Antenna Radiation Patterns

Key Takeaways

• Antenna radiation patterns depict the radiation properties of an antenna. 

• Radiations from an antenna are plotted as a function of radial distance and angular position from the antenna.

•  In the 3D view, the omni-directional antenna radiation pattern forms a doughnut shape.

Antennas are intermediate elements between a transmitter and receiver system. They radiate electromagnetic waves into the free space (transmitter antenna) or receive signals from the free space to the guiding device (receiver antenna). The radiation from an antenna can be quantified using parameters such as radiation field strength, directivity, gain, phase, and radiation efficiency—all of which influence how the antenna performs for a specific application.

The performance of an antenna can be improved by working on the antenna parameters. Engineers can get a basic idea of which antenna parameters they need to start with by looking at antenna radiation patterns—pictorial representations of the electromagnetic radiation spreading from an antenna.

Radiation in Antennas

The capability to radiate and receive radiations is a key feature of an antenna. In certain cases, in transmitter and receiver systems, the same antenna multitasks for the transmission and reception of signals.

In the case of a transmitter antenna,  a voltage of a specified frequency is applied to it. The voltage applied and the current flowing in the antenna generates an electric and magnetic field. The electromagnetic field radiates to the free space.

In receiver antennas, the electromagnetic field interacting with the antenna induces a voltage in the antenna elements. The receiver antenna acts as a signal source for the guiding device connected to it. 

Antenna Radiation Patterns

In both transmitter and receiver antennas, the radiation energy assumes different shapes, which are generally called antenna radiation patterns. An antenna radiation pattern is the mathematical representation of the radiation energy distribution with respect to the directional coordinates of the electromagnetic wave departure.

Antenna radiation patterns help engineers understand the behavior and performance of an antenna. They are a basic requirement in any antenna application, as these patterns provide information about the spatial distribution of electromagnetic energy, which helps to identify antenna type and give clarity on antenna pointing directions. With these patterns, one can point an antenna in any direction and see how the antenna performs in that particular direction. This pattern is the primary antenna property considered when the operating frequency is defined.

Antenna radiation patterns depict the radiation properties of an antenna, including:

• Radiation intensity - The radiation intensity is important in antenna design, as it gives the power radiated per unit solid angle.
• Power flux density - By examining the radiation pattern, engineers can identify the direction at which the power flux density is maximized and can direct the antenna there.
• Directivity - If the direction of an antenna is not strictly specified, the direction at which directivity is maximum is considered for pointing the antenna. The parameter-directivity describes the ratio of antenna radiation intensity in a given direction to the antenna radiation intensity in all directions.
• Bandwidth - The comparison of antenna radiation patterns taken at different frequencies can provide information about the bandwidth at which the antenna operates correctly.
• Field strength - The field strength of the antenna radiation pattern in various directions from the antenna at a constant distance can be obtained from the antenna radiation pattern. It is possible to compare the reach of different antennas for a given distance using field strength data. 
• Polarization - For maximum power transfer, the polarization of the transmitter antenna and receiver antenna need to be the same. A polarization mismatch can be easily distinguished from the antenna radiation patterns of the transmitter and receiver antennas. 
• Phase - The phase of electromagnetic wave transmission from an antenna influences the direction of the radiation pattern. The concept of measuring phase is important when using electronic control rather than mechanical sweep control of an antenna. 
• Radiation efficiency - This is particularly useful when the antenna under consideration is a transceiver. 
• Gain - The gain of an antenna compares the antenna of interest with an ideal antenna in the same direction. Engineers can understand how much offset there is in the antenna power gain compared to the ideal antenna.
• Effective aperture - The effectiveness of the antenna as a receiver can be measured from its effective aperture value. 

An antenna radiation pattern is a function of space coordinates. However, for antennas in wireless telecommunication systems, the radiation pattern is defined at the far-field region, which makes it independent of the distance from the antenna. Usually, the spherical coordinate system is used to represent the antenna radiation pattern. The antenna is considered the origin or point of source of the antenna radiation pattern. The radiations from the antenna are plotted as a function of radial distance and angular position from the antenna. 

Power Patterns and Amplitude Field Patterns

The plot of the radiations at the constant radial distance or radius from the antenna positioned at origin is called a power pattern, whereas the spatial distribution of the electric and magnetic fields along a constant radius forms an amplitude field pattern. Usually, power patterns are graphically represented as a function of the square of electric and magnetic fields and field patterns are represented as a function of electric and magnetic fields. In both power and field patterns, the fields are plotted as a function of the angular space on a logarithmic scale. 

Types of Antenna Radiation Patterns

An antenna radiation pattern is a 3D plot, and 2D sections can be obtained by dividing the 3D plot into horizontal and vertical planes. The resulting patterns are called horizontal patterns and vertical patterns, respectively. Some common antenna radiation patterns are:

• Non-directional patterns or omni-directional patterns - In the 3D view, an omni-directional pattern forms a doughnut shape. In 2D, it is a figure-of-eight pattern. This radiation is similar to an isotropic radiation pattern, which is the referential point of antenna radiation patterns. This pattern is uniformly distributed in the given plane, which gives it the merit of a large coverage area.

• Fan-beam patterns - Resembles a fan shape. This pattern is commonly used in defense radars, marine navigation radars, and long-range and ultra-long-range radars.

• Pencil-beam patterns - Resembles a sharp directional pencil-shaped pattern. If you are in need of highly directional antennas, choose pencil-beam patterns. 

• Shaped beam patterns - Any antenna radiation pattern that is patternless and non-uniform falls under the shaped-beam pattern. The shaped beam antenna is the classic choice of antenna pattern when the antenna is illuminating a selected geographical area.  

Antenna characteristics can be determined by looking at antenna radiation patterns. By interpreting the antenna parameters from the radiation pattern, engineers have the opportunity to modify or change the antenna design and make it suitable for any given application in terms of antenna parameters such as field strength, directivity, radiation efficiency, etc. Cadence software offers EM simulation software that allows engineers to perform antenna analysis. 

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