An antenna is a device that serves as an interface between an electric circuit and space, designed to transmit and receive electromagnetic waves in a certain frequency range in accordance with its own dimensions and shape. It is made of metal, mainly copper or aluminum, transmitting antennas can convert electric current to electromagnetic radiation and vice versa. Each wireless device contains at least one antenna.
Wireless Radio Waves
When there is a need for wireless communications, an antenna is needed. It has the ability to send or receive electromagnetic waves for communication, where it is impossible to install a wired system.
An antenna is a key element of this wireless technology. Radio waves are easy to create and widely used for both indoor and outdoor communications because of the ability to travel through buildings and travel long distances.
Key features of transmitting antennas:
- Since the radio transmission is omnidirectional, the need for physical coordination of the transmitter and receiver is not required.
- Radio frequency determines many transmission characteristics.
- At low frequencies, waves can easily pass through obstacles. However, their power decreases with the inverse square relative to the distance.
- Higher wave frequencies are more prone to absorption, and they are reflected in obstacles. Due to the long range of transmission of radio waves, interference between transmissions is a problem.
- In the VLF, LF, and MF bands, wave propagation, also called ground waves, follows the curvature of the Earth.
- The maximum transmission ranges of these waves are of the order of several hundred kilometers.
- Transmitting antennas are used for low bandwidth transmissions, such as amplitude modulated (AM) radio transmissions.
- HF and VHF transmissions are absorbed by the atmosphere located near the surface of the Earth. However, part of the radiation, called the sky wave, propagates outward and upward to the ionosphere in the upper atmosphere. The ionosphere contains ionized particles formed by solar radiation. These ionized particles reflect the waves of the sky back to Earth.
Wave propagation
- Propagation of line of sight. Among all distribution methods, this is the most common. The wave travels to the minimum distance that can be seen with the naked eye. Next, you need to use the transmitter of the amplifier to increase the signal and transmit it again. Such propagation will not be smooth if there is any obstacle in its transmission path. This gear is used for infrared or microwave transmissions.
- Earth wave propagation from a transmitting antenna. The wave propagates to the ground along the contour of the Earth. Such a wave is called a direct wave. The wave sometimes bends due to the Earth's magnetic field and enters the receiver. Such a wave can be called a reflected wave.
- A wave propagating through the earth's atmosphere is known as the earth. Forward wave and reflected wave together give a signal at the receiving station. When the wave reaches the receiver, the delay stops. In addition, the signal is filtered to avoid distortion and gain for clear output. Waves are transmitted from one place and where they are received by many transceiver antennas.
Antenna Measurement Coordinate System
When looking at flat models, the user will encounter indicators of the azimuth of the plane and the height of the plane of the pattern. The term azimuth usually occurs in relation to the "horizon" or "horizontal", while the term "height" usually refers to the "vertical". In the figure, the xy plane is the azimuthal plane.
The azimuthal plane diagram is measured when the measurement is performed by moving the entire xy plane around the transceiver antenna under test. The elevation plane is the plane orthogonal to the xy plane, for example, the yz plane. The elevation plane plan bypasses the entire yz plane around the antenna under test.
Samples (azimuths and elevation charts) are often displayed as graphs in polar coordinates. This gives the user the ability to easily visualize how the antenna emits in all directions, as if it was already “aimed” or mounted. It is sometimes useful to draw patterns in Cartesian coordinates, especially when there are several side lobes in patterns and where side lobe levels are important.
Key Communication Features
Antennas are the main components of any electrical circuit, as they provide the relationship between the transmitter and the free space, or between the free space and the receiver. Before talking about the types of antennas, you need to know their properties.
Antenna Array - The systematic deployment of antennas that work together. Individual antennas in an array are usually of the same type and are located in close proximity, at a fixed distance from each other. The array allows you to increase directivity, control the main radiation beams and side beams.
All antennas are characterized by passive gain. Passive gain is measured by dBi, which is associated with a theoretical isotropic antenna. It is believed that it transfers energy equally in all directions, but does not exist in nature. The gain of an ideal half-wave dipole antenna is 2.15 dBi.
EIRP, or equivalent isotropic radiated power of a transmitting antenna, is a measure of the maximum power that a theoretical isotropic antenna would radiate in the direction of maximum gain. EIRP takes into account losses from power lines and connectors and includes the actual gain. EIRP allows you to calculate the actual power and field strength values if the actual transmitter gain and output power are known.
Directional antenna gain
It is defined as the ratio of the power gain in a given direction to the power gain of the reference antenna in the same direction. It is standard practice to use an isotropic emitter as a reference antenna. In this case, the isotropic emitter will be lossless, emits its energy equally in all directions. This means that the gain of the isotropic emitter is G = 1 (or 0 dB). It is usually customary to use a dBi block (decibels relative to an isotropic emitter) to amplify against an isotropic emitter.
The gain expressed in dBi is calculated using the following formula: GdBi = 10 * Log (GNumeric / GIsotropic) = 10 * Log (GNumeric).
Sometimes a theoretical dipole is used as a reference, therefore, the unit dBd (decibels relative to the dipole) will be used to describe the gain with respect to the dipole. This unit is typically used when it comes to amplifying omni-directional antennas with a higher gain. In this case, their gain is higher by 2.2 dBi. Therefore, if the antenna has a gain of 3 dBc, the overall gain will be 5.2 dBi.
Beam Width 3 dB
This beam width (or beam width of half power) of the antenna is usually determined for each of the main planes. A beam width of 3 dB in each plane is defined as the angle between the points of the main lobe, which are reduced from the maximum gain by 3 dB. A beam width of 3 dB is the angle between two blue lines in the polar region. In this example, a 3 dB beamwidth in this plane is about 37 degrees. Broad-beam antennas usually have a low gain, and narrow-beam antennas have a higher gain.
Thus, an antenna that directs most of its energy into a narrow beam in at least one plane will have a higher gain. The forward-backward ratio (F / B) is used as an indicator of dignity, which attempts to describe the level of radiation from the back of a directional antenna. Basically, the forward-backward ratio is the ratio of peak gain in the forward direction to a gain of 180 degrees behind the peak. Of course, on a DB scale, the forward-backward relationship is simply the difference between the peak gain in the forward direction and the gain of 180 degrees behind the peak.
Antenna Classification
There are many types of antennas for various applications, such as communication, radar, measurement, electromagnetic pulse simulation (EMP), electromagnetic compatibility (EMC), etc. Some of them are designed to work on narrow frequency bands, while others are designed for radiation / receive transient pulses. Characteristics of transmitting antennas:
- The physical structure of the antenna.
- Frequency ranges of work.
- Application mode.
The following are the types of antennas in accordance with the physical structure:
- wire;
- aperture
- reflective;
- lens antennas;
- microstrip antennas;
- massive antennas.
The following are the types of transmitting antennas depending on the frequency of operation:
- Very low frequency (VLF).
- Low frequency (LF).
- Medium Frequency (MF).
- High frequency (HF).
- Very high frequency (VHF).
- Ultra High Frequency (UHF).
- Super high frequency (SHF).
- Microwave wave.
- Radio wave.
The following are transmitting and receiving antennas in accordance with the application modes:
- Point-to-point relationship.
- Broadcast applications.
- Radar communication.
- Satellite connection.
Design Features
Transmitting antennas create radio-frequency radiation propagating in space. Receiving antennas perform the reverse process: they receive radio frequency radiation and convert them into the required signals, for example, sound, image in television transmitting antennas and mobile phone.
The simplest type of antenna consists of two metal rods and is known as a dipole. One of the most common types is a monopole antenna, consisting of a rod located vertically to a large metal board, which serves as a grounded plane. Installation on vehicles is usually a monopole, and the metal roof of the vehicle serves as a ground. The device of the transmitting antenna, its shape and size determine the operating frequency and other characteristics of the radiation.
One of the important attributes of an antenna is its directivity. In connection between two fixed targets, as well as in connection between two fixed transmission stations, or in radar applications, an antenna is required to directly transmit the transmission energy to the receiver. Conversely, when the transmitter or receiver is not stationary, as in cellular communications, an omnidirectional system is required. In such cases, an omnidirectional antenna is required, which uniformly receives all frequencies in all directions of the horizontal plane, and in the vertical plane, the radiation is uneven and very small, as in the HF transmit antenna.
Transmitting and receiving sources
The transmitting device is the main source of radio frequency radiation. This type consists of a conductor, the intensity of which varies with time and converts it into radio frequency radiation propagating in space. A receiving antenna is a device for receiving radio frequencies (RF). It performs the reverse transmission performed by the transmitter, receives radio frequency radiation, converts it into electric currents in the antenna circuit.
Television and broadcasting stations use transmitting antennas to transmit certain types of signals that travel through the air. These signals are detected by receiving antennas, which convert them into signals and are received by an appropriate device, for example, a TV, radio, mobile phone.
Radio receiving and television receiving antennas are designed exclusively for receiving radio frequency radiation, and they do not produce radio frequency radiation. Cellular devices, such as base stations, repeaters and mobile phones, are equipped with designated transmitting and receiving antennas that emit radio frequency radiation and serve cellular networks in accordance with communication network technologies.
The difference between analog and digital antenna:
- The analog antenna has a variable gain and operates in the 50 km range for DVB-T. The farther the user is from the signal source, the worse the signal.
- To receive digital TV - the user receives either a good image, or an image in general. If it is far from the signal source, it does not receive any image.
- The transmitting digital antenna has built-in filters to reduce noise and improve image quality.
- The analog signal is transmitted directly to the TV, while the digital signal must first be decoded. This allows you to fix errors, as well as data such as signal compression to obtain additional functions as additional channels, EPG, Pay TV, interactive games, etc.
Dipole transmitters
Dipole antennas are the most common omnidirectional type and distribute radio frequency (RF) energy 360 degrees in the horizontal plane. These devices are designed to be resonant with a half or a quarter wavelength of the applied frequency. It can be as simple as two pieces of wire, the right length, or it can be encapsulated.
Dipole is used in many corporate networks, small offices and for home use (SOHO). It has a typical impedance, allowing it to be matched with a transmitter for maximum power transfer. If the antenna and the transmitter do not match, reflections will occur on the transmission line that degrade the signal or may even damage the transmitter.
Directional focus
Directional antennas focus the radiated power on narrow beams, providing significant gains in this process. Its properties are also mutual. Characteristics of a transmitting antenna, such as impedance and gain, also apply to a receiving antenna. That is why the same antenna can be used both for sending and for receiving a signal. Amplification of a strongly directional parabolic antenna serves to amplify a weak signal. This is one of the reasons why they are often used for long distance communications.
A commonly used directional antenna is the Yagi-Uda array called Yagi. It was invented by Shintaro Uda and his colleague Hidetsugu Yagi in 1926. A yagi antenna uses several elements to form a directional array. One controlled element, usually a dipole, distributes radio frequency energy, elements located directly in front of and behind the driven element re-emit radio frequency energy in phase and out of phase, amplifying and slowing down the signal, respectively.
These elements are called spurious elements. The element behind the slave is called the reflector, and the elements in front of the slave are called directors. Yagi antennas have a beam width in the range of 30 to 80 degrees and can provide more than 10 dBi of passive gain.
A parabolic antenna is the most familiar type of directional antenna. A parabola is a symmetrical curve, and a parabolic reflector is a surface that describes a curve with 360-degree rotation - a plate. Parabolic antennas are used for long-distance communication lines between buildings or large geographical areas.
Semi-directional sectional emitters
A patch antenna is a semi-directional emitter using a flat metal strip mounted above the ground. Radiation from the back of the antenna is effectively cut off by the ground plane, increasing forward focus. This type of antenna is also known as a microstrip antenna. It is usually rectangular and enclosed in a plastic case. This type of antenna can be made using standard PCB methods.
A patch antenna can have a beam width of 30 to 180 degrees and a typical gain of 9 dB. Sectional antennas are another type of semi-directional antenna. Sector antennas provide a radiation pattern and are usually mounted in an array. The beam width for a sector antenna can range from 60 to 180 degrees, with typical 120 degrees. , 360 .
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