raises the signal and the noise together, so it improves a level problem and worsens a quality problem.
Where the shortfall is caused by loss, amplifying at the head end often works because the signal-to-noise ratio entering the amplifier is still good.
Where the shortfall is caused by a reflection at a bad termination, amplification changes nothing about the underlying fault and can make the symptoms less predictable.
Fix the passive path first, then consider amplification for what remains.
Where a run is marginal, measure before adding hardware. A measurement at the receiver documents the starting point and gives the next change something to be judged against.
Without that baseline, an improvement cannot be distinguished from a change in conditions.
Keep the budget with the drawing. A design that was computed is a design that can be checked when the service changes.
Related reading: Coax cable for cable modem — blogs/news/coax-cable-for-cable-modem
Related reading: 75 ohm vs 50 ohm coax — blogs/news/75-ohm-vs-50-ohm-coax
Related reading: Coax cable bend radius — blogs/news/coax-cable-bend-radius
A loss figure quoted without a frequency is not a specification. It is half of one.
Frequently Asked Questions
Does coax loss depend on frequency?
Yes, and steeply. The same cable loses noticeably more at higher channel frequencies, which is why a run can be fine for one service and marginal for another.
Why is my measured loss worse than the datasheet?
Usually the terminations. A poor connector produces reflections that appear as extra loss when measured at the far end.
How do I know whether to replace cable or connectors?
A single sharp degradation points at a termination; a gradual shortfall across the run points at the cable. Check connectors first because they are cheaper.
Does a thicker cable always reduce loss?
It reduces conductor loss, which is one contribution. Dielectric loss and length still apply, so the improvement is not proportional to diameter.
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