Feedline Loss, Attenuation & Frequency Effects

Feedline loss, also known as attenuation, is one of the most important factors affecting amateur radio station performance. As radio signals travel through coaxial cable, a portion of the signal energy is lost as heat. This loss increases with frequency and cable length, directly reducing transmitted power and received signal strength.

Understanding how attenuation works allows operators to make informed decisions about coax selection, run length, and station layout to maximize efficiency.

What Is Feedline Attenuation?

Attenuation is the reduction in signal strength as it travels through a transmission line. It is typically measured in decibels (dB) per unit length and increases as frequency rises. Even small amounts of loss can significantly impact performance, especially at VHF and UHF.

A loss of 3 dB represents approximately half of the transmitted power reaching the antenna.

Frequency vs. Loss

Higher frequencies experience greater attenuation in coaxial cable. A feedline that performs well on HF may introduce substantial loss on VHF or UHF if the same cable type and length are used.

• HF frequencies generally tolerate higher-loss coax
• VHF frequencies benefit from lower-loss feedline
• UHF frequencies demand high-quality, low-loss coax

Cable Length Effects

As feedline length increases, total attenuation increases proportionally. Longer runs require careful cable selection to prevent excessive signal loss.

• Short runs may allow more flexibility in coax choice
• Longer runs amplify the importance of low-loss cable
• Excessive adapters and connectors add additional loss

Real-World Impact on Transmit & Receive

Feedline loss affects both transmitted and received signals. Loss in the feedline reduces effective radiated power on transmit and weakens incoming signals on receive. Improving feedline quality can often yield better results than increasing transmitter power.

Balancing Performance & Practicality

While the lowest-loss coax offers the best performance, practical factors such as cost, flexibility, and installation constraints must also be considered. Selecting the right balance ensures reliable operation without unnecessary expense.

Related Coax & Feedline Guides

→ How to Choose the Right Coaxial Cable
→ Coaxial Cable Types & Characteristics
→ Installing & Weatherproofing Coaxial Cable

This guide is part of the Elmer Reference Library and is intended to help amateur radio operators understand feedline loss and make informed decisions that improve station efficiency.