Explanation
Overview Feed-line radiation occurs when a transmission line, typically a coaxial cable, unintentionally radiates RF energy instead of carrying it exclusively between the transmitter and the antenna. In a properly operating coaxial feed line, RF energy should remain confined between the center conductor and the inside surface of the shield. When common-mode current develops on the outside of the shield, the feed line itself can become part of the antenna. Common Causes Common-mode current. Unbalanced antenna systems. Missing or ineffective current choke. Poor feed-point design. Asymmetrical antenna installation. Possible Effects Distorted radiation patterns. Higher received noise. RF interference in nearby equipment. Unstable SWR readings. RF in the operating position. Reducing Feed-Line Radiation Install a common-mode choke at the antenna feed point. Improve antenna symmetry. Route coax away from the antenna whenever practical. Verify proper feed-point construction. Inspect feed-line connections. Applied to Chameleon Products Many Chameleon antenna systems benefit from the use of a properly selected common-mode choke to minimize feed-line radiation, stabilize antenna performance,
The exact result depends on the complete station: frequency, geometry, feed line, matching network, return-current path, environment, operating power, and the reference plane of any measurement. A low SWR establishes an impedance relationship at that point; it does not by itself prove efficiency, radiation pattern, compatibility, or safety.
What to Verify
- Use the newest official product guide or primary service documentation.
- Confirm the exact model, revision, components, configuration, and operating conditions.
- Begin tests 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
- Antenna Measurement Reference Planes
- 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, services, software, specifications, availability, and safety requirements against current primary sources.
