Antennas, Transmission Lines, and RF Safety
From feedline to field
Extra questions connect antenna theory to practical safety. You must understand characteristic impedance, standing wave ratio (SWR), feedline loss, antenna gain and directivity, and the FCC RF exposure rules in §97.13(c) and §1.1310.
Concepts to master
- Transmission lines: characteristic impedance Z₀ depends on conductor geometry and dielectric, not on length. SWR = (1 + |Γ|)/(1 − |Γ|), where Γ is the reflection coefficient. A 1:1 SWR means all power is delivered; higher SWR reflects power back and increases loss.
- Antenna gain: dBi is gain relative to an isotropic radiator; dBd is gain relative to a dipole (dBi = dBd + 2.15). Gain and directivity trade off against beamwidth.
- Feedline loss: coaxial cable loss increases with frequency. Ladder line has lower loss than coax at HF. A tuner can match impedance but cannot recover power lost in the feedline.
- RF safety: the FCC sets Maximum Permissible Exposure (MPE) limits based on frequency, averaging time, and whether the exposure is controlled or uncontrolled. Amateurs must perform an exposure evaluation; most stations are exempt, but high-power or close-in antennas may not be.
Traps
Questions often test the difference between gain and ERP (ERP = transmitter power × antenna gain relative to a dipole), and the fact that SWR does not cause feedline loss directly - mismatch loss plus line loss does. Another trap: near-field vs far-field exposure; MPE limits apply in the far field, and the near field requires different evaluation.
Worked example
A 100 W transmitter feeds an antenna with 6 dBd gain. ERP = 100 W × 10^(6/10) = 100 × 4 = 400 W relative to a dipole. If the feedline loses 3 dB, delivered power is 50 W, so ERP = 50 × 4 = 200 W.