Radio direction finder appeared in the early 20th century. In the 1920s, radio enthusiasts in the United States used the received radio waves to find the sending station and started Amateur radio direction finding activities. Due to the size and weight of the equipment at that time, it was only used for navigation. In the Second World War, Germany successfully developed a small direction finder and installed it on an airplane, and used radio broadcasts for navigation and bombing of London. During the war, the warring parties competed to develop and improve the airborne direction-finding equipment, which greatly promoted the development of direction-finding technology.
In recent years, due to the emergence and extensive use of more advanced navigation instruments, such as Roland, Omega, and radar, they have the advantages of simple operation and high orientation accuracy compared with radio direction finder. It gradually replaced the radio direction finder in many aspects, making the radio direction finder in an auxiliary position in the radio navigation equipment. However, the radio direction finder still plays a role with its unique advantages, especially the ability to determine the position of the radio transmitter is still unique.
Radio direction finder appeared in the early 20th century. It uses two or more omnidirectional radios on the shore to determine the direction of the ship’s direction of incoming waves. It can be used to determine the ship’s position. The location of the target. In the 1920s, radio enthusiasts in the United States used the received radio waves to find sending stations and began amateur radio direction finding activities. Due to the limitations of objective factors such as the size and weight of the equipment at that time, the radio direction finder was only used for navigation. In World War II, this technology was widely used.
The wireless electromagnetic indicator is a composite indicator. It has a fixed triangle mark to indicate the heading direction of the aircraft; it has a circular dial that can be rotated within 360 degrees, driven by the electrical signal of the aircraft magnetic compass, and the scale aligned with the fixed mark indicates the magnetic heading of the aircraft; There is a thin, rotatable pointer, driven by the electrical signal of the radio compass. The scale of the dial indicated by the arrow indicates the magnetic position of the ground navigation station relative to the aircraft, and the angle between the pointer and the fixed mark indicates the station heading of the aircraft. There is also a rotatable hollow pointer, driven by the electrical signal of the Volt (or Tacan) receiver. The scale of the dial indicated by the arrow indicates the magnetic position of Vol (or Tacan) ground navigation station relative to the aircraft’s radio station. The angle between the pointer and the fixed mark indicates the aircraft's station heading. The hollow pointer can also be used by another radio compass like a thin solid pointer to indicate the position and heading of another radio station. Once the pointer is aligned with the fixed mark, it means that the aircraft is flying at the selected navigation station.
The radio direction finder is an instrument that uses a directional wireless receiving source (target) signal to determine its direction. The radio direction finder can be installed on the coast, aircraft and ships. According to the band, it can be divided into three types: medium wave, short wave and ultrashort wave. Medium-wave and short-wave radio direction finder are used to determine the direction of a radio transmitting station. Ultrashort-wave radio direction finder can not only determine the direction of the transmitting station, but also can be used to determine the direction of a radar station. According to the direction finding method, it can be divided into two categories: the direction of the radiation source can be measured without turning the antenna during the search and the radio direction finder with a follower device. The direction finder for antenna rotation has high accuracy. The radio direction finder consists of an antenna feeder device and a receiving indicator. The antenna feeder is used to receive the radio waves sent by the measured target. The reception indicator is used to transform and amplify the signals sent by the antenna feeder. These signal parameters include information on the angle of arrival of the radio waves and azimuth information. Indicators include earphones, pointer instruments and digital displays.
Ultrashort wave radio direction finder has a range of 350 kilometers; short-range shortwave radio direction finder has a range of 600-1000 kilometers; medium wave radio direction finder has a range of 1200-2400 kilometers; The range of long-range shortwave radio direction finder can reach 5000-6000 kilometers. The accuracy of azimuth measurement is 0.7°-3°.
Two or more radio direction finders measure the radio waves emitted by a target at the same time, and the position of the target can be determined by means of azimuth rendezvous. The direction finding should be closely integrated with listening and reconnaissance.
The errors of the radio direction finder to determine the azimuth mainly include radio deviation and deviation.
Radio deviation is also called coast "refraction" error. This error occurs when radio waves pass through the coastline during their propagation.
The reason for the deviation can be explained as follows: During the propagation of the electromagnetic wave emitted by the launching station, the contour of the electromagnetic field strength is a circle centered on the launching station. Assuming that a single loop rotating antenna direction finder is used to determine the direction of the launching station, in order to obtain the minimum signal, the plane of the loop antenna should be consistent with the equipotential line. At this time, the normal direction of the antenna indicates the correct direction of the launching station.
When the electromagnetic wave passes through the coastline, the equipotential line is deformed near the coast. The surveyor still makes the antenna plane and the equipotential line consistent. As a result, the antenna normal direction does not point to the direction of the launching station, but deviates At one angle, this is the radio deviation.
Radio deviation cannot be eliminated, but its impact can be avoided or reduced. For this purpose, the following points should be considered when selecting a direction finding station: (1) The distance between the ship and the coast should be greater than 10 times the emission wavelength of the transmitting station, at this time, the deviation disappears;
(2) Try to choose a station close to the coast;
(3) The connection between the ship and the station should be as perpendicular to the coast as possible.
Under the action of electromagnetic fields, metal hulls, masts, chimneys, cables, etc., generate induced currents of the same frequency within them, and can also radiate electromagnetic waves outward, that is The so-called secondary radiation. These metallic conductors are called secondary radiators. At this time, the loop antenna of the direction finder is simultaneously affected by the main electromagnetic field and the secondary radiated electromagnetic field emitted by the launching station, causing an error in the measured position of the station. This error is called radio error.
The size of the radio error is closely related to the condition and position of the metal conductor on the ship. In order to reduce the radio error, the antenna of the direction finder should be placed as high as possible, so that most of the metal equipment on the ship is symmetrical to the antenna, and the cables and other accessories should be well insulated.
There is a device to eliminate the radio error in the direction finding instrument, which can partially eliminate the radio error. The remaining radio error is measured by the method of observation, and then the radio error table is listed or drawn as the error Curve for use in radio direction finding and positioning.
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