Introduction
Transmitting or receiving antennas that work in the short-wave band (1~30MHz) are collectively called short-wave antennas. Shortwave transmission is divided into two types according to different transmission paths. One is reflection through the ionosphere, which is called sky wave. Because solar activity will have a certain impact on the ionosphere, the wavelength transmitted in this way will also change with the strength of solar activity; the other is ground waves transmitted on the ground. This type of transmission is affected by the relative permittivity and electrical conductivity and causes loss. The relative permittivity and conductivity of seawater are relatively large, so the loss is small, and it is more used in maritime communications.
Because shortwave communication does not require a relay station like satellite, optical fiber, and cable communication, it is widely used. It is one of the important means of modern long-distance radio communication.
Whip antennas (Whip) are often used for short-wave antennas for ultra-long-distance (500km+) communication.
When communicating in short-wave blind areas (20-100km), short-wave antennas are mostly used as non-blind-area antennas (NVISantenna), which are mostly ring-shaped.
Types
External short-wave antennas
There are many types of short-wave antennas, among which symmetrical antennas, in-phase horizontal antennas, double-wave antennas, and angular antennas are the most widely used. , V-shaped antenna, diamond antenna, fishbone antenna, etc. Compared with long-wave antennas, short-wave antennas have a large effective height, large radiation resistance, high efficiency, good directivity, high gain, and bandwidth.
Built-in shortwave antenna
The terminal needs a built-in antenna. The radio uses a magnetic antenna with a wire wound on a magnetic rod. This type of antenna is generally made of two types of magnetic materials.
One is the traditional sintered ferrite winding. The characteristic is that the gain is high, but too close to the circuit board will cause high noise, and the noise of the circuit board is also picked up by the ferrite, resulting in a decrease in the signal-to-noise ratio of the whole machine. It is not anti-vibration and is easily broken. The second is the winding manufacturing of micro-hang series of organic magnetic materials. The characteristic is that the magnetism is not closely related to the frequency, the signal-to-noise ratio is higher than the 2-3db of the ferrite, and it is resistant to drop.
Working principle
When a high-frequency current is passed through a conductor, an electric field and a magnetic field will be generated in the surrounding space. According to the distribution characteristics of electromagnetic field in space, it can be divided into near zone, middle zone and far zone. Let R be the distance between a point in space and the conductor, in
It must be pointed out that when the length L of the wire is much smaller than the wavelength, the radiation is very weak; when the length L of the wire is increased to be comparable to the wavelength, the current on the wire will greatly increase, thus forming Strong radiation.
The transmitting antenna just uses this property of the radiation field, so that the transmitted signal can fully radiate to the space after passing through the transmitting antenna. How to make the conductor an effective radiator system? Here we first analyze the situation on the transmission line. In order to transmit only energy without radiation on a parallel double-line transmission line, the two-line structure must be symmetrical, the current magnitude and direction of the corresponding points on the line are opposite, and the distance between the two lines "Π. In order to effectively radiate the electromagnetic field, the symmetry of the transmission line must be broken. For example, the two conductors are separated at a certain angle, or one side is removed, etc., which can destroy the symmetry of the conductor and generate radiation.
Frequently asked questions
Antenna element spacing
In short-wave communications, when designing the spacing of the array elements, if the spacing is selected as the half-wavelength of the high frequency band, Then, good directional characteristics can be obtained in the high frequency band, but the distance is only 1/20～1/10 wavelengths relative to the low frequency band, which will bring a strong coupling effect, so that the antenna array loses electrical scanning and Directional ability; if the spacing of the array elements is selected as the half-wavelength of the lower frequency band, then it can have a better effect in the low frequency band, but for the high frequency band, the spacing may be several wavelengths, which will produce many grating lobes. Disperse the power and reduce the power in the main direction, which in turn makes the antenna array meaningless.
Therefore, when setting up an antenna array, the distance between the array elements is a problem that needs to be carefully considered.
Ground influence
Generally, when studying antenna characteristics, the antenna is placed in a free space for analysis. In actual situations, especially short-wave communications, antennas are always set up on the ground, and the influence of the ground is closely related to the performance of the antenna. When the antenna is installed on the actual ground, the electromagnetic field excited by the antenna will cause a current on the ground, and the current will excite a secondary field in space. The space field is the superposition of the field directly excited by the antenna and the secondary field. Therefore, it will cause The antenna's directional characteristics, impedance characteristics, antenna efficiency, etc. have undergone great changes.
In actual work, you can use the mirror image method to estimate the influence of the ground on the antenna: regard the ground as a mirror surface, and replace the mirror surface with a mirror image antenna at a symmetrical position in the mirror surface, and consider the actual system as a free space For the system composed of the middle mirror antenna and the actual antenna, the performance of the equivalent system in the upper half of the space is calculated. In addition, in order to improve the efficiency of the antenna, it is also possible to lay a ground net.
Impedance mismatch
The output impedance of the radio transmitter should be consistent with the impedance of the feeder, and the impedance of the feeder and antenna. When the impedance is matched, the maximum power transmission will be obtained; if the impedance values are inconsistent, the high-frequency energy output by the transmitter will not be all emitted by the antenna. This part of the energy that is not emitted will be reflected back to produce a standing wave, which will cause damage to the insulation layer of the feeder and the final power amplifier tube of the transmitter.