Explanation
Overview Feed-line loss , also called transmission line loss , is the amount of RF energy that is dissipated as heat while traveling through the transmission line between the radio and the antenna. Every coaxial cable has some loss. The objective is not to eliminate feed-line loss—because that is impossible—but to minimize it so that as much RF power as possible reaches the antenna. Why Feed-Line Loss Matters If a transmitter produces 100 watts but the feed line loses 20 watts before the signal reaches the antenna, only 80 watts are available for radiation. The same loss also affects received signals because weak incoming signals are attenuated before reaching the receiver. What Determines Feed-Line Loss? Operating frequency Cable length Cable diameter Cable construction Dielectric material Connector quality SWR on the transmission line As frequency increases, feed-line loss generally increases. Cable Length Longer cables have greater loss because RF energy travels farther through the conductor and dielectric material. Whenever practical, use only the cable length required for the installation. Excess coax should not be added simply for convenience. Cable Size Larger-diameter coaxial cables generally have lower attenuation than smaller cables because they use larger conductors and lower-loss dielectric materials. However, larger cables are heavier, more expensive, and less flex
Interpret this concept within the complete antenna and station system. Frequency, geometry, feed line, matching network, return-current path, ground, nearby conductors, operating power, and measurement reference plane can change the observed result. A low SWR alone does not prove radiation efficiency, pattern, compatibility, or safety.
What to Verify
- Confirm the exact product, revision, configuration, and newest primary instructions.
- Measure at a known reference plane and record the field geometry.
- Begin at low power and change one variable at a time.
- Do not infer compatibility from connector or thread fit.
Learn Next
- Antenna Selection: A Mission-First Decision Guide
- Engineering Design Tradeoffs in Portable HF Antennas
- Feedline Loss and Overall System Efficiency
- Understanding Common-Mode Current
Source note: Independently synthesized with reference to The ARRL Handbook for Radio Communications, 99th edition (2022), and The ARRL Antenna Book for Radio Communications, 24th edition (2019). Verify changing regulations, product specifications, and safety requirements against current primary sources.
