Coaxial Cable Characteristics — Loss, Impedance, Shielding & RF Performance
Coaxial cable is the most widely used feedline in amateur radio, and understanding its characteristics is essential for building efficient and reliable stations. This guide explains the key performance factors that determine how coax behaves in real-world installations: loss, impedance, shielding effectiveness, construction, and general RF performance.
Loss Characteristics
Loss occurs as RF energy is absorbed by the cable’s dielectric and conductor materials. Important factors include:
- Attenuation per 100 ft or per 100 m at various frequencies
- Dielectric quality — solid PE, foam PE, PTFE
- Copper vs copper-clad aluminum center conductor
- Frequency dependence — loss increases as frequency rises
Characteristic Impedance
Most ham-radio coax is standardized at 50 ohms, balancing power handling capability and low-loss characteristics. Impedance mismatch between transmitter, feedline, and antenna can increase SWR and reflected power.
Shielding & Noise Rejection
Shielding quality determines how well the coax minimizes interference and prevents RF leakage. Types include:
- Single braid
- Double braid
- Braid + foil combinations
- Quad-shielded designs
Effective shielding reduces common-mode noise, received interference, and stray RF coupling into nearby equipment.
Velocity Factor
Velocity factor indicates the speed of RF propagation through the dielectric compared to free space. Foam dielectrics generally provide higher velocity factors, making precise cable length calculations more predictable for matching systems and phasing lines.
Mechanical & Environmental Considerations
- UV resistance of outer jacket
- Flexibility (stranded vs solid core)
- Direct burial ratings
- Temperature stability for high-power or outdoor installations
Summary
The performance of your station is strongly influenced by the coaxial cable you choose. By understanding loss, impedance, shielding, and velocity factor, amateur radio operators can pick the right feedline for their power level, frequency range, and environmental needs.
Continue exploring the Feedline section for more articles on ladder line behavior, SWR and mismatch loss, connectors, and weatherproof installation techniques.
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