Antenna Impedance
Understanding Antenna Impedance
Antenna impedance determines how efficiently power is transferred between the transmission line and the radiating structure. Understanding and matching antenna impedance is essential for system performance.
Antenna Impedance Components
- Radiation resistance (Rr): The real part that accounts for radiated power. Higher Rr = more efficient radiation.
- Loss resistance (Rl): Conductor and dielectric losses. Reduces efficiency.
- Reactance (Xa): Stored energy near the antenna. Zero at resonance. Capacitive below resonance, inductive above.
Impedance of Common Antennas
| Antenna | Za at resonance |
|---|---|
| Half-wave dipole | 73 + j0 ohms |
| Quarter-wave monopole | 36 + j0 ohms |
| Microstrip patch | 100-400 ohms (edge-fed) |
| Folded dipole | 300 + j0 ohms |
| Small loop | < 1 ohm + jX (very reactive) |
Frequently Asked Questions
What is antenna impedance?
Antenna impedance Za = Ra + jXa is the feed point impedance. Ra includes radiation resistance and loss resistance. At resonance, reactance Xa = 0. A dipole is 73 ohms at resonance. Must be matched to the feed line for efficient power transfer.
Why is 50 ohms the standard?
50 ohms is a compromise between maximum power handling (~30 ohms for coax) and minimum loss (~77 ohms for coax). At 50 ohms, the loss is only about 10% more than minimum, while power handling is significantly better than 75 ohms.
What if the antenna is not matched?
Mismatch causes power reflection (mismatch loss), standing waves on the feed line, and potentially PA damage. A matching network (LC, stub, quarter-wave transformer) transforms the antenna impedance to 50 ohms.