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Chameleon Knowledge Base · The Complete Online HF Antenna Handbook

What Is Velocity Factor?

Learn what velocity factor is, why radio waves travel slower inside coaxial cable, and how velocity factor affects antenna construction and measurements.

Getting Started HF Fundamentals Reviewed 2026-07-14
Short Answer: Learn what velocity factor is, why radio waves travel slower inside coaxial cable, and how velocity factor affects antenna construction and measurements.

Explanation

Overview Velocity Factor (VF) describes how fast RF energy travels through a transmission line compared to the speed of light in free space. Because the dielectric material inside coaxial cable slows electromagnetic waves, signals travel at a fraction of the speed of light. Understanding Velocity Factor A velocity factor of 0.66 means RF travels at approximately 66% of the speed of light inside the cable. A velocity factor of 0.85 means RF travels at approximately 85% of the speed of light. Different coaxial cables use different dielectric materials, resulting in different velocity factors. Why Velocity Factor Matters Velocity factor becomes important whenever electrical cable length is critical, including: Quarter-wave matching sections Half-wave phasing lines Stub filters Transmission-line transformers Some antenna matching systems Physical Length vs. Electrical Length Two cables of identical physical length may have different electrical lengths because of different velocity factors. This is why accurate antenna construction often requires using the manufacturer's published velocity factor. Applied to Chameleon Products Most Chameleon antenna users will not need to calculate velo

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.