Coax and Feedlines — Getting Power to the Antenna
What Feedlines Do
The feedline carries RF from your transmitter to the antenna (and received signals back). The most common type is coaxial cable (coax) — a centre conductor surrounded by insulation, then a shield, then a jacket.
Characteristic Impedance
Every feedline has a characteristic impedance (Z₀) determined by its physical construction:
| Type | Z₀ | Loss (per 30m @ 150 MHz) | Best for |
|---|---|---|---|
| RG-58 | 50 Ω | ~6 dB (high) | Short runs, low power, patch leads |
| RG-213 | 50 Ω | ~3 dB | General HF use, moderate runs |
| LMR-400 | 50 Ω | ~1.5 dB (low) | VHF/UHF where loss matters |
| 450 Ω ladder line | 450 Ω | ~0.3 dB (very low) | Balanced antennas, lowest loss |
Velocity Factor
RF travels slower in a feedline than in free space. The velocity factor tells you how much slower:
- Solid polyethylene coax: ~0.66 (signal travels at 66% of light speed)
- Foam dielectric coax: ~0.80
- Air-spaced line: ~0.95
Why this matters: If you need a quarter-wave matching section of coax, you must account for the velocity factor. A λ/4 section at 14 MHz in solid coax is shorter than you'd expect: (300/14/4) × 0.66 = 3.54 metres, not 5.36 metres.
SWR — Standing Wave Ratio
When your antenna's impedance doesn't match the feedline's impedance, some power reflects back. This creates standing waves on the feedline, measured as SWR:
| SWR | Reflected Power | What it means |
|---|---|---|
| 1.0:1 | 0% | Perfect match (theoretical) |
| 1.5:1 | 4% | Excellent — don't touch anything! |
| 2.0:1 | 11% | Good — most tuners handle this easily |
| 3.0:1 | 25% | Needs attention — use a tuner |
| ∞:1 | 100% | Open or short circuit — nothing radiates |