Coax Cable Loss Calculator

Calculate feed line attenuation for common coaxial cable types. Determine matched loss, SWR mismatch loss, and how much power actually reaches your antenna.

Select the coaxial cable type used in your feed line.
Operating frequency in megahertz (1–3,000 MHz).
Total length of the coaxial cable run.
If provided, calculates watts lost and watts delivered to the antenna.
Standing Wave Ratio at the antenna. Leave blank or 1.0 to skip mismatch loss.
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Common Scenarios — Quick Reference

Scenario Cable Freq Length
50 ft RG-213 on 20m (HF)
Typical HF station feed line		 RG213 14.2 MHz 50 feet
100 ft LMR-400 on 2m (VHF)
Tower-mounted VHF antenna		 LMR400 146.0 MHz 100 feet
25 ft RG-58 on 70cm (UHF)
Short UHF run — loss adds up fast at UHF		 RG58 440.0 MHz 25 feet
30 m LMR-600 on 23cm
Microwave feed line — every dB counts		 LMR600 1296.0 MHz 30 meters

Understanding Coaxial Cable Loss

Why does cable loss matter? Every decibel of loss in your feed line is power that never reaches your antenna. A 3 dB loss means half your power is wasted as heat in the cable. On VHF and UHF, where cable loss increases significantly with frequency, choosing the right cable can make the difference between a strong signal and a marginal one.

Matched loss vs. mismatch loss: Matched loss (also called "flat loss" or "attenuation") occurs even with a perfect 1.0:1 SWR. It increases with frequency and cable length. Mismatch loss is the additional power reflected back due to an imperfect impedance match (SWR > 1.0). In practice, moderate SWR (under 2:1) adds relatively little additional loss — usually less than 0.5 dB.

How to choose cable: For HF (below 30 MHz), even RG-8/213 performs well for runs under 100 feet. For VHF (2m), LMR-400 or equivalent is a strong choice for tower runs. For UHF (70cm) and above, every foot of cable matters — use the lowest-loss cable you can afford and keep runs as short as practical. Tower-mounted preamps and remote antenna tuners can help compensate for unavoidable loss.

Real-world factors: Published loss specs assume new cable at 68°F (20°C) with properly installed connectors. Actual loss increases with cable age, UV exposure, moisture ingress, tight bends, and poor connector workmanship. Measured loss in an installed cable run is typically 10–20% higher than book values. When in doubt, measure your actual cable loss with an antenna analyzer.

75-ohm cable note: RG-6 and other 75-ohm cables are designed for TV/CATV use. Connecting them to 50-ohm amateur equipment creates a baseline 1.5:1 SWR from the impedance mismatch alone, before any antenna mismatch is factored in. They can work for receive-only applications but are not recommended for transmitting.

Related tools: Use the RF Exposure Calculator to evaluate your station's MPE compliance using the feed line loss calculated here. Calculate antenna element dimensions with the Antenna Length Calculator, then determine how much power your feed line will deliver. Check your operating privileges and power limits with the Band Plan Checker.

This tool was created by Ben Crittenden, an IT professional with experience in web development, systems administration, and project management.