Near Field vs. Far Field Antenna Radiation Pattern

This page covers antenna radiation pattern basics and also describes the difference between near-field and far-field radiation patterns.

Figure 1 depicts the radiation pattern of a directional antenna.

As shown in the figure, the radiation pattern will have a main lobe, side lobes, and a back lobe.

• Main lobe: The direction in which the antenna radiates maximum energy into the air.
• Side lobes: The directions away from the maximum energy direction.

Radiation patterns help determine the gain and directivity of an antenna.

There are two fields of the antenna based on the distance from it.

The directivity of the antenna is referred to as the antenna’s ability to radiate an EM-wave in a particular direction. Beamwidth is used to mention directivity. It is the angular distance between two points on the main lobe. These points are 3dB less than the maximum power on the radiation pattern.

Antenna Gain (G) can be calculated using the following formula:

$G = \frac{4 \times \pi \times A_e}{\lambda^2} = \frac{4 \times \pi \times f^2 \times A_e}{c^2}$

Near Field Region

The region up to the distance of $2D^2/\lambda$ from the antenna is referred to as the near-field region. Where:

• D is the antenna diameter.
• $\lambda$ (Lambda) is the wavelength of free space.

Far Field Region

The region beyond the distance of $2D^2/\lambda$ is referred to as the far-field region. This region is also called the Fraunhofer region. In this region, power radiated from the antenna decays inversely proportional to the square of the distance ($1/R^2$).

The near-field of the antenna will have more energy compared to the far-field. This is because of the proximity of the EM wave to the antenna radiator part. Power in the near-field region follows $1/R^4$, and hence the power intensity falls off very rapidly.

Example: Near Field Distance Calculation

Let’s assume that an antenna has a length of about 7 cm and is radiating a 14 cm wavelength signal. What will be the near-field distance?

Answer: The near-field region extends up to 7 cm from the antenna.