Basic knowledge of antenna working principle, classification, bandwidth and direction (1) What is the working principle and function of antenna? Answer: Antenna is an indispensable part of wireless communication. Its basic function is to radiate and receive radio waves. When transmitting, it converts high-frequency current into electromagnetic waves; when receiving, it converts electromagnetic waves into high-frequency current. (2) How many types of antennas are there? Answer: There are many types of antennas, which can be mainly classified in the following ways: According to their use, they can be divided into base station antennas and mobile portable antennas. According to their working frequency band, they can be divided into ultra-long wave, long wave, medium wave, short wave, ultra-short wave and microwave; according to their direction, they can be divided into omnidirectional and directional antennas. (3) How to choose an antenna? Answer: Antenna is an important part of the communication system. Its performance directly affects the indicators of the communication system. When choosing an antenna, users must first pay attention to its performance. Specifically, there are two aspects. The first is to choose the antenna type; the second is to choose the electrical performance of the antenna. The significance of selecting the antenna type is whether the directional pattern of the selected antenna meets the requirements of the radio wave coverage in the system design; the requirements for selecting the electrical performance of the antenna are whether the electrical indicators such as the frequency bandwidth, gain, and rated power of the selected antenna meet the requirements of the system design. Therefore, it is best for users to contact the manufacturer for consultation when choosing an antenna. (4) What is the gain of an antenna? Answer: Gain is one of the main indicators of an antenna. It is the product of directivity and efficiency, and is a manifestation of the size of the radio waves radiated or received by the antenna. The choice of gain depends on the requirements of the system design for the radio wave coverage area. Simply put, under the same conditions, the higher the gain, the farther the radio wave propagates. Generally, base station antennas use high-gain antennas, and mobile station antennas use low-gain antennas. (5) What is the voltage standing wave ratio? Answer: When the antenna input impedance and the characteristic impedance of the feeder are inconsistent, the magnetic wave generated by the superposition of the reflected wave and the incident wave on the feeder is the ratio of the maximum and minimum values of the adjacent voltages. It is the basis for testing the transmission efficiency of the feeder. The voltage standing wave ratio is less than 1.5, and the voltage standing wave ratio at the operating frequency is less than 1.2. If the voltage standing wave ratio is too large, the communication distance will be shortened, and the reflected power will return to the transmitter power amplifier part, which is easy to burn out the power amplifier tube and affect the normal operation of the communication system. Voltage standing wave ratio 1.0 1.1 1.2 1.5 2.0 3.0 Reflected power % 0 0.2 0.8 4.0 11.1 25.0 Transmitted power % 100 99.8 99.2 96 88.9 75.0 (6) What is the directivity of an antenna? Answer: An antenna has different radiation or receiving capabilities in different directions in space. This is the directivity of an antenna. The directivity of an antenna is usually measured by the radiation pattern. On the horizontal plane, an antenna with no maximum radiation or reception direction is called an omnidirectional antenna, and an antenna with one or more maximum directions is called a directional antenna. Omnidirectional antennas are mostly used in point-to-multipoint communication hubs because they are non-directional. Directional antennas have the maximum radiation or reception direction, so their energy is concentrated and their gain is higher than that of omnidirectional antennas. They are suitable for long-distance point-to-point communications. At the same time, due to their directionality, they have strong anti-interference capabilities. (7) How do you understand the working frequency bandwidth of an antenna? Answer: The electrical parameters of the antenna are generally related to the operating frequency. The frequency variation range that ensures the electrical parameter indicators is allowed is the working frequency bandwidth of the antenna. Generally, the working bandwidth of an omnidirectional antenna can reach 3-5% of the working frequency range, and the working bandwidth of a directional antenna can reach 5-10% of the working frequency. (8) How to select the cable and cable length? Answer: Mobile communication systems often use coaxial cables with a characteristic impedance of 50 ohms as feeders. In order to effectively transmit radio waves to the antenna interface, the transmission loss of the feeder should be minimized. The transmission loss depends on the diameter and length of the cable. At the same frequency, the larger the cable diameter, the smaller the loss, and the longer the cable, the greater the loss. In principle, the transmission loss of the cable should not exceed 3 decibels. The following table lists the attenuation values (db/m) of commonly used cables. Users can reasonably select the cable model and length according to their own situation. Frequency model 150MHz 400MHz 900MHz SYV-50-7 0.121 0.203 0.295 CTC-50-7 0.060 0.100 0.165 CTC-50-9 0.050 0.085 0.135 CTC-50-12 0.040 0.060 0.105 Imported 10D-FB 0.040 0.070 0.110 (9) How to choose the installation site of the antenna? Answer: Due to the influence of terrain and environment, the electromagnetic waves received by the antenna are the superposition of direct waves, reflected waves and scattered waves. The result determines the field strength amplitude and phase at the receiving point and directly affects the application effect of the antenna. Therefore, the following aspects should be noted when selecting the antenna installation location: 1. The transmitting or receiving direction of the antenna should avoid obstacles (buildings, towers, bridges, etc.); 2. The antenna installation location should be as far away from interference sources (high-voltage lines, flight routes, towers, roads, etc.); 3. The antenna should be installed at a nearby commanding height as much as possible: 4. If several antennas are working on the same tower, special attention should be paid to the left-right and up-down spacing between them to prevent mutual coupling from affecting system performance. (10) How should the antenna feed system be installed? Answer: First, assemble the antenna, feeder and supporting parts according to the requirements of the product instructions, and then fix them to the antenna bracket of the tower with a clamp at the antenna support position, and make the parallel spacing between the antenna and the tower greater than the wavelength used to reduce the impact of the tower on the antenna performance. At the antenna port, connect the feeder line to the antenna with a connector (or cable head), bend a ring with a diameter of about fifty times the diameter of the feeder line and fix it on the antenna bracket to prevent the connector from being directly stressed and causing breakage or damage. (11) How to waterproof the antenna feed system? Answer: The antenna and feeder are mainly connected by connectors. Self-adhesive rubber sealing tape is used. After stretching, it is wrapped around the connector in a half-lap form to achieve good sealing and waterproofing. In addition, a return bend is made at the point where the feeder enters the room to prevent rainwater from entering the indoor equipment along the feeder. (12) How to detect the antenna feeder system? Answer: After the antenna feeder system is set up, it should be tested by professional technicians using special testing instruments. Usually, a through-type power meter can be connected in series between the transmitter and the antenna feeder system to check the transmitter power and reflected power of the equipment to determine whether the system is working properly.
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Published on 2006-8-11 18:56
I don't know what kind of small antenna you are talking about. How to define "small"? Please give a comprehensive introduction. I don't know if the following is what you mean. Discussion on the principle of small antenna. Most shortwave listening enthusiasts believe the myth that the higher the sensitivity, the more stations you can receive. On the shortwave band, this is not entirely true. When you use a modern receiver with a noise index of less than 10 dB (equivalent to a 10 dB signal-to-noise ratio sensitivity of 1 microvolt for AM at 30MHz), the myth is completely wrong. The reason involves atmospheric noise, especially man-made noise. On the longwave, mediumwave and shortwave bands, noise is the main problem. Atmospheric noise is mainly generated by tens of thousands of lightning strikes every day, which is particularly serious during tropical storms; solar noise generated by the interaction between the solar wind and the earth also becomes atmospheric noise. Atmospheric noise is quite large in the low-end frequency range, and gradually decreases with increasing frequency. At night, solar noise disappears, but there is still a small amount of galactic noise from outer space. Man-made noise "pollution" was already quite serious in the 1990s, especially in urban areas. Heavy-loaded electrical equipment is basically a noise source. Although some governments have taken measures to enforce electrical interference below certain levels, these regulations only protect local stations and do not protect distant, weak shortwave signals. For example, all trams, electric trains, and many electrical motors generate sparks, sometimes hundreds of miles away. There is also the noise from neon lights, fluorescent lights, dimmers, televisions, computers, monitors, etc., all of which add up to the background noise floor. Man-made noise is particularly severe in industrial areas - especially in Asian and Latin American cities. To get good reception, you have to live hundreds of kilometers away from those industrial cities where the man-made noise is lower than the atmospheric noise. The noise floor generated by man-made noise and atmospheric noise combined can induce 3 to 20 microvolts on a dipole antenna in the 1-10 MHz frequency band, and even higher on longwave. The noise floor masks the weak broadcast signals you wish to hear. In fact, if you want to clearly distinguish the broadcast content in the signal from a distant transmitter, the signal must be 10 dB (about 3 times) stronger than the noise. If it is 10 times stronger (20 dB), it will be easily audible. This leads to an interesting conclusion: the reception effect and resolution (signal-to-noise ratio) does not depend on the sensitivity of the receiver or the type of antenna, but is related to the ratio of the strength of the station to be heard to the total noise strength!
Let's take the following example. Suppose you use a standard dipole antenna (1/2 wavelength) to receive broadcast or SSB voice signals. Voice is just intelligible, so the signal-to-noise ratio is 10 dB. This means that in your receiving area, the signal is 3 times stronger than the noise. What happens if you shorten the antenna? The capture area of the antenna is reduced, so the signal sensed by the antenna is reduced. But as the signal sensed by the antenna is reduced, the total noise sensed is also reduced! And the reduction is in the same proportion. The signal is still 3 times stronger than the total noise, which means that the intelligibility is the same compared to the standard dipole antenna. Only the S-meter reading has dropped. You can continue to shorten the antenna section by section until the signal strength drops below the 10 dB sensitivity of the receiver. So in the shortwave bands, where atmospheric and man-made noise are high, you can make the antenna very small until the station content becomes indistinguishable. In the VHF and UHF bands, where atmospheric and man-made noise levels are very low, the situation is completely different. When you make the antenna smaller, a new problem arises: impedance matching.
Impedance matching theory states that for energy (signal) to be transferred from the energy source (antenna) to the load (receiver) with minimal loss, the internal impedance of the energy source must be equal to the impedance of the load. Let's take an example: the internal impedance (radiation resistance) of a 1/2 wavelength dipole antenna is about 50 ohms. The characteristic impedance of the coaxial cable is 50 ohms, and the antenna input impedance of the receiver is also 50 ohms. All impedances are the same, so the signal is picked up by the antenna and sent to the receiver only by the natural loss of the coaxial cable. If the impedances are not equal, mismatches will cause additional losses. To explain further, if you use a high impedance antenna, such as an end-fed long wire antenna with an impedance of about 600 ohms, connecting it directly to the 50 ohm antenna input of the receiver is too heavy a load, and the low impedance receiver shorts out the high impedance antenna, and the reception effect will be very poor. You will encounter the same impedance matching problem when you shrink the antenna. A very short antenna like a telescopic rod is equivalent to a small capacitor and a very small resistor in series. The resistor is less than 1 ohm and can be ignored. The capacitance value is related to the length of the antenna. A 1-meter antenna corresponds to a capacitance of about 10 picofarads, which means that the impedance of a 1-meter antenna at 150 kHz is 100 kilo-ohms, and the impedance at 15 MHz is also 1000 ohms. The impedance of a small antenna is very high and frequency-dependent. This is why a small antenna directly connected to the 50-ohm antenna port of a receiver has very poor reception. The antenna has been short-circuited. The solution is impedance matching. You have to transform the high impedance of the antenna into the low impedance of the receiver. There are two ways to do this: The first is to use an antenna tuner. An antenna tuner usually consists of two capacitors and an inductor. With such a network, you can achieve impedance matching. But the antenna tuner has disadvantages: since the antenna impedance changes with frequency, you have to adjust it all the time, and a computer or memory-based quick adjustment is not possible. Commercial antenna tuners are also not very suitable for very short antennas and very low reception frequencies. Another important point is that the antenna cannot be connected to the antenna tuner with a coaxial cable, which will be interfered by more artificial noise. The second method is to use an electronic amplifier with very high input impedance and very low output impedance. The high input impedance matches the small antenna, and the low output impedance drives the receiver easily. The combination of a small antenna and an electronic impedance transformer is called an "active" antenna.
The following is a small antenna ALX1800-OA-2-I main technical parameters Frequency range (MHz) 1710-1880 Bandwidth (MHZ) 170 Gain (dBi) ＞2 Standing wave ratio VSWR ＜2 Polarization mode vertical polarization Input impedance (Ω) 50 Maximum power (W) 50 Connector form SMA-J or TNC-J Antenna length (mm) 210 (unbent), 185 (bent) Weight (Kg) 0.025 Cover color gray or black Use environment Indoor working temperature (℃) -35~+60 Product Description: Applicable to terminal equipment of GSM1800 system
This is another small wall-mounted antenna DZD-2400B-8 wall-mounted antenna, designed with broadband technology, suitable for signal forwarding of 2.4GHz spread spectrum communication system. The antenna has a small structure and beautiful appearance, and can be installed on the wall as needed. Frequency range: 2400~2483MHz Bandwidth: 83MHz Gain: 8dBi Horizontal plane lobe width: 80° Vertical plane lobe width: 60° Voltage standing wave ratio: ≤1.5 Nominal impedance: 50Ω Polarization: vertical Maximum power: 50W Connector model: N-seat or user-specified Dimensions: 145×97×38mm Weight: 210g
Basic knowledge of antenna working principle, classification, bandwidth and direction
(1) What is the working principle and function of antenna?
Answer: Antenna is an indispensable part of wireless communication. Its basic function is to radiate and receive radio waves. When transmitting, it converts high-frequency current into electromagnetic waves; when receiving, it converts electromagnetic waves into high-frequency current.
(2) How many types of antennas are there?
Answer: There are many types of antennas, which can be mainly classified in the following ways: According to their use, they can be divided into base station antennas and mobile portable antennas. According to their working frequency band, they can be divided into ultra-long wave, long wave, medium wave, short wave, ultra-short wave and microwave; according to their direction, they can be divided into omnidirectional and directional antennas.
(3) How to choose an antenna?
Answer: Antenna is an important part of the communication system. Its performance directly affects the indicators of the communication system. When choosing an antenna, users must first pay attention to its performance. Specifically, there are two aspects. The first is to choose the antenna type; the second is to choose the electrical performance of the antenna. The significance of selecting the antenna type is whether the directional pattern of the selected antenna meets the requirements of the radio wave coverage in the system design; the requirements for selecting the electrical performance of the antenna are whether the electrical indicators such as the frequency bandwidth, gain, and rated power of the selected antenna meet the requirements of the system design. Therefore, it is best for users to contact the manufacturer for consultation when choosing an antenna.
(4) What is the gain of an antenna?
Answer: Gain is one of the main indicators of an antenna. It is the product of directivity and efficiency, and is a manifestation of the size of the radio waves radiated or received by the antenna. The choice of gain depends on the requirements of the system design for the radio wave coverage area. Simply put, under the same conditions, the higher the gain, the farther the radio wave propagates. Generally, base station antennas use high-gain antennas, and mobile station antennas use low-gain antennas.
(5) What is the voltage standing wave ratio?
Answer: When the antenna input impedance and the characteristic impedance of the feeder are inconsistent, the magnetic wave generated by the superposition of the reflected wave and the incident wave on the feeder is the ratio of the maximum and minimum values of the adjacent voltages. It is the basis for testing the transmission efficiency of the feeder. The voltage standing wave ratio is less than 1.5, and the voltage standing wave ratio at the operating frequency is less than 1.2. If the voltage standing wave ratio is too large, the communication distance will be shortened, and the reflected power will return to the transmitter power amplifier part, which is easy to burn out the power amplifier tube and affect the normal operation of the communication system. Voltage standing wave ratio 1.0 1.1 1.2 1.5 2.0 3.0 Reflected power % 0 0.2 0.8 4.0 11.1 25.0 Transmitted power % 100 99.8 99.2 96 88.9 75.0
(6) What is the directivity of an antenna?
Answer: An antenna has different radiation or receiving capabilities in different directions in space. This is the directivity of an antenna. The directivity of an antenna is usually measured by the radiation pattern. On the horizontal plane, an antenna with no maximum radiation or reception direction is called an omnidirectional antenna, and an antenna with one or more maximum directions is called a directional antenna. Omnidirectional antennas are mostly used in point-to-multipoint communication hubs because they are non-directional. Directional antennas have the maximum radiation or reception direction, so their energy is concentrated and their gain is higher than that of omnidirectional antennas. They are suitable for long-distance point-to-point communications. At the same time, due to their directionality, they have strong anti-interference capabilities.
(7) How do you understand the working frequency bandwidth of an antenna?
Answer: The electrical parameters of the antenna are generally related to the operating frequency. The frequency variation range that ensures the electrical parameter indicators is allowed is the working frequency bandwidth of the antenna. Generally, the working bandwidth of an omnidirectional antenna can reach 3-5% of the working frequency range, and the working bandwidth of a directional antenna can reach 5-10% of the working frequency.
(8) How to select the cable and cable length?
Answer: Mobile communication systems often use coaxial cables with a characteristic impedance of 50 ohms as feeders. In order to effectively transmit radio waves to the antenna interface, the transmission loss of the feeder should be minimized. The transmission loss depends on the diameter and length of the cable. At the same frequency, the larger the cable diameter, the smaller the loss, and the longer the cable, the greater the loss. In principle, the transmission loss of the cable should not exceed 3 decibels. The following table lists the attenuation values (db/m) of commonly used cables. Users can reasonably select the cable model and length according to their own situation. Frequency model 150MHz 400MHz 900MHz SYV-50-7 0.121 0.203 0.295 CTC-50-7 0.060 0.100 0.165 CTC-50-9 0.050 0.085 0.135 CTC-50-12 0.040 0.060 0.105 Imported 10D-FB 0.040 0.070 0.110
(9) How to choose the installation site of the antenna?
Answer: Due to the influence of terrain and environment, the electromagnetic waves received by the antenna are the superposition of direct waves, reflected waves and scattered waves. The result determines the field strength amplitude and phase at the receiving point and directly affects the application effect of the antenna. Therefore, the following aspects should be noted when selecting the antenna installation location: 1. The transmitting or receiving direction of the antenna should avoid obstacles (buildings, towers, bridges, etc.); 2. The antenna installation location should be as far away from interference sources (high-voltage lines, flight routes, towers, roads, etc.); 3. The antenna should be installed at a nearby commanding height as much as possible: 4. If several antennas are working on the same tower, special attention should be paid to the left-right and up-down spacing between them to prevent mutual coupling from affecting system performance.
(10) How should the antenna feed system be installed?
Answer: First, assemble the antenna, feeder and supporting parts according to the requirements of the product instructions, and then fix them to the antenna bracket of the tower with a clamp at the antenna support position, and make the parallel spacing between the antenna and the tower greater than the wavelength used to reduce the impact of the tower on the antenna performance. At the antenna port, connect the feeder line to the antenna with a connector (or cable head), bend a ring with a diameter of about fifty times the diameter of the feeder line and fix it on the antenna bracket to prevent the connector from being directly stressed and causing breakage or damage.
(11) How to waterproof the antenna feed system?
Answer: The antenna and feeder are mainly connected by connectors. Self-adhesive rubber sealing tape is used. After stretching, it is wrapped around the connector in a half-lap form to achieve good sealing and waterproofing. In addition, a return bend is made at the point where the feeder enters the room to prevent rainwater from entering the indoor equipment along the feeder.
(12) How to detect the antenna feeder system?
Answer: After the antenna feeder system is set up, it should be tested by professional technicians using special testing instruments. Usually, a through-type power meter can be connected in series between the transmitter and the antenna feeder system to check the transmitter power and reflected power of the equipment to determine whether the system is working properly.
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