What Is Cable Attenuation and Why Does It Matter?

What Is Cable Attenuation?

Attenuation is the reduction in signal power that occurs as a signal travels along a cable. Every cable — no matter how well-made — loses a fraction of the signal it carries. The electrical energy that was launched into one end of the cable arrives at the other end weaker, because some of it was converted to heat by the cable’s internal resistance and by dielectric losses in the insulating material.

Attenuation is expressed in decibels per unit length (dB/m or dB/100m). This tells you how much loss you’ll accumulate for every metre of cable in your run.

Because the decibel scale is logarithmic, these losses add up quickly. Every 3 dB of loss halves the signal power. Every 10 dB of loss removes 90% of it. A 20 dB loss means only 1% of the original power reaches the far end.

This is why attenuation matters: a cable run that seems modest in length can strip out most of your signal if you’ve chosen the wrong cable type — and because the scale is logarithmic, there’s no gradual warning. You go from working to not working in a surprisingly short distance.

How Attenuation Is Measured and Specified

Manufacturers specify attenuation in dB per 100 metres at a set of standardised frequencies. A typical datasheet entry for LMR-400 looks like this:

• At 450 MHz: 4.6 dB/100m
• At 1 GHz: 6.8 dB/100m
• At 2.4 GHz: 11.0 dB/100m
• At 5.8 GHz: 17.5 dB/100m
• At 12 GHz (Ku-band): ~30 dB/100m

To find the loss for your specific run, multiply the attenuation figure by the run length in metres and divide by 100:

Cable loss (dB) = (attenuation dB/100m × run length in metres) ÷ 100

Example: 35m of LMR-400 at 1 GHz:
Loss = (6.8 × 35) ÷ 100 = 2.38 dB

If the signal is at 12 GHz (Ku-band, not down-converted IF):
Loss = (30 × 35) ÷ 100 = 10.5 dB — a dramatic difference for the same cable and same distance.

Why Frequency Makes Attenuation Worse

This is the single most important thing to understand about cable attenuation: loss increases as frequency increases, and it does so steeply.

Two physical mechanisms drive this. Skin effect: at higher frequencies, current concentrates into a thin layer at the surface of the conductor. Less cross-sectional area carries the current, so resistance increases. Dielectric loss: the insulating material between the centre conductor and the shield absorbs a small amount of energy as the electromagnetic field oscillates through it — this absorption increases with frequency.

Both effects scale roughly with the square root of frequency. Go from 1 GHz to 12 GHz and attenuation goes up by roughly 3.5–4×. This is why Ku-band IFL specifications are so unforgiving compared to L-band.

Cable Attenuation Comparison: LMR Series vs Legacy Coax

Not all coaxial cables are equal. The main variable is cable diameter: larger cables have lower attenuation because they have a bigger centre conductor (lower resistance) and a thicker dielectric. This is the core trade-off — larger cable, lower loss, harder to handle and route.

Values are approximate and vary by manufacturer. Always verify against the specific datasheet for the cable in use.

The difference between LMR-400 and RG-214 — two cables that look broadly similar — is dramatic. At 1 GHz, RG-214 loses more than twice as much signal per metre. For any professional RF installation, LMR-400 or better is the minimum acceptable specification.

Attenuation and VSAT IFL Runs: Real-World Limits

In a VSAT terminal, the cable connecting the indoor unit (modem) to the outdoor unit (BUC and LNB on the dish) is called the IFL (Interfacility Link). This is almost always coaxial, and attenuation directly sets the maximum usable run length.

Most VSAT systems allow a total IFL loss budget of roughly 8–12 dB, depending on the modem manufacturer’s specification. Exceed this, and the modem can no longer lock to the carrier — or it locks but at a degraded signal quality that causes errors under rain fade.

Note that VSAT IFL cables carry L-band IF signals (950 MHz – 2,150 MHz), not raw Ku-band — which is why the actual usable run lengths are much longer than a raw 12 GHz attenuation figure would suggest.

The Other Sources of Signal Loss: Don’t Forget Connectors

Cable attenuation gets all the attention, but every connector junction in your system also introduces loss. A well-terminated N-type connector adds approximately 0.1–0.15 dB per connection. In a typical run with a connector at each end, that’s 0.2–0.3 dB — small but real.

A poorly made connector is a different story. A bad crimp, a loose centre pin, or oxidised contact surfaces can add 0.5–2 dB per connector — easily as much loss as metres of cable. Well-terminated N-type: 0.1–0.15 dB. Well-terminated SMA: 0.1–0.2 dB. BNC at L-band: 0.15–0.2 dB. Each adapter (N-to-SMA, etc.): add 0.2–0.3 dB. Poorly made connector: 0.5–2.0 dB.

For a long IFL run, minimise the number of connections. Run a single cable from modem to LNB/BUC where possible, and use weatherproof sealant on all outdoor connections to prevent moisture ingress, which dramatically increases connector loss.

How to Reduce Cable Attenuation

You can’t eliminate attenuation from a cable, but you can manage it effectively:

1. Choose a larger cable diameter. LMR-600 has roughly 40% lower attenuation than LMR-400 at the same frequency. Where run length is pushing your budget, step up a cable size.

2. Shorten the run. Every extra metre adds loss. Position the indoor unit close to the cable entry point, and use short patch cables rather than routing a single long run around obstacles.

3. Use quality connectors and terminate properly. A well-made crimp connection loses 0.1 dB. A poor one can lose 2 dB. Use the correct die for the connector, and inspect the finished crimp before sealing.

4. Seal outdoor connectors. Moisture in a connector or cable jacket multiplies attenuation significantly. Self-amalgamating tape over all outdoor connections is non-negotiable.

5. Avoid sharp bends. Exceeding a cable’s minimum bend radius compresses the dielectric and increases attenuation. LMR-400 has a minimum bend radius of 25mm; LMR-600 is 38mm.

6. Check for impedance mismatches. Mixing 50Ω and 75Ω cables or connectors creates reflection losses. In RF systems, keep everything 50Ω (VSAT, two-way radio). In broadcast distribution, keep everything 75Ω.

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