Skip to content
RFrftools.io
RF

Coaxial Cable Loss Calculator

Calculate RF cable attenuation for LMR-400, RG-58, RG-213, and more. Enter cable type, frequency, and length to get insertion loss in dB. Free, instant results.

Loading calculator...

Formula

Loss=α(f)×L100\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

Coax loss calculator computes signal power loss for any cable type, run length, and frequency — RF engineers, broadcast system designers, and antenna installers use this to calculate link budgets and select appropriate cable for their frequency range. Total loss combines conductor loss (proportional to sqrt(f) due to skin effect) and dielectric loss (proportional to f due to dielectric heating): alpha_total = alpha_c sqrt(f) + alpha_d f, per ITU-R P.525 and transmission line theory in Pozar's 'Microwave Engineering' (4th ed.).

At HF frequencies (3-30 MHz), conductor loss dominates — a 100m run of RG-58 loses 4.2 dB at 30 MHz versus 1.3 dB at 3 MHz. Above 1 GHz, dielectric loss becomes significant: solid polyethylene (tan_delta = 0.0002) adds 0.8 dB/100m at 1 GHz while PTFE (tan_delta = 0.0001) adds only 0.4 dB/100m. Low-loss cables like LMR-400 use foamed polyethylene (er = 1.5, tan_delta = 0.0001) achieving 6.8 dB/100m at 1 GHz versus 21.5 dB/100m for RG-58.

Velocity factor VF = 1/sqrt(er) directly correlates with loss: foam dielectrics (VF = 0.85) have 30-40% lower loss than solid PE (VF = 0.66) at the same frequency because the electromagnetic field travels through more air. Temperature increases loss by approximately 0.2%/C for copper conductors due to increased resistivity.

Worked Example

Problem: Calculate total loss for a 75m LMR-400 run at 915 MHz (LoRa frequency) and determine power delivered to antenna from a 1W (30 dBm) transmitter.

Solution using manufacturer specifications and ITU-R methodology:

  1. LMR-400 attenuation at 900 MHz: 6.0 dB/100m (Times Microwave datasheet)
  2. Cable loss: 6.0 * (75/100) = 4.5 dB
  3. Add connector losses: 2x N-type connectors at 0.15 dB each = 0.3 dB
  4. Total system loss: 4.5 + 0.3 = 4.8 dB
  5. Power at antenna: 30 - 4.8 = 25.2 dBm = 331 mW
  6. Efficiency: 10^(-4.8/10) = 33.1% — acceptable for a 75m run
Comparison: RG-58 at 915 MHz has 22 dB/100m loss, yielding 16.5 dB total loss and only 2.2% efficiency (22 mW at antenna). LMR-400 delivers 15x more power for the same installation.

Practical Tips

  • ✓Select cable where total run loss is < 3 dB for transmit systems (50% power efficiency) and < 1 dB for receive systems where every dB affects noise figure directly
  • ✓For runs over 30m at UHF (400+ MHz), upgrade from RG-58/RG-8X to LMR-400 or equivalent — the 3-4x cost difference is justified by 3-4x lower loss
  • ✓Use hardline (7/8" or 1-5/8") for permanent installations over 50m at cellular/microwave frequencies — Andrew LDF4-50A achieves 1.6 dB/100m at 900 MHz versus 6.0 dB for LMR-400

Common Mistakes

  • ✗Using room-temperature specifications for outdoor installations — cable loss increases 0.2%/C; at 60C ambient, add 8% to datasheet values. A 10 dB link margin can shrink to 9.2 dB on a hot day
  • ✗Ignoring connector losses in link budget — each N connector adds 0.1-0.15 dB at 1 GHz, SMA adds 0.15-0.2 dB, and PL-259 (UHF) adds 0.3-0.5 dB. Four connectors in a typical installation add 0.5-1.0 dB
  • ✗Not accounting for VSWR mismatch loss on top of cable loss — a 2:1 VSWR adds 0.51 dB mismatch loss; with cable loss, this compounds: 3 dB cable loss + 2:1 VSWR = 3.51 dB total, not 3 dB
  • ✗Comparing cables using different length units — always normalize to dB/100m or dB/100ft; LMR-400 at 4.69 dB/100m sounds better than 1.43 dB/100ft until you realize they're the same cable

Frequently Asked Questions

At HF (< 30 MHz), both cables have similar loss under 2 dB/100m — the upgrade provides minimal benefit. At VHF/UHF, the difference is dramatic: at 450 MHz, LMR-400 is 4.7 dB/100m versus RG-213 at 10.5 dB/100m. For a 30m run, LMR-400 loses 1.4 dB while RG-213 loses 3.2 dB — that 1.8 dB difference doubles your effective transmit power. Rule of thumb: upgrade when frequency * length (MHz * meters) exceeds 10,000.
Add per-connector loss based on frequency and connector type. At 1 GHz: N-type 0.1 dB, SMA 0.15 dB, BNC 0.2 dB, PL-259/SO-239 (UHF) 0.3-0.5 dB. Losses increase with frequency — at 6 GHz, N-type is 0.2 dB and SMA is 0.25 dB. For precision measurements, mated pair loss should be verified per IEEE 287-2007. Worn or contaminated connectors can add 0.5+ dB — inspect and clean before critical operations.
At 2.4 GHz, losses are high for all cables. LMR-200: 26 dB/100m (keep runs under 5m for < 1.3 dB loss). LMR-400: 11.5 dB/100m (acceptable to 10m). LMR-600: 8.0 dB/100m (acceptable to 15m). For runs over 15m, use LMR-900 (5.5 dB/100m) or install the access point closer to the antenna. At 5 GHz WiFi, losses are 40-50% higher — halve the maximum distances above.
Cable loss between antenna and LNA adds directly to system noise figure: NF_system = NF_cable + NF_LNA. A 3 dB cable loss before the LNA degrades sensitivity by 3 dB (halves detectable signal power). For receive-critical applications (radio astronomy, weak-signal amateur, GPS), mount the LNA at the antenna feedpoint and run DC power up the coax. A 0.5 dB LNA at the antenna followed by 10 dB cable loss yields NF_system = 0.5 + 10/10 = 1.5 dB. The same LNA after the cable yields NF_system = 10 + 0.5 = 10.5 dB.

Related Articles

Antenna Design

Calculate EIRP Within FCC, ETSI, ISM Limits

Learn how to calculate EIRP and ERP for regulatory compliance. Worked examples for FCC Part 15, ETSI 2.4 GHz, and ISM 433 MHz with margin analysis.

Protocols

Will Your Ethernet Cable Work? Calculate Attenuation

Calculate Ethernet cable attenuation, max length, and pass/fail status for Cat5e–Cat8 runs. Includes worked examples and real engineering numbers.

RF Engineering

VSWR, Return Loss & Reflected Power Guide

Learn how VSWR relates to return loss, reflection coefficient & mismatch loss. Includes worked examples and an online calculator for RF engineers.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

SMA Connectors

Standard SMA RF connectors for board-to-cable connections

Amazon →

RF Coaxial Cables

Coaxial cable assemblies for RF signal routing

Amazon →

TinySA Spectrum Analyzer

Compact handheld spectrum analyzer for RF measurement up to 960 MHz

Amazon →

Related Calculators

RF

Coax Impedance

Calculate coaxial cable characteristic impedance (Z0), capacitance, inductance per meter, and TE11 cutoff frequency from conductor dimensions. Free, instant results.

RF

Link Budget

Free RF link budget calculator: enter Tx power, antenna gains, frequency, and distance to get received signal level, link margin, and max range. Covers satellite, terrestrial, and IoT links.

RF

VSWR / Return Loss

Convert between VSWR, return loss, reflection coefficient, and mismatch loss instantly. Get reflected and transmitted power percentages. Free, instant results.

RF

Link Margin

Calculate RF link margin from TX power, antenna gains, path loss, and receiver sensitivity. Determine max range and fade margin for wireless links. Free, instant results.