An antenna is a metal device (such as a rod, wire, or arrangement of wires) used to transmit or receive radio waves.

A component used in radio equipment to transmit or receive electromagnetic waves. Engineering systems such as radio communications, broadcasting, television, radar, navigation, electronic countermeasures, remote sensing, and radio astronomy, which use electromagnetic waves to transmit information, rely on antennas to work. In addition, in the use of electromagnetic waves to transmit energy, non-signal energy radiation also requires an antenna. Generally, antennas are reversible, that is, the same antenna can be used as both a transmitting antenna and a receiving antenna. The basic characteristic parameters of the same antenna as transmitting or receiving are the same. This is the reciprocity theorem of antennas.

"Antenna Picture"

The radar concept was formed in the early 20th century. Radar is the transliteration of English radar, which means radio du detection and ranging. It is an electronic device that uses microwave band electromagnetic waves to detect targets.

The specific purpose and structure of various radars are different, but the basic form is the same, including five basic components: transmitter, transmitting antenna, receiver, receiving antenna and display. There are also auxiliary equipment such as power supply equipment, data acquisition equipment, and anti-interference equipment.

The role of radar is similar to that of eyes. Of course, it is no longer a masterpiece of nature. At the same time, its information carrier is radio waves. In fact, whether it is visible light or radio waves, they are essentially the same thing, they are all electromagnetic waves, and the speed of propagation is the speed of light C. The difference is that they occupy different bands. The principle is that the transmitter of the radar equipment shoots the electromagnetic wave energy to a certain direction in space through the antenna, and the object in this direction reflects the electromagnetic wave encountered; the radar antenna receives the reflected wave and sends it to the receiving device for processing, and extracts relevant information. Some information of the object (the distance from the target object to the radar, the rate of change of range or radial velocity, azimuth, altitude, etc.).

Measuring distance is actually measuring the time difference between the transmitted pulse and the echo pulse. Since electromagnetic waves travel at the speed of light, it can be converted into the precise distance of the target.

The measurement target azimuth is measured by the sharp azimuth beam of the antenna. The elevation angle is measured by a narrow elevation beam. The target height can be calculated based on the elevation angle and distance.

Measuring speed is based on the principle of the frequency Doppler effect generated by the radar based on the relative movement between itself and the target. The target echo frequency received by the radar is different from the radar transmitting frequency, and the difference between the two is called the Doppler frequency. One of the main information that can be extracted from the Doppler frequency is the rate of change of the distance between the radar and the target. When the target and the interference clutter exist in the same spatial resolution unit of the radar at the same time, the radar uses the difference in Doppler frequency between them to detect and track the target from the interference clutter.

"Radar Picture"

The advantage of radar is that it can detect long-distance targets during the day and night, and is not blocked by fog, clouds and rain. It has the characteristics of all-weather and all-weather, and has a certain penetration ability. Therefore, it has not only become an indispensable electronic equipment in the military, but also widely used in social and economic development (such as weather forecasting, resource detection, environmental monitoring, etc.) and scientific research (astronomical research, atmospheric physics, ionospheric structure research, etc.). Spaceborne and airborne synthetic aperture radars have become very important sensors in remote sensing today. Radars that target the ground can detect the precise shape of the ground. Its spatial resolution can reach several meters to tens of meters, and has nothing to do with distance. Radar has shown good application potential in flood monitoring, sea ice monitoring, soil moisture survey, forest resource inventory, and geological survey.