Look beyond an antenna’s size and gain; its radiation pattern holds the key to its performance. Understanding this can unlock a whole new perspective on wireless communication.
You need to know more than simply antenna gain to accurately evaluate the performance of antennas. You should know everything about their radiation patterns. Essentially, the gain of an antenna measures the intensity of the beam it produces. Similar to a flashlight, the more focused the beam, the greater its range and the need for accuracy in orientation.
Radiation pattern vs gain
Antenna gain has a straightforward numerical value and is expressed as dBi. The pattern in which an antenna emits radio frequency energy is known as its radiation pattern. The radiation from an antenna depicts how the antenna performs for a specific application in terms of radiation field strength, directivity, gain, phase, and radiation efficiency as a function of radial distance and angular position from the antenna. An antenna radiation pattern is the mathematical graphical representation of the radiation energy distribution into all directions of space and hence is three-dimensional, expressed in the spherical coordinate system.
Two principal planes
Characterising a well-behaved antenna’s radiation properties with two plane patterns in a coordinate system involves azimuth plane pattern and elevation plane pattern. Assuming that the antenna is measured in the orientation in which it will be used, as shown in Fig. 1(a), the x-y plane or ‘the horizontal’ (θ=90 deg) is the azimuth plane, and the pattern is measured by traversing the entire x-y plane. The plane orthogonal to the x-y plane, say the y-z plane or “the vertical” (φ=90 deg), is the elevation plane in which the pattern is made by traversing the entire y-z plane around the antenna under test. The elevation plane is oriented at right angles to the azimuth plane. In short, the principal plane patterns are:
- • Azimuth/XY/horizontal plane (top-down view) (φ)
- • Elevation/vertical/YZ plane (side–on view) (90-θ)
2D and 3D view
The radiation patterns can be three-dimensional (3D), i.e., a function of (θ, ϕ, r), or two-dimensional (2D). In 2D, the radiation pattern represents a cut of the 3D radiation pattern, for given angles θ = θ0 or ϕ = ϕ0. These two planar patterns (azimuth and elevation plane patterns) are used to describe radiation in a 3D pattern using spherical coordinates, as in Fig. 1(b), and are frequently shown as 2D plots using polar coordinates, as shown in Fig. 1(c).
Below is a summary comparing 3D and 2D patterns. A 2D polar plot can be preferred for symmetrical directional antennas, but for omni-directional or newly designed evaluation, a 3D spherical pattern is used. The preference and comparison between 2D and 3D are shown in Table 1.
| Table 1 Relation between 2D and 3D | |
| 2D Radiation Pattern | 3D Radiation Pattern |
| Resolution is high only in the selected plane | Lower resolution in the entire radiation plane |
| Projects sharp peaks and nulls in directional antennas | Reveals unexpected pattern and distortion |
| Radiation efficiency cannot be calculated since entire sphere is not covered | Radiation efficiency vs frequency is possible |
| Best suited for monopole, log periodic high gain directional antennas | Best suited for PIFA, loop, embedded, and omni directional antennas |
E and H plane
The orientation of the electrical and magnetic components of electromagnetic waves, called polarisation from a radiating antenna, is in a single linear direction for linearly polarised antennas. When representing the radiation patterns of linearly polarised antennas, the E and H planes can be used and compared with azimuth/elevation plane as in Table 2.
| Table 2 Relation between azimuth/elevation and E/H plane | |
| Electric field or E plane | Magnetic field or H plane |
| Plane containing the electric field vector and the direction of maximum radiation | Plane containing the magnetic field and the direction of maximum radiation |
| Called as E aperture | Called as H aperture |
| Determines the polarisation or orientation of the radio wave | Lies at a right angle to the ‘E’ plane |
| For horizontal polarised antenna | |
| Coincides with the horizontal/Azimuth/XY plane | Coincides with the vertical/Elevation plane |
| For vertical polarised antenna | |
| Coincides with the vertical/YZ/Elevation plane | Coincides with the horizontal/Azimuth plane |
| Relation between E and H plane: (a) Horizontally polarised-directional antenna (Yagi-Uda), (b) vertically polarised–omni directional antenna (dipole) | |
An antenna with a poor radiation pattern but high gain may perform less reliably and effectively than an antenna with an excellent radiation pattern but lower gain. Antenna gain can only act on the antenna’s existing radiation pattern, which is again determined by the quality and details of the antenna’s design.
Dr S. Suganthi is Professor at Ece Department, K. Ramakrishnan College of Technology, Samayapuram, Trichy
