Coaxial Cable Loss Calculator

Calculate RF attenuation for common coaxial cables (LMR-400, RG-58, RG-213, and more). Enter cable type, frequency, and run length to get insertion loss in dB.

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Formula

\text{Loss} = \alpha(f) \times \frac{L}{100}

Reference: Times Microwave LMR cable datasheets; Belden cable catalog

α(f)— Cable attenuation at frequency f (dB/100m)

L— Cable length (m)

How It Works

Coaxial cable attenuation arises from two loss mechanisms: conductor loss (proportional to √f, due to skin effect) and dielectric loss (proportional to f, due to the dielectric material). Total attenuation is α(f) = α₁√f + α₂f.At HF and VHF frequencies conductor loss dominates; at microwave frequencies dielectric loss becomes significant. This is why low-loss cables like LMR-400 use foamed polyethylene dielectric — lower dielectric loss tangent.Velocity factor (VF) is related to the dielectric constant: VF = 1/√εᵣ. Foam dielectrics have VF ≈ 0.85 vs solid PE at VF ≈ 0.66. Higher VF means lower loss per unit length.

Worked Example

A 50 m run of LMR-400 at 435 MHz (70 cm amateur band): α = 4.69 dB/100 m → Loss = 4.69 × 50/100 = 2.35 dB. Power at antenna = 10^(-2.35/10) × 100% = 58.3%. Acceptable for a 50 W transmitter (29.1 W reaches the antenna). Replacing with 7/8" hardline (Andrew LDF4-50A): α = 1.55 dB/100 m → Loss = 0.775 dB → 83.7% efficiency.

Common Mistakes

✗Using room-temperature specs for outdoor runs — loss increases ~0.4%/°C
✗Ignoring connector losses — an N connector adds ~0.1 dB at 1 GHz, a PL-259 adds up to 0.5 dB
✗Not accounting for VSWR mismatch — a 2:1 VSWR adds 0.51 dB mismatch loss on top of cable loss
✗Comparing cables by dB/ft rather than dB/100m — always normalize to the same length unit

Frequently Asked Questions

LMR-400 vs RG-213: when is the upgrade worth it?

At HF (below 30 MHz), the difference is small — both are under 2 dB/100m. At 450 MHz, LMR-400 is 4.69 dB/100m vs RG-213 at 10.5 dB/100m — LMR-400 is dramatically better for VHF/UHF. For runs over 20 m at 435 MHz or above, LMR-400 or better is strongly recommended.

How do I account for connector losses?

Add approximately 0.1 dB per N connector at 1 GHz, 0.15 dB per SMA at 1 GHz, and up to 0.5 dB per PL-259 (UHF connector) at 144 MHz. For low-power receive applications connector quality matters more than for transmit.

What cable for a 2.4 GHz WiFi antenna extension?

LMR-200 is the minimum practical choice — at 2.4 GHz it has 26 dB/100m. Keep runs under 5m for less than 1.3 dB loss. LMR-400 gives 11.5 dB/100m, acceptable for runs up to 10m. Anything longer should use a powered antenna or active extender.

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