dB/km
Reading Attenuation as a Rate Over Distance
The decibel by itself describes a ratio of two power levels and says nothing about how far a signal traveled to lose that power. dB/km closes that gap by dividing the total loss by the length of the medium, producing a per-unit-length figure engineers can scale to any run. A cable rated at 100 dB/km loses 1 dB over a 10 m patch and 50 dB over a 500 m feeder; the rate stays constant as long as the medium is uniform. This is the same idea captured by the attenuation constant, expressed in field-engineering units rather than nepers per meter.
Two properties make the unit convenient. First, loss in decibels adds linearly along a path, so concatenated sections simply sum: 300 m of coax at 200 dB/km plus a connector contributing 0.2 dB gives 60.2 dB total. Second, the rate isolates the frequency behavior of the medium. For coaxial cable the conductor (skin-effect) loss grows as the square root of frequency while dielectric loss grows linearly, so a datasheet lists dB/km at spot frequencies such as 100 MHz, 1 GHz, and 10 GHz rather than a single number. Optical fiber instead shows a loss-versus-wavelength curve with minima near 1310 nm and 1550 nm.
Confusion usually comes from mixing length units. A figure given in dB/km is one thousand times the dB/m value and roughly 32.8 times the dB/100 ft value common on North American coax datasheets. Always carry the reference length and the frequency together; a "30 dB" cable specification is meaningless until both the length it spans and the test frequency are stated.
Conversion Formulas
L(dB) = αdB/km × dkm
Length-unit conversions:
αdB/m = αdB/km ÷ 1000
αdB/100ft ≈ αdB/km × 0.03048
Nepers per kilometer (field units):
αNp/km ≈ αdB/km ÷ 8.686
Where α = specific attenuation, d = path length. Example: a 30 m run of coax rated 220 dB/km at 2.4 GHz gives L = 220 × 0.030 ≈ 6.6 dB. The factor 8.686 = 20 / ln(10) links the field-based neper to the power decibel.
Typical Specific-Attenuation Values
| Medium | Frequency / Wavelength | Attenuation (dB/km) | Equivalent (dB/100 ft) | Notes |
|---|---|---|---|---|
| Single-mode fiber | 1550 nm | ~0.2 | ~0.006 | Long-haul minimum loss |
| Single-mode fiber | 1310 nm | ~0.35 | ~0.011 | Zero-dispersion window |
| LMR-400 coax | 2.4 GHz | ~220 | ~6.7 | Flexible 50 Ω line |
| RG-58 coax | 1 GHz | ~720 | ~22 | Thin coax, short runs |
| 7/8 in heliax hardline | 2 GHz | ~40 | ~1.2 | Cellular tower feeders |
| Clear-air absorption | 60 GHz | ~15 | ~0.46 | Sea-level O2 absorption peak |
Frequently Asked Questions
How do I convert dB/km to dB/m or dB/100 ft?
Divide by 1000 for dB/m, so 30 dB/km equals 0.030 dB/m. For the datasheet unit dB/100 ft, multiply dB/km by 0.03048, giving 0.914 dB/100 ft; multiply dB/100 ft by 32.808 to return to dB/km. The decibel scale is linear with distance, so these are pure unit changes, valid at one frequency in a uniform medium.
Why is cable attenuation specified in dB/km instead of total dB?
A maker cannot know how long a run an installer will cut, so it publishes a per-kilometer rate to multiply by actual length. LMR-400 at roughly 220 dB/km at 2.4 GHz yields 6.6 dB over 30 m. The normalized rate also exposes frequency dependence: coax loss scales with √f from skin effect plus a linear dielectric term, so datasheets list dB/km at several spot frequencies.
What is a typical dB/km value for single-mode fiber versus RF coax?
Single-mode fiber reaches about 0.2 dB/km at 1550 nm and 0.35 dB/km at 1310 nm, letting links span tens of kilometers unamplified. RF coax is far lossier, from tens of dB/km at HF to several hundred dB/km in the microwave band, where hardline or waveguide takes over. That huge gap is why long-haul backbones are optical, not coaxial.