Characteristics of Lens antenna

  • Source: eahison
  • Date:04/18/2019

Characteristic

Compared with common antennas, lens antennas have unique characteristics: first, the side and back lobes of the lens antenna are small, and the pattern is better; second, the lens antenna does not require high precision in manufacturing the lens, and thus the manufacturing is relatively simple and convenient. However, the lens antenna also has its own shortcomings: first, the production efficiency is low; second, it is difficult to construct, more complicated, and third, the cost is relatively high.

microwave-antenna

A microwave antenna that concentrates the radiation energy into a narrow beam by using the lens in front of the vibrator or horn radiator.

Lens antennas have the following advantages:

1. The side lobes and the back lobes are small, so the pattern is better;

2. The precision of manufacturing the lens is not high, so the manufacturing is convenient. The disadvantages are low efficiency, complex structure and high price.

3. The lens antenna is used in microwave relay communication.

The basic principle of the lens:

A dielectric lens is placed in front of electromagnetic radiators of various shapes to concentrate electromagnetic radiation into a narrow beam. A lens is an electromagnetic spherical wave that can be released by a "lens" electromagnetic radiation source whose refractive index is not equal to 1 when an electromagnetic wave passes through a "lens" and can be converted into a plane wave to obtain a tapered or cylindrical beam. The refractive index of the lens also tends to vary and can be a function of position. The structure of the lens affects its mouth-surface distribution.

Before making the lens, the refractive index and shape of the lens can be determined in advance according to the needs of use. When a material medium with a refractive index greater than 1 is selected, the lens is concentrated, usually called a deceleration lens; the refractive index of the lens material is less than 1 The role of the lens is divergent and accelerated, commonly referred to as an accelerating lens.

When both sides of the lens are refracting surfaces, they are called double-sided lenses. When only the illuminating surface is a refracting surface, it is called a single-sided lens.

According to the optical theory, the concept of the basic principle of "lens" is clearer, and the design idea becomes clearer. In the early days, some innovative antennas (such as single-pulse radar antennas) often used "lens" as a model to illustrate the working principle, but the "lens" deficiency was cumbersome, material gradients and interface reflections often caused losses. Therefore, there are not many practical lens antennas, such as the luneburg ball lens. (as the picture shows)

luneburg-ball-lens

A microwave antenna equipped with a lens in front of the vibrator or horn radiator to concentrate the radiant energy into a narrow beam

A lens is a three-dimensional structure that can pass electromagnetic waves and whose refractive index is not equal to one. A spherical wave or a cylindrical wave from a point source or a line source can be transformed into a plane wave through a lens to obtain a pen-shaped or fan-shaped beam. The refractive index of the lens may be a function of position, and the shape of the lens determines its surface field distribution. 

The lens may be made of a natural medium having a refractive index n greater than 1, or may be an artificial medium structure composed of a metal grid or a metal sheet (n>1 or n<1). n=c/v φ (where c is the speed of light; v φ is the phase velocity in the medium), a lens with n greater than 1 is called a deceleration lens, and a lens with n less than 1 is called an acceleration lens. When both sides of the lens are refracting surfaces, they are called double-sided lenses. When only the illuminating surface is a refracting surface, it is called a single-sided lens.

When a plane wave is incident on the lens, it is focused by the lens to the other end of the diameter perpendicular to the plane wave front. Therefore, placing a feed at this point creates a plane wave on the surface of the ball antenna. The beam can be scanned 360° as long as the feed is moved on the sphere. A structure that can transform the input field distribution into a desired field distribution, also known as a lens. For example, the input surface and the output surface are respectively arrays of equal number of radiating elements, and the corresponding units are connected by a transmission line. The length of each transmission line is determined according to the output field distribution requirements of the output port. It is also possible to access the phase shifter in each transmission line to obtain a phased scan.

The characteristics of the lens antenna's pattern and impedance are related to the frequency response of the lens refractive index n in addition to the feed characteristics, lens shape, and the like. In the deceleration lens, the change in the wavelength λ has little effect on the refractive index n, but in the accelerating lens (metal plate lens), n has a close relationship with λ, and thus the band is narrow, only a few percent. In a lens composed of a TEM transmission line, n is independent of frequency, and the frequency band is determined by other factors.

 

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