Coax Attenuation: Why Loss Rises With Frequency

Ask for a coax cable’s loss and you will get a table, not a figure. That is not evasiveness — it is how the cable behaves. Loss depends on frequency, and a single quoted number cannot describe a cable that carries a television signal, a satellite band and a camera feed.

Reading the table at the right frequency is most of what cable selection for coax comes down to.

Quick answer

Question	Answer
Does loss rise with frequency?	Yes, for every coaxial cable
Is it a straight line?	No. It rises faster at low frequencies and more slowly higher up
Where is the loss figure I should use?	At the frequency your system actually carries
What changes the curve?	Center conductor size, dielectric material and construction
Why is RG6 better than RG59?	Larger conductor and dielectric, so lower loss at every frequency
Is a headline figure useful?	Only as a rough comparison between cables measured the same way

Why loss rises with frequency

Two mechanisms do it, and both get worse as frequency climbs.

Skin effect. At higher frequencies, current stops using the whole cross-section of the center conductor and concentrates near its surface. The effective conducting area shrinks, so resistance rises with frequency.

Dielectric loss. The insulating material between the conductor and the shield absorbs a small amount of energy on every cycle, and more cycles per second means more absorption. Foam dielectrics absorb less than solid ones, which is why foam is standard in cables that need low loss.

Together they produce a curve that climbs steeply at first and then flattens. The practical consequence is that a cable which performs comfortably at television frequencies may be marginal several hundred megahertz higher.

Reading an attenuation table

A coax datasheet lists loss at a series of frequencies, usually in dB per 100 meters or per 100 feet. Three habits make the table usable.

Find your frequency first. Not the highest one listed, and not the lowest. A CCTV baseband signal, a television channel, a satellite band and a MoCA network all sit in different parts of the spectrum, and the difference between the loss figures can be substantial.

Read the units. Per 100 meters and per 100 feet differ by a factor of about 3.3. Comparing one cable quoted per 100 feet against another quoted per 100 meters will lead you to the wrong conclusion.

Multiply by your run length. The table gives loss per unit length; the number that decides whether the system works is that figure times the distance.

Why the same cable passes one job and fails another

Consider a cable whose loss at television frequencies is modest. Send a television signal along it and the run works. Send a satellite intermediate-frequency band along the same cable and the loss at that higher frequency may be two or three times greater, while the signal arriving from the dish is far weaker to begin with.

Nothing about the cable changed. The signal did.

This is why "is RG6 good enough" has no single answer. Good enough for what, over what distance, with what margin at the far end?

The loss budget

The useful way to think about a coax run is as a budget with a few line items.

Item	What it contributes
Source level	What the camera, antenna or LNB delivers
Cable loss	The figure from the table, at your frequency, times the length
Splitter loss	An unavoidable division, plus a small insertion loss
Connector and joint loss	Small individually, meaningful in quantity, larger when badly made
Receiver requirement	The minimum the equipment needs to work reliably

The run works when the source, minus everything the signal loses on the way, still leaves the receiver more than its minimum. The margin that remains is what protects the system against a slightly longer route, a small increase in loss, or a connector that is not perfect.

Splitter loss has its own arithmetic, and in a distribution system it often contributes more than the cable does.

What to do when the budget does not close

Four options, in order of preference.

Shorten the run, or reroute it to avoid a longer path.

Use a lower-loss cable. Moving from a thinner coax to RG6, or from RG6 to a larger construction, lowers the figure across the whole curve. The RG6 and RG59 comparison covers the common case.

Remove loss from the chain. Fewer splitters, one fewer joint, a shorter patch lead at each end.

Amplify at the right point. An amplifier placed at the receiving end amplifies the signal and the noise the cable added, and does nothing for the signal-to-noise ratio. Placed at the source end, before most of the loss, it improves the ratio. That is why antenna preamps go at the mast, and why a distribution amplifier belongs at the head end of the system rather than at the far end of a long run. The antenna case is covered here.

Scenarios

A CCTV camera 60 m away on baseband video. The loss at baseband is the lowest on the curve, which is why modest coax can carry a surprising distance on analog video.

A satellite run of the same length. The signal is much weaker and sits much higher in frequency, so the same cable and the same distance are a different problem.

A MoCA network added to an existing coax installation. The MoCA band sits above the television services, so it sees a higher loss than the television signal does — and an old splitter may not pass the band at all.

A distributor asking whether a cable is suitable. The answer needs the frequency, the run length and the receiver’s requirement. Without those, no table can answer it.

FAQ

Does coax lose more signal on longer runs? Yes, in proportion to length. Loss per unit length multiplied by the distance gives the total.

Does higher frequency mean more loss? Yes. That is the shape of every coax attenuation curve.

Why is loss quoted at several frequencies? Because no single number describes the cable. The set of figures lets you read the one that applies to your signal.

Is RG6 loss the same as RG59 loss? No. RG6 has a larger center conductor and dielectric, so its loss is lower at every frequency.

Can I compensate for loss with an amplifier? You can restore level, but an amplifier at the far end cannot restore the signal-to-noise ratio. Amplify early if you are amplifying at all.

How much margin should I leave? Enough that the run still works if it is slightly longer than planned or a connector is less than perfect. The equipment’s own specification is the starting point.