What Is a Directional Antenna?

Directional antenna (Directional antenna) refers to an antenna that emits and receives electromagnetic waves in one or several specific directions, and emits and receives electromagnetic waves in other directions that are zero or extremely small. The purpose of using a directional transmitting antenna is to increase the effective utilization of radiated power and increase confidentiality; the main purpose of using a directional receiving antenna is to enhance signal strength and increase anti-interference ability.

Directional antennas are often used to cover longer streets and have the following advantages:

1.Large forward gain

2, can suppress backward

(1) Binary directional antenna array

Consists of two identical radiating elements, the distance between the two elements is generally a quarter wavelength. This is a weak directional antenna with low gain and wide half-power angle. Passive reflectors are generally used because the input impedance of the antenna changes more smoothly than with a director. Usually only feed the radiator, and a variable reactor is connected to the reflector. By changing this reactance value, the forward (by the radiator direction) radiation is the highest or the backward (by the reflector direction) radiation is the weakest. . But there are also two antenna elements feeding at the same time, using the network to adjust the current amplitude and phase of the two elements to make the radiation pattern of the antenna array meet the needs.

(2) Quaternary directional antenna array

The quaternary directional antenna array is composed of four identical radiating elements, and its plane layout and the relationship between the currents are shown in the figure. The distance d between two radiating elements or two reflecting elements. Generally a half wavelength, the distance d between the radiating element and the reflecting element. Generally 1/4 wavelength.

Antennas have different radiating or receiving capabilities for different directions in space. This is the directivity of the antenna. There are two types of antennas: omnidirectional and directional. The following mainly explains the differences between them and related parameters.

Omnidirectional antenna

An omni-directional antenna, that is, it displays 360 ° uniform radiation on the horizontal pattern, which is usually referred to as non-directional. In general, the smaller the lobe width, the greater the gain. Omnidirectional antennas are generally used in communication systems with short distances, large coverage areas, and low prices. The gain is generally below 9dB. The following figure shows the signal radiation pattern of an omnidirectional antenna.

The radiation range of an omnidirectional antenna is more like an apple.

Directional antenna

A directional antenna appears as a certain angular range of radiation on the horizontal pattern, which is usually referred to as directional. As with omnidirectional antennas, the smaller the lobe width, the greater the gain. Directional antennas are generally used in communication systems for long communication distances, small coverage areas, high target densities, and high frequency utilization.

We can also think about the relationship between omnidirectional antennas and directional antennas: omnidirectional antennas emit signals in all directions, and they can receive signals from front to back, left and right. The directional antenna is like a bowl-shaped reflective surface behind the antenna. The signal can only be transmitted to the front, and the signal directed to the back is blocked by the reflecting surface and reflected to the front, which strengthens the signal strength in the front. The following figure shows the signal radiation pattern of a directional antenna. ^[1]

Directional antenna

The main radiation range of a directional antenna resembles an inverted, incomplete cone.

Difference between the two

Through the above we can visually understand what is an omnidirectional antenna and what is a directional antenna, so what should we pay attention to in practical applications?

Antenna purchase: If multiple sites need to be met, and these sites are distributed in different directions of the AP, an omnidirectional antenna needs to be used; if concentrated in one direction, a directional antenna is recommended; in addition, it is also necessary to consider whether the antenna connector is in Whether the matching with the AP and the gain of the antenna meet your needs;

Antenna installation: For outdoor antennas, lightning protection equipment needs to be added between the antenna and the wireless AP; the directional antenna should pay attention to the front of the antenna facing the far site; the antenna should be installed as high as possible, between the antenna and the site as far as possible Possibly satisfying sight distance (visible to the naked eye, avoiding obstacles in the middle).