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Antenna design glossary

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This glossary defines the core terms of antenna design, following IEEE Std 145 usage where applicable. Each definition links to a full article with formulas and sources.[1]

What do the key antenna terms mean?

Active reflection coefficient
Reflection seen at an array port with all elements excited; varies with scan angle. More
Antenna factor
Ratio of incident field strength to the voltage at the antenna terminals, used in EMC measurement. More
Aperture efficiency
Ratio of effective aperture to physical aperture area. More
Array factor
Pattern of an array of isotropic elements, set by positions and excitations. More
Axial ratio
Ratio of major to minor axes of the polarization ellipse; 0 dB is perfect circular polarization. More
Balun
Device connecting a balanced antenna to an unbalanced line. More
Beamwidth (HPBW)
Angle between the half-power points of the main lobe. More
Bode–Fano limit
Bound on the bandwidth over which a reactive load can be matched. More
Boresight
Direction of maximum radiation or the antenna’s mechanical axis. More
Chu limit
Minimum radiation Q of an antenna enclosed in a sphere of radius a. More
Circular polarization
Polarization in which the field vector rotates at constant magnitude. More
Common-mode current
Current on the outside of a coaxial shield that makes the feed line radiate. More
Cross-polarization
Field component orthogonal to the intended polarization. More
dBd
Gain in decibels relative to a half-wave dipole; 0 dBd = 2.15 dBi. More
dBi
Gain in decibels relative to an isotropic radiator. More
Directivity
Radiation intensity in a direction relative to the average over all directions. More
E-plane
Pattern plane containing the electric field and the direction of maximum radiation. More
Effective aperture
Area that, multiplied by incident power density, gives delivered power; A_e = λ²G/4π. More
EIRP
Transmitter power times antenna gain relative to isotropic. More
Electrically small antenna
Antenna fitting inside a sphere with ka < 1. More
Envelope correlation coefficient
Measure of similarity between two antennas’ patterns for diversity and MIMO. More
ERP
Effective radiated power relative to a half-wave dipole. More
Far field
Region beyond about 2D²/λ where the pattern is independent of distance. More
FDTD
Finite-difference time-domain method for solving Maxwell’s equations. More
FEM
Finite element method, solving the wave equation in a meshed volume. More
Fresnel zone
Ellipsoidal region around a path where secondary paths differ by multiples of λ/2. More
Friis equation
Free-space received power: P_r = P_t G_t G_r (λ/4πR)². More
Front-to-back ratio
Ratio of forward to rearward gain. More
G/T
Receive gain divided by system noise temperature, in dB/K. More
Gain
Directivity multiplied by radiation efficiency. More
Grating lobe
Spurious main-beam replica caused by large element spacing. More
Ground plane
Conducting surface acting as the image plane or counterpoise of a monopole-type antenna. More
H-plane
Pattern plane containing the magnetic field and the direction of maximum radiation. More
Input impedance
Ratio of voltage to current at the antenna terminals. More
Isotropic radiator
Hypothetical lossless antenna radiating equally in all directions. More
Method of moments (MoM)
Integral-equation method solving for currents on conductors. More
MIMO
Multiple-input multiple-output: multiple antennas at both ends of a link. More
Mismatch loss
Power lost to reflection: −10·log10(1−|Γ|²). More
Mutual coupling
Electromagnetic interaction between nearby antennas. More
Near field
Region close to an antenna where reactive or distance-dependent fields dominate. More
Phase center
Apparent point from which the radiated spherical wave emanates. More
Phased array
Array steered electronically by controlling element phases or delays. More
Polarization
Orientation and rotation of the radiated electric field. More
Polarization loss factor
Fraction of power received due to polarization mismatch. More
Q factor
Ratio of stored to radiated energy per radian; inversely related to bandwidth. More
Radiation efficiency
Ratio of radiated power to accepted power. More
Radiation pattern
Angular distribution of radiated power or field. More
Radiation resistance
Resistance representing radiated power at the antenna terminals. More
Radome
RF-transparent protective cover for an antenna. More
Realized gain
Gain reduced by impedance mismatch loss. More
Reciprocity
Principle that transmit and receive properties of a passive antenna are the same. More
Return loss
−20·log10|Γ|, the ratio of incident to reflected power in dB. More
SAR
Specific absorption rate: RF power absorbed per unit mass of tissue. More
Sidelobe level
Peak of the largest sidelobe relative to the main beam. More
Skin depth
Depth at which current density falls to 1/e of its surface value. More
Smith chart
Graphical plot of impedance on the reflection-coefficient plane. More
TIS
Total isotropic sensitivity: receiver sensitivity averaged over the sphere. More
TRP
Total radiated power integrated over the sphere. More
VSWR
Voltage standing-wave ratio, (1+|Γ|)/(1−|Γ|). More
Wheeler cap
Conducting enclosure used to measure small-antenna efficiency. More

Frequently asked questions

What is the difference between gain and directivity?

Gain equals directivity multiplied by radiation efficiency; directivity ignores losses.

What is the difference between dBi and dBd?

dBi is relative to an isotropic radiator; dBd is relative to a half-wave dipole, 2.15 dB higher than isotropic.

Where are antenna terms officially defined?

In IEEE Std 145, the IEEE Standard for Definitions of Terms for Antennas.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
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