The essential antenna terms. Learn about E-fields, H-fields, ports, and radiation patterns. Understand polarization, impedance match, and bandwidth.
Common Antenna Terms
Common terms used in describing antenna performance:
E-field: Electric field component
H-field: Magnetic field component
Port: Unique interface for input or output
Radiation pattern: The way that an antenna distributes energy in space
Bandwidth: Frequency range over which an antenna operates
Directivity: The extent to which an antenna focuses energy
Impedance match: Measure of how well an antenna converts input power to radiated power
Polarization: Description of E-field vector
B and H are often used interchangeably when referring to magnetic fields, though there is a subtle difference.H-fields are used to represent the magnetic field due to current flow and B‐fields the magnetic field density (in other words, H is a bit like the number of magnetic field lines and B is how tightly packed they are). Port refers to the signal input/output connection of the antenna. Radiation pattern can be further quantified by directivity. Bandwidth defines the frequency range over which the antenna operates as designed. Directivity is a value (calculated from the antenna pattern) that quantifies the extent to which the antenna focuses energy. The higher the directivity, the more focused the antenna beam. Impedance match is a measure of how well the antenna converts input power to radiated power. Polarization describes the orientation of the E‐field vector as the energy propagates through free space.
The E and H fields define two orthogonal reference planes
The electric field (E‐plane) determines the polarization orientation
E-field horizontal: antenna is horizontally polarized
E-field vertical: antenna is vertically polarized
E-field rotates: antenna is circularly polarized
Radiated antenna energy is comprised of electric and magnetic field components, E and H, which define two orthogonal reference planes. The polarization of an antenna is determined by the E‐field plane orientation. When the E‐field lies in the horizontal plane, the antenna is considered horizontally polarized. Likewise, when the E‐field is in the vertical plane, the antenna is vertically polarized. The E‐field can also be rotating as a function of time and distance. In such a case, the antenna is said to be circularly polarized.
Measurement parameters used to define polarization:
Axial Ratio (AR)
Tau (Elliptical Tilt)
Polarization of a radiated wave results in the electric field vector varying in time in the direction of propagation and is the curve traced by the end point of an arrow representing the instantaneous electric field. If the E‐field rotates as a function of time along the direction of propagation, the electromagnetic wave is said to be elliptically polarized. The E‐field vector at any instance in time along the direction of propagation can be broken into a horizontal and a vertical component. An electromagnetic wave that has only a horizontal or vertical component is considered to be linearly polarized.
- Spatially selective collectors of energy
- Frequency selective collectors of energy
The term bandwidth is used to describe the frequency range over which the antenna
will operate satisfactorily:
- There is NO unique definition
- It defines the bandwidth over which selected antenna characteristics (depending on the antennas application) meet a certain specification
- e.g., 40% is broadband, 20% is typical for OEWG probes
Antenna bandwidth is used to describe the frequency selectivity of an antenna and is often represented by a percentage value. Several different design parameters can be evaluated to define the antenna bandwidth.
Antenna Bandwidth Definitions
Radiation pattern bandwidth defines a frequency range over which a certain beam width and/or sidelobe and/or cross-pol is maintained to specified levels.
Gain bandwidth defines a frequency range over which the antenna gain is maintained within a specified range
Input impedance bandwidth defines a frequency range over which its reflection coefficient is less than a given level, e.g. -10dB.
Antenna bandwidth can be quantified using radiation pattern characteristics, gain, and/or input impedance. Which measured value(s) are used is typically defined by the application and which performance metrics are deemed most important. For example, in wireless applications the antenna gain (which includes transfer of input to radiated energy) is typically more important than the overall pattern characteristics.
Gain, Directivity & Efficiency
Gain / Directivity: The extent to which an antenna focuses energy
In general gain is measured and directivity is calculated
Efficiency (dB) = Directivity (dB) – Gain (dB)
We already introduced the term antenna “gain”, but what does this really represent? Gain, much like directivity, is used to quantify the extent to which an antenna focuses energy. Unlike directivity, gain is a measured parameter whereas directivity is calculated.
Therefore, gain takes into account additional IR and thermal losses incurred during the transmit of input power to radiated output power. These losses define antenna efficiency, which is the difference between directivity and gain. Realized gain differs from (IEEE) gain in that it is "reduced by its impedance mismatch factor." This mismatch induces losses above the dissipative losses described above; therefore, realized gain will always be less than gain.
Antenna Measurement Units
An isotropic radiator is a theoretical point source of electromagnetic energy that radiates uniformly in all directions.
dB – decibel, logarithmic unit used to express the ratio of two values
dBm - decibel relative to one milliwatt (mW) - For example, 0 dBm = 1 mW
dBi – decibel relative to an isotropic radiator
Up until this point we have been discussing relative antenna measurement parameters, which are represented in dB values. To obtain an absolute value, the measured parameter needs to be referenced to a known quantity. For absolute power, values are referenced to watts (dBW) or milli‐watts (dBm). When measuring gain, the absolute used is an isotropic radiator, which is a theoretical point source of EM energy that radiates uniformly in all direction (i.e. the perfect omni‐directional antenna). The measured gain relative to this isotropic radiator is expressed in values of dBi.
30cm x 30cm Aperture Directivity
A is the aperture area in m2
λ is wavelength in m and
η is the aperture efficiency
Directivity can be calculated using the antenna pattern or from the antenna design parameters using the equation above. As this value does not include radiated antenna losses (i.e. antenna efficiency), directivity should always be greater than the actual measured gain.
For a uniformly illuminated square aperture, 30cm x 30cm, one can therefore plot directivity as a function of frequency as shown above.