There are many ways to classify antennas, and each classification is based on a property. Let’s start with the antenna band and discuss antennas in different bands and their representative categories. For the division of bands, please see the figure below.
Microwave Antenna
- General definition
Transmitting or receiving antennas working in the meter wave, centimeter wave, millimeter wave and other bands are collectively called microwave antennas. Microwaves are mainly propagated by space waves. In order to increase the communication distance, the antenna is set up higher.
- Common antenna types
Among microwave antennas, the most widely used ones are parabolic antennas, horn parabolic antennas, horn antennas, lens antennas, slotted antennas, dielectric antennas, periscope antennas, etc.
3. Points to note when setting up microwave antennas
1) Avoid obstacles in the transmitting or receiving direction of the antenna.
2) Install the antenna as far away from interference sources as possible.
3) Install the antenna at a nearby high point as much as possible.
4) Pay attention to the left-right and top-down spacing between antennas and the operating frequency.
- Explanation of parabolic antenna
Definition: Parabolic antenna is a directional microwave antenna, which consists of a parabolic reflector and a radiator. The radiator is installed at the focus or focal axis of the parabolic reflector. The electromagnetic waves emitted by the radiator are reflected by the parabola to form a highly directional beam.
Method: The parabolic reflector is made of highly conductive metal. There are four main types: rotating parabola, cylindrical parabola, cut rotating parabola and elliptical edge parabola. The most commonly used are rotating parabola and cylindrical parabola. Radiators generally use half-wave oscillators, open waveguides, slotted waveguides, etc.
Advantages: Parabolic antenna has the advantages of simple structure, strong directivity and wide operating frequency band. This antenna is widely used in microwave relay communication, tropospheric scatter communication, radar and television.
Shortwave Antenna
- General definition
Shortwave antenna refers to a transmitting or receiving antenna that works in the shortwave frequency band (1~30MHz).
- Common antenna categories
Including Yagi antenna, log-periodic antenna, vehicle-mounted mobile antenna, etc.
- Shortwave antenna selection considerations
Long/short distance communication between fixed stations
Since the communication direction between fixed stations is fixed, high-gain, highly directional shortwave antennas are generally used. For communication distances of 1000-3000 kilometers, high-gain, low-elevation log-periodic antennas (LP) can be used, but the antennas are expensive.
Since the communication direction is not fixed during the movement of the mobile station, the antenna of the central station should be an omnidirectional antenna. The central station can also use a whip antenna. The whip antenna has a low elevation angle, which makes short-distance (20-100 kilometers) communication difficult. As long as the frequency is suitable for long-distance (500-3000 kilometers), the communication effect is better.
Due to the limitation of the installation surface, the mobile station antenna mostly uses a whip antenna. In China, grid, double-loop, and triple-loop antennas are sometimes used. For long-distance communication, the whip antenna is vertical, and for short-distance communication, it can be placed in an inverted “L” shape. This increases the vertical radiation surface of the antenna and improves the transmission efficiency.
If the communication quality requirements are not too high, relatively cheap antennas such as Yagi antennas and long-line antennas can also be used, but long-line antennas require antenna tuning. When the distance is within 600 kilometers, the use of horizontal dipole antennas can achieve better results, but horizontal dipole antennas occupy a large area, and the central station has many radio stations and is not suitable for antenna arrays.
Communication between fixed and mobile stations
Antenna Selection in Interference Environments
Radio interference is very different from interference caused by other natural conditions. It is very random and unpredictable. In the case of electronic interference, the use of high-gain, highly directional logarithmic periodic antennas can achieve certain results.
Of course, overcoming interference mainly improves the performance of shortwave radio stations (transmitting power, receiving sensitivity, etc.) or adopts frequency adaptation and shortwave broadband frequency hopping technology.
- Explanation of Yagi Antenna
Definition: Yagi antenna is an end-fire antenna composed of an active vibrator (generally a folded vibrator), a passive reflector and several passive directors arranged in parallel.
Method: It consists of several metal rods, one of which is a radiator, the longer one behind the radiator is a reflector, and the shorter ones in front are directors. The radiator usually uses a folded half-wave oscillator. The maximum radiation direction of the antenna is the same as the direction of the director.
Advantages: The advantages of the Yagi antenna are simple structure, light and strong, and easy feeding. It is used in ultra-short wave communication and radar.
Medium wave antenna
- Conventional definition
Medium wave refers to electromagnetic waves with a frequency of 300KHz-3MHz and a corresponding wavelength of 1km-100m.
- Common antenna categories
Mainly civil broadcast antennas.
- Characteristics of medium wave antennas
Medium waves can propagate in the form of surface waves or sky waves, just like long waves. However, long waves penetrate the ionosphere very shallowly and can be reflected at the lower interface of the ionosphere. Medium waves have a higher frequency than long waves, so they need to be reflected in a deeper ionosphere.
Radio communications with a wavelength of 3000-2000 meters use wireless or surface wave propagation, and the receiving field strength is very stable, which can be used to complete reliable communications, such as ship communications and navigation. Medium and short waves with a wavelength of 2000-200m are mainly used for broadcasting, so this band is also called the broadcast band.
Long wave antenna
- General definition
Long wave refers to electromagnetic waves with a frequency of 100-300KHz and a corresponding wavelength of 3-1km.
- Common antenna categories
Generally include vertical radiating antennas, top-loaded monopole antennas, T-type and inverted L-type antennas, horizontal branch antennas, diamond antennas and diamond-based composite diamond antennas.
- Characteristics of long wave antennas
Long transmission distance, less information attenuation
Low frequency, less information
Used for communication on the ocean, medium and long distance communication, underground rock communication, and long-distance navigation.
Not suitable for transmitting signals with large amounts of information.
Explanation of inverted L-type and T-type antennas
(1) Inverted L-type antenna
An antenna formed by connecting a vertical down conductor to one end of a single horizontal conductor. Because its shape resembles the English letter L inverted, it is called an inverted L-type antenna. The Russian letter Γ is exactly the inverted English letter L. Therefore, it is more convenient to call it a Γ-type antenna. It is a form of vertical grounded antenna. In order to improve the efficiency of the antenna, its horizontal part can be composed of several wires arranged on the same horizontal plane. The radiation generated by this part can be ignored, and it is the vertical part that generates radiation. Inverted L antennas are generally used for long-wave communications. Its advantages are simple structure and easy installation.
(2) T-type antenna
In the center of the horizontal conductor, a vertical down conductor is connected, and its shape resembles the English letter T, so it is called a T-type antenna. It is the most common type of vertical grounded antenna. The radiation of its horizontal part can be ignored, and it is the vertical part that generates radiation. In order to improve efficiency, the horizontal part can also be composed of multiple wires. The characteristics of the T-type antenna are the same as those of the inverted L-type antenna. It is generally used for long-wave and medium-wave communications.
