Ham Radio Tools
Coax Loss Calculator
Estimate how much transmit power is lost in a coaxial feedline before it reaches the antenna. Enter the operating frequency, coax type, cable length, and transmitter power to calculate line loss, antenna-end power, efficiency, and heat lost in the cable.
Calculator
Results
Enter values to estimate feedline performance.
| Band | Frequency | Loss | Antenna power | Efficiency |
|---|
How the Calculation Works
Coaxial cable attenuation is usually specified in dB per 100 ft or dB per 100 m at several frequencies. This calculator estimates the attenuation at your selected frequency, scales it by cable length, and converts the dB loss into a power ratio.
Rules of Thumb
- 1 dB loss is usually acceptable for many HF stations, but it still loses about 20 percent of power.
- 3 dB loss means roughly half your power is gone in the feedline.
- Long RG-58 runs can work on HF, but they become lossy on VHF and UHF.
- For 2 m, 70 cm, and above, shorter cable and lower-loss coax often matter more than a small power increase.
- Real installations also include connector loss, adapters, water ingress, cable age, and mismatch effects.
Notes for Real Stations
This tool estimates matched cable loss. If the antenna has a high SWR, the feedline can see additional loss because reflected power travels back through the cable. For serious work, measure the antenna system with an antenna analyzer or VNA at the shack and, when possible, near the antenna feed point.
Built-in cable data is compiled from common attenuation charts and manufacturer-style datasheets. Treat it as a planning estimate; for final engineering work, use the exact datasheet for the cable brand and construction you buy.