Impedance, SWR and matching
Every antenna presents a feedpoint impedance — the load your transmitter and feedline actually see.
The numbers
A resonant half-wave dipole is about 73 ohms; a quarter-wave monopole about 36 ohms; a folded dipole about 300 ohms. Common coax is 50 ohms, so most antennas need a reasonable, if imperfect, match.
SWR
When the antenna impedance does not equal the feedline impedance, some power reflects back. The Standing Wave Ratio (SWR) measures this:
- 1:1 — perfect match, no reflected power
- 2:1 — about 11% of power reflected; usually fine
- 3:1 and up — increasingly poor; many transmitters fold back power
Fixing a mismatch
A matching network (a tuner, transformer, or matching section) transforms the antenna impedance toward 50 ohms. A balun additionally manages the transition from balanced antenna to unbalanced coax. None of this makes a bad antenna good — it just lets a good antenna accept power efficiently.