What Is a Random Wire Antenna?

A wire antenna is an antenna composed of one or more metal wires whose wire diameter is much smaller than the wavelength and whose length can be compared with the wavelength. It is mainly used in long, medium, short wave and ultra short wave bands as transmitting or receiving antennas.

A wire antenna is an antenna composed of one or more metal wires whose wire diameter is much smaller than the wavelength and whose length can be compared with the wavelength. It is mainly used in long, medium, short wave and ultra short wave bands as transmitting or receiving antennas.

Chinese name

Wire antenna

Foreign name

linear antenna

history

Early theory: HR Hertz was the founder of antenna theory. From 1887 to 1888 he first established the most basic and simple theory of capacitive antennas. But antenna theory has been slow to advance. In 1897, HC Bocklinton established an integral equation for a thin-line antenna and proved that the current on the thin-line antenna is close to a sinusoidal distribution. The current and charge waves on the antenna propagate forward at the speed of light. From then until the 1930s, antenna and antenna array theories were based on these two results by Pocklinton. Hertz's solution enables people to find the electromagnetic field and radiation pattern for a given current distribution, plus the results of Pocklinton and the law of conservation of energy, which can solve many practical antenna problems. However, due to the mathematical difficulties at that time, it was impossible to solve the Pocklinton's integral equation, and the antenna theory in this period was approximate. For example, when determining the input impedance of an antenna, first assume that the current distribution on the antenna is sinusoidal, and then use Poynting's theorem to find the power emitted by the antenna surface divided by the square of the maximum current (half-wave oscillator) Get the input impedance. This method is called the induced electromotive force method, and its approximation is that the field generated by the sinusoidal current distribution on the antenna surface does not meet the boundary conditions.

Circuit theory or integral equation theory: Since about the 1930s, in order to find out the accurate current distribution and input impedance, some scholars have sought a rigorous solution to the wire antenna. In 1931, E. Helen established his integral equation for an infinitely thin ideal metal thin tube antenna fed by a rotationally symmetric-function source in the middle, and obtained a strict solution in 1938. Later, R. Jin et al. Conducted a large number of theoretical analysis, numerical calculations, and design experiments on solid thin-line antennas based on Helen's linearized integral equation, and obtained a large number of curves and numerical results. They first transformed Maxwell's equation into an integral equation with the current distribution on the antenna as the function to be sought according to the boundary conditions, and then appropriately processed the latter in order to obtain a series solution using the stepwise approximation method. Although the linearized integral equation used by R. Jin et al. Is itself approximate, the results of approximate calculations based on the integral equation still have practical significance for thin-line antennas.

Field theory or differential equation theory: In 1941, JA Stratton and Zhu Lancheng used long ellipsoidal coordinates to conduct a theoretical analysis of a long ellipsoidal antenna with an eccentricity close to 1 fed in the middle. The Maxwell equation is directly solved according to the boundary conditions to obtain its field, and the current distribution and input impedance on the antenna are obtained from the latter.

From 1941 to 1945, SA Shekmunmunov used spherical coordinates to theoretically analyze a pair of top-fed double-cone antennas fed by the center, applied the separation variable method, and directly solved the Maxwell equations according to the boundary conditions to obtain the field and antenna current Distribution and input impedance. He divided the linear antenna theory into a linear antenna resonator theory and a linear antenna mode theory. The former regards the antenna as a leaky resonator; the latter regards the antenna as a biconical waveguide with aperture scattering.

In 1950, H. Julte used cylindrical coordinates to theoretically analyze an infinite number of coaxial thin tube antenna arrays, studied the inverse feeding between adjacent array elements, applied the separation variable method, and solved the scalar Hai according to the boundary conditions. Mholtz equation, and then the distance between adjacent array elements tends to be infinitely large to obtain the field, current distribution and input impedance of a single round tube thin antenna.

definition

A wire antenna is an antenna composed of one or more metal wires whose wire diameter is much smaller than the wavelength and whose length can be compared with the wavelength.

working principle

The working principle of the wire antenna is based on the field strength superposition principle. A single-wire antenna can be considered to be composed of many infinitely short small line segments. These infinitely short small line segments are called current elements. The radiation fields of many current elements are added together to form the total radiation field of the entire antenna.

If an antenna consists of several wires, its total radiation pattern is the superposition of the radiation patterns of these wires.

An antenna array (or array antenna) is formed by arranging antennas of the same type in a certain position. Each pair of antennas in the antenna array is called a unit of the antenna array, or an array element. Units arranged on a straight line are called linear antenna arrays (referred to as linear arrays); arrayed on a plane are called planar antenna arrays (referred to as area arrays). The radiation pattern of the entire antenna array is a superposition of the radiation pattern of each array element antenna. Therefore, it is related to the type, relative position and current distribution of each array element antenna. Selecting and adjusting the type, relative position, and current magnitude and phase of each array element antenna can make the total pattern of the antenna array superimposed to meet various needs.

Main type

The main types of line antennas are dipole antenna, half-wave antenna, cage antenna, monopole antenna, whip antenna, tower antenna, spherical antenna, magnetic antenna, V-shaped antenna, diamond antenna, fishbone antenna, Yagi Antennas, log-periodic antennas, antenna arrays, etc. (see non-directional antennas, weak directional antennas, strong directional antennas).

classification

Line antennas can be classified into standing wave antennas and traveling wave antennas if they are classified according to the current distribution on the antennas. For example, standing wave antennas commonly used in short-wave communications are cage antennas, and traveling wave antennas are diamond antennas.

The working band width of the standing wave antenna is relatively narrow, and a large change in the operating frequency of the antenna will affect the characteristic parameters of the antenna. Conversely, a traveling wave antenna has a wider frequency band and can work in a wider band, but the efficiency of a standing wave antenna is higher than that of a traveling wave antenna. Therefore, standing wave antennas are generally used for fixed services with constant frequency, while traveling wave antennas can be used for services with frequent frequency changes. For example, in short-wave communications, the available frequency should be changed accordingly because the ion density of the ionosphere is different day and night. If a standing wave antenna is used, two to three antennas must be prepared, but if a traveling wave antenna is used, it is not necessary to replace the antenna.

Main parameter

The main parameters of a line antenna are the directivity pattern, main lobe width and elevation angle, sub-braid level and distribution, directivity coefficient, gain, efficiency, input impedance, standing wave ratio, frequency band width, polarization pattern, equivalent length, etc. Effective area (caliber), etc. (see the main parameters of the line antenna).

Gain, efficiency, and bandwidth are three important parameters related to antenna performance. The main method to increase the gain is to increase the antenna length, or form many antennas into the antenna array, such as Yagi antenna, in-phase antenna array, and so on. The main methods to increase the frequency bandwidth are to use a traveling wave antenna. The second is to increase the wire diameter of the wire antenna. For example, a single wire is changed to a cage antenna composed of multiple wires in parallel, or a single wire is changed to a board. A metal-shaped metal bar, such as a bat-wing antenna used in television broadcasting, and the third is to use a periodic structure (such as a log-periodic antenna) or a frequency-independent antenna structure (such as an equiangular spiral antenna) from the antenna type. The main method to improve efficiency is to improve the impedance matching between the feeder and the antenna and various connectors on the feeder so that the standing wave ratio on the feeder is as close to 1 as possible, so as to avoid electromagnetic energy from reflecting back and forth on the feeder and causing losses.