Coax Cable Loss Calculator

Common Scenarios — Quick Reference

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. The dominant loss in most installations is matched loss from the cable itself.

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.

Connector quality counts: A poorly installed PL-259 or BNC connector can introduce as much loss as 10–20 feet of cable. Silver-plated connectors with proper solder joints and weatherproofing (coax seal, self-amalgamating tape) on outdoor connections are well worth the extra effort.

Related radio tools: Feed line loss directly affects your RF exposure evaluation — use the total loss figure calculated here as the feed line loss input in the RF Exposure Calculator for an accurate effective radiated power calculation. To determine what antenna you're feeding, use the Antenna Length Calculator to get element dimensions for your operating frequency. And before transmitting, verify that your frequency, mode, and power level are within your license privileges using the Band Plan Checker.