What are the fundamental antenna parameters?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

An antenna converts guided waves into radiated waves and back. It is characterized by its radiation pattern, directivity and gain, radiation efficiency, input impedance and bandwidth, polarization, and effective aperture A_e = Gλ²/(4π). IEEE Std 145 provides the standard definitions.^[1]^[2]

What are the near-field and far-field regions?

The reactive near field extends to about 0.62√(D³/λ); the radiating near field (Fresnel region) extends to the far-field distance R = 2D²/λ for an antenna of largest dimension D. Patterns and gain are defined in the far field, where fields fall as 1/r and the angular distribution is independent of distance.^[2]

What is radiation resistance?

The part of the input resistance that accounts for radiated power: P_rad = ½|I|²R_rad (peak current). Loss resistance accounts for ohmic heating. Radiation efficiency is e_r = R_rad/(R_rad + R_loss); small antennas have low R_rad and therefore low efficiency unless losses are minimized.^[2]

What pattern parameters are specified?

Half-power beamwidth, first-null beamwidth, sidelobe level, front-to-back ratio, and for arrays, scan range and grating lobes. Patterns are plotted as E-plane and H-plane cuts or full 3D measurements; see antenna measurement.

How does reciprocity apply?

For antennas made of linear, passive, isotropic materials, transmit and receive patterns and impedances are identical, so either mode can be measured.^[2]

Frequently asked questions

What is an isotropic antenna?

A hypothetical lossless antenna radiating equally in all directions; the 0 dBi reference.

Where does the far field begin?

Conventionally at 2D²/λ for electrically large antennas, and at least several wavelengths away.

Is antenna bandwidth only about impedance?

No; pattern, gain and polarization bandwidths also matter.

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)”. IEEE. 2013 revision.
“Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.

Categories: Antennas and propagation

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