What is antenna input impedance?
Input impedance is the impedance presented by an antenna at its terminals, ZA = RA + jXA, where RA = Rr + RL combines radiation and loss resistance.[1][2] An antenna is resonant where XA = 0; a thin half-wave dipole is about 73 + j42.5 Ω at exactly λ/2 and becomes resistive when shortened by a few percent.[2]
What determines input impedance?
Impedance depends on geometry, feed position, frequency, conductor thickness and the environment: ground planes, nearby objects and other antennas (mutual coupling). Feeding a dipole off-center or a patch at different inset depths changes the resistance seen, in proportion to the local current and voltage.[3]
What are typical impedances?
| Antenna | Approximate resonant impedance |
|---|---|
| Half-wave dipole (thin) | ≈ 73 Ω |
| Folded dipole | ≈ 4 × 73 ≈ 290 Ω |
| λ/4 monopole, infinite ground | ≈ 36.5 Ω |
| Rectangular patch, edge fed | ≈ 150–300 Ω |
| Half-wave slot | ≈ several hundred ohms (Babinet) |
| Small loop | milliohms to ohms + large inductive reactance |
From Balanis.[2]
Why does impedance matter?
Maximum power transfer and low VSWR require the antenna impedance to match the line (usually 50 Ω), either directly by geometry or through a matching network. Reflected power is lost to the system and can stress transmitters.[4]
How is impedance measured?
With a calibrated vector network analyzer, referencing the measurement plane to the feed point by port extension or de-embedding, and taking care to suppress common-mode currents on the cable.[5]
Frequently asked questions
Why is a dipole cut shorter than λ/2?
To cancel its inductive reactance; resonance typically occurs at about 0.47–0.48λ depending on thickness.
Does impedance depend on height above ground?
Yes; a horizontal dipole’s resistance oscillates around 73 Ω with height because of the ground image.
Is 50 Ω special for antennas?
No; it is a coaxial-line convention. Antennas are matched to it.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Antenna Theory and Design, 3rd ed.”. W. L. Stutzman and G. A. Thiele, Wiley. 2012.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “Applying Error Correction to Network Analyzer Measurements (Application Note 1287-3)”. Keysight Technologies. application note.