Lens Antenna Basics: Types, Operation, and Design

This page covers the basics of lens antennas, including their functions and operation. It also describes the two main types of lens antennas: dielectric and metal plate.

Definition: A lens antenna is a three-dimensional electromagnetic device with a refractive index other than unity. It consists of an electromagnetic lens and a feed mechanism. This antenna is similar to a glass lens used in optics.

Here are the primary functions of a lens antenna:

• Generates a plane wavefront from a spherical wavefront.
• Focuses an incoming wavefront at its focal point.
• Creates directional characteristics.
• Collimates electromagnetic rays.
• Controls aperture illumination.

Working Operation of a Lens Antenna

Figure 1: Working operation of lens antenna

As shown in Figure 1, a feed antenna is positioned at the focal point of the lens. Two different modes of lens antenna operation are illustrated: transmit mode and receive mode.

In transmit mode, diverging rays are collimated, forming a plane wavefront after they pass through the lens. Collimation occurs due to refraction within the lens, which has a refractive index of less than one.

In receive mode, parallel rays converge at the focal point after passing through the lens, also due to refraction.

While lens antennas can be used with a point source, horn antennas are often used at the focal point in practical applications.

Types of Lens Antennas

There are two primary types of lens antennas:

• Dielectric Lens Antenna
• Metal Plate Lens Antenna

Dielectric Lens Antenna

Figure 2: Dielectric lens antenna

Figure 2 illustrates a dielectric lens antenna. As shown, the spherical wavefront produced by the primary feed antenna is converted into a plane wavefront by the dielectric lens.

This is also known as a delay lens antenna because the outgoing electromagnetic rays are collimated and delayed by the lens material.

Here are some features of dielectric lens antennas:

• More useful at higher frequencies. They can become heavy and bulky at frequencies less than 3 GHz.
• Typically made of materials such as polystyrene, lucite, or polyethylene.

The frequency limitation mentioned above can be mitigated by using stepped or zoned dielectric lenses. The width of a stepped lens can be expressed as:

Width (w) = λ / (n - 1)

where:

• n = refractive index of the lens
• λ = wavelength

The bandwidth of a zoned lens antenna is expressed as:

Bandwidth = (50 * n) / (1 + K * n)

where:

• K = Number of zones
• n = refractive index

Metal Plate Lens Antenna

Figure 3: Metal plate lens antenna

Figure 3 shows a metal plate lens antenna. In this type, a spherical wavefront is converted into a plane wavefront, but the outgoing wavefront is speeded up by the material of the lens. The figure illustrates this operation.

Advantages and Disadvantages

The main advantage of lens antennas is that the feed antennas in the design do not obstruct the aperture.

The main disadvantage of lens antennas is that they tend to be more expensive for the same gain and bandwidth compared to reflector antennas.