What Is a Horn Antenna?

The horn antenna is a surface antenna, and a circular or rectangular cross-section microwave antenna with a gradually expanding opening at the waveguide terminal is the most widely used type of microwave antenna. Its radiation field is determined by the size and propagation of the mouth of the horn. Among them, the effect of the horn wall on radiation can be calculated using the principle of geometric diffraction. If the length of the horn remains the same, the phase difference between the mouth surface size and the quadratic phase will increase as the horn opening angle increases, but the gain will not change with the mouth surface size. If the frequency band of the speaker needs to be extended, the reflection at the neck and mouth of the speaker needs to be reduced; the reflection will decrease as the size of the mouth increases. The structure of the horn antenna is relatively simple, and the directional pattern is relatively simple and easy to control. Generally, it is used as a medium directional antenna. Parabolic reflector horn antennas with high bandwidth, low sidelobes, and high efficiency are often used in microwave relay communications.

The horn antenna is a surface antenna, and a circular or rectangular cross-section microwave antenna with a gradually expanding opening at the waveguide terminal is the most widely used type of microwave antenna. Its radiation field is determined by the size and propagation of the horn. Among them, the effect of the horn wall on radiation can be calculated using the principle of geometric diffraction. If the length of the horn remains the same, the phase difference between the mouth surface size and the quadratic phase will increase with the increase of the horn opening angle, but the gain will not change with the mouth surface size. If the frequency band of the speaker needs to be extended, the reflection at the neck and mouth of the speaker needs to be reduced; the reflection will decrease as the size of the mouth increases. The structure of the horn antenna is relatively simple, and the directional pattern is relatively simple and easy to control. Generally, it is used as a medium directional antenna. Parabolic reflector horn antennas with high bandwidth, low sidelobes, and high efficiency are often used in microwave relay communications.

Horn antenna structure and classification

The horn antenna is a widely used microwave antenna. Its advantages are simple structure, high frequency bandwidth, large power capacity, and easy adjustment and use. Reasonable choice of speaker size

, Can obtain good radiation characteristics: quite sharp main lobe, smaller side lobe and higher gain. Therefore, the horn antenna is widely used in military and civilian applications, and it is a common test antenna.

The basic form of the horn antenna is formed by gradually expanding the opening surfaces of the rectangular waveguide and the circular waveguide. As the opening surface of the waveguide is gradually enlarged, the matching between the waveguide and free space is improved, and the reflection coefficient in the waveguide is small, that is, Most of the transmitted energy is radiated by the horn, and the reflected energy is very small.

In principle, the waveguide open end and the horn antenna are very simple antennas, but it is quite difficult to strictly solve their aperture and external fields. First, the field distribution on the waveguide end face and the horn mouth surface is different from the field distribution in an infinitely long waveguide, and the TEM waves propagating in space are also different, which are relatively complex structures. Secondly, in addition to the incident wave, there are reflected waves on the mouth surface. Again, in addition to the main wave on the mouth, there are higher-order wave patterns. In addition, due to the open structure of the waveguide and the horn, currents exist at the edges of the waveguide opening and the horn opening and on the outer wall, and they also participate in radiation. ^[1]

Horn antennas can be divided into four categories:

The horn fed by circular waveguide is generally a cone horn;

The horn fed by the rectangular waveguide is divided into three types of horns according to different forms of expansion, namely E-face fan horn (formed by expanding its narrow side); H face fan horn (formed by expanding its wide side); Wide and narrow sides are formed by expanding);

TEM horn;

Ridge waveguide speakers.

Horn antenna principle and characteristics

Horn antenna

The radiation field of the horn antenna can be calculated from the mouth-to-surface field using Huygens principle. The mouth-to-surface field is determined by the mouth size and the propagation pattern of the horn. Geometric diffraction theory can be used to calculate the effect of the horn wall on the radiation, so that the calculated pattern and the measured value can agree well up to the far side lobe. Its radiation characteristics are determined by the size and field distribution of the mouth, and the impedance is determined by the neck of the horn (the discontinuity at the beginning) and the reflection from the mouth. When the length of the horn is constant, if the opening angle of the horn is gradually increased, the phase difference between the mouth surface and the quadratic phase is also increased at the same time, but the gain does not increase in synchronization with the mouth surface size, and there is a Mouth size, a speaker with this size is called the best speaker.

Conical and pyramidal horns propagate spherical waves, while cylindrical horns propagate in fan-shaped horns with one surface (E or H-plane) spread. The horn mouth surface field is a field with a quadratic phase difference, and the magnitude of the quadratic phase difference is related to the length of the horn and the size of the mouth surface.

In order to extend the frequency band of the speaker, it is necessary to reduce the reflection at the neck and mouth of the speaker. As the size of the mouth surface increases, the reflection decreases. In addition, making the transition between the waveguide and the horn as smooth as possible can also reduce the reflection there. Due to the small size of the speakers near this location, higher-order modes cannot be propagated, and single-mode transmissions are generally transmitted. In order to control the radiation pattern, sometimes a multi-mode field distribution is required on the mouth. At this time, a device capable of generating high-order modes should be introduced in an appropriate position in the speaker. This horn is called a multi-mode horn and can be used as a monopulse radar or high efficiency antenna feed. Due to the different phase speed of each mode in the horn, the frequency band of the multi-mode horn is narrower than that of the conventional horn. ^[1]

Horn antenna pattern

Horn Antenna Application

Horn antenna

Due to the simple structure of the horn antenna and easy control of the directional pattern, it is usually used as a medium directional antenna, such as a standard horn, and the most common is used as a feed for the reflective surface (see Antenna Feed). When used as a stand-alone antenna, a phase-correcting reflector or lens is usually added. The horn-parabolic reflector antenna has the characteristics of frequency bandwidth, low side lobes, and high efficiency. It is often used in microwave relay communications. Because of its heavy weight and complicated structure, the lens has been rarely used as the phase correction of the horn.

Horn antennas are often used in the following areas: 1 feeds for large radio telescopes, reflective antenna feeds for satellite ground stations, reflective antenna feeds for microwave relay communications; 2 phased array unit antennas; 3 antennas In the measurement, the horn antenna is often used as a universal standard for calibration and gain testing of other high-gain antennas.