BS 2 stationary HF traveling wave antenna. THEM. Vorobiev. Equipment and operation of radio stations. Traveling wave antenna with active communication element BS2
2.5. ANTENNAS FOR RADIO COMMUNICATION LINES USING SPATIAL RADIO WAVES
General information.
When radio communication lines operate using spatial radio waves, electromagnetic waves are emitted relative to the earth's surface at an angle of 7 ... 20°. Such radiation is provided by horizontally suspended antennas of the decameter wavelength range.
The directional pattern in the vertical plane of these antennas has zero radiation in the direction of the earth's surface, which reduces energy losses in the ground and increases efficiency.
On radio communication lines of short R1 and long R2 lengths, energy radiation is required at large and small angles, respectively, with respect to the earth's surface (Fig. 2.7).
The most common types of antennas that are installed at radio stations when operating with spatial radio waves are: VG - horizontal vibrator; IOP -
horizontal range vibrator; VGDSH is a horizontal range shunt vibrator, RGD is a horizontal double rhombus and BS2 is a traveling wave antenna with active communication elements.
These types of antennas, excluding BS2, are used for both radiation and reception of electromagnetic waves. The physical processes occurring in these antennas, their technical characteristics and designs are discussed below.
Symmetrical horizontal vibrator VG.
A symmetrical horizontal vibrator is the simplest and most common type of antenna (Fig. 2.8).
When radio frequency oscillations are applied to the vibrator from the transmitter, standing waves are created along the wires, and current nodes / and charge antinodes q (voltage) are created at the ends of the vibrator.
A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.
The vibrator has a symbol
L
To reduce the induction of large currents in the ropes 3 supporting the vibrator, stick insulators 4 are installed at a distance of 3 m from the ends of the vibrator. The distance between the antenna masts is L=2l+ (5 ... 6) m. The reduction of the vibrator 5 is two-wire and is made in the same way with the same wire as the vibrator (bimetal with a diameter of 3 ... 6 mm), with a distance between the wires of 300 mm.
The vibrator is suspended on insulators 2 and connected to the transmitter through a two-wire feeder with a characteristic impedance of 600 Ohms and is used to operate on a fixed wave λ 0 =4l but is actually operated in the wave range 1.6l≤λ≤4l i.e. λmin=1.6l, and λmax=4l.
In Fig. Figure 2.9 shows the radiation patterns of a symmetrical vibrator in the horizontal plane, from which it can be seen that they are all determined by the ratio l / λ.
The vibrator emits in both directions from its axis: the greatest radiation is perpendicular to the axis, along the axis the radiation is zero. The radiation diagram is shaped like a figure eight (see Fig. 2.9, a, b).
A symmetrical vibrator with l = 0.63λ or λ = 1.6l has the highest efficiency (Fig. 2.9, c), has a sharper radiation pattern in the horizontal plane, low levels of side lobes, therefore, when operating on one fixed wave, it is most suitable for use . When l >0.63λ.
The antenna's directivity deteriorates.
A horizontal vibrator with a suspension height of h=Q.25X is used on short-distance radio communication lines (up to 250 km), and with /i=0.65X - long-distance (up to 1500 km).
Horizontal range IOP vibrator.
In the decameter wave range, a symmetrical vibrator with reduced wave resistance, called a Nadenenko dipole, is widely used (Fig. 2.11).
The antenna is suspended on two masts at a height of 13 ... 22 m, the vibrator arm is made in the form of a cylinder with a diameter of 1 ... 2 m, along which six wires are stretched. The antenna uses a bimetallic wire with a diameter of 4 mm. To facilitate the design of the antenna, rings 2 are made of copper or aluminum pipes.
The wires to these rings are soldered or secured with special bolts.
The design of the antenna with an increased diameter of the vibrator makes it possible to obtain a characteristic impedance in the range of 200 ... 400 Ohms.
At the point where the reduction is connected, due to the mutual influence of the vibrators, an increased distributed capacitance is formed, which leads to a deterioration in the matching of the wave impedances of the vibrator and the reduction. To eliminate this, the ends of the cylinders are made in the form of cones with a length of l 1 = 1 m, l 2 = 3 m.
The radiation patterns of the VGD antenna have the same shapes as those of the VG antennas. On radio communication lines up to 400 km long, the VOP antenna can be used as a directional one.
A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.
The antenna is used when working with transmitters with a power of 1 ... 5 kW or more. To cover the wave range from 13 to 120 m, a set of three VOP antennas is used.
L
The wave range in which the antenna can be effectively used is determined by the ratio 1.6l≤λ≤4l.
Antenna symbol -
IOP= --- d.
where l is the length of the vibrator arm, m; h is the height of the antenna above the ground, m; d - diameter of the vibrator ring, m.
At the midpoint of the vibrator, a current antinode and a charge node are formed (U = 0), therefore the active component of the input resistance Rin at this point is zero. At the extreme points of the vibrator B, B1 there is a charge antinode q and a current node I, therefore, with a wave impedance of the vibrator W = 800 Ohm, the active component of the input resistance at these points is Rin = W 2 /R Σ = 800 2 /73.1 = 8750 Ohm.
Consequently, on the vibrator between points B, B1 one can find two symmetrical points A, A 1 in which the active component of the input resistance, with appropriate selection of the distance l 2, would be equal to the characteristic impedance of the two-wire feeder line. Thanks to this, a traveling wave mode is ensured in the feeder line.
When making a vibrator from a wire with a diameter of 1.5 ... 5 mm, the best matching is obtained with the following dimensions: /, where λ is a fixed working wave.
The exact value of the length l 1 is selected by adjusting it and simultaneously measuring the traveling wave coefficient. The value of l 1 is selected at which the traveling wave coefficient is greatest.
A wire shunt symmetrical vibrator VGDSH (Fig. 2.13), proposed by G. Z. Eisenberg and V. D. Kuznetsov, is used as a band antenna. The vibrator is made of six wires, a two-wire reduction is connected to only four wires of the vibrator, and two wires form a loop. The antenna is suspended on two masts.
The shunt symmetrical vibrator is operated in the wave range from 1.6l to 6l, expanding the range towards longer waves.
Good matching of the wave impedance of the vibrator with the wave impedance of the feeder is ensured in a wide frequency band with a traveling wave coefficient in the feeder of no worse than 0.3.
For radio communication lines up to 400 km long, the VGDSh antenna has circular radiation.
VGDSh antennas with a vibrator length l equal to 6, 8, 12 and 16 m are made of six wires, and VGDSh antennas with a vibrator length l = 24 m are made of nine wires to reduce the diameter of the vibrator and maintain wave impedance; in this case, power is supplied to six wires. For the manufacture of vibrators, bimetallic wire d=4 mm is used.
In areas with increased thunderstorm activity, the middle part of the VGDSh antenna shunt is grounded at point A with a metal wire d=6 mm (see Fig. 2.13).
Antennas are rhombic horizontal RG and RGD.
A highly directional wide-range antenna in the decameter wavelength range is a rhombic antenna, the radiating elements of which are wires located in a horizontal plane on the sides of the rhombus.
The antenna is suspended on four masts and is oriented towards the correspondent with a large diamond diagonal (Fig. 2.14). The principle of operation of the antenna is based on the flow of a traveling wave of current around a long wire.
It is known that the radiation pattern of a wire with a traveling current wave in any plane passing through the axis of the wire has two main lobes. These petals are located symmetrically relative to the wire and are inclined in the direction of the movement of the current wave.
To obtain unidirectional radiation and a traveling wave mode in the antenna, a load resistance (absorption line) is connected to the wires of the acute corner of the rhombus facing the correspondent. A two-wire reduction is connected to the other acute corner of the diamond, through which radio frequency oscillations are supplied through a horizontal feeder from the transmitter. The reduction has a characteristic impedance equal to the characteristic impedance of a rhombic antenna (600 ... 700 Ohms). The efficiency of the antenna is 60 ... 80% - The main dimensions of a rhombic antenna are: the length of the side of the rhombus /, half of the obtuse angle F and the height of the suspension h.
The required signal level at the receiving location for a given transmitter power depends on the angle of energy emission, the latter
determined by the height of the antenna suspension h. The set of operating frequencies for a radio communication line determines the optimal wavelength λ 0, and therefore the length of the side of the rhombus l.
For short-distance radio communication lines (600 ... 2000 km), rhombic antennas with dimensions l = 1.7λ 0, Ф = 57º and l = 2.8λ 0, Ф = 65º are used; for medium-length lines (2000 ... ... 4000 km) - l =4λ 0, Ф=65º;
for long lines (4000 ... 6000 km or more) l =6λ 0, Ф=70º.
Symbol for rhombic antennas -
where RG is a single horizontal rhombus; F - half an obtuse angle; a=l/λ 0 -, b-h/λ 0 ; I is the length of the side of the rhombus; h is the average height of the antenna above the ground;
Symbol for double rhombic antenna -
The radiation patterns in the horizontal and vertical planes of a rhombic antenna depend on its geometric dimensions.
ditch (l, Ф, h). To ensure round-the-clock operation of the radio communication line for the entire period of 11 years of solar activity, a set of two or three rhombic antennas is used. In this case, each antenna is used in a limited portion of the radio wave range (0.8...2) λ 0, i.e. one antenna operates during the day, another at night, and the third during intermediate hours of the day.
To maintain the value of the antenna's wave impedance along its length, a single diamond wire is replaced by two wires diverging at the vertices of obtuse angles (see Fig. 2.16). Wires at obtuse corners are suspended one above the other at a distance of 2.5 m.
Rhombic antennas are used when working with transmitters with a power of 1 kW and higher.
Traveling wave antenna with active communication element BS2.
The traveling wave antenna with active communication elements BS2 is widely used in receiving radio stations. It is a horizontally suspended four-wire collecting line, oriented towards the correspondent and connected through resistors to horizontal symmetrical vibrators (Fig. 2.17).
The end of the collecting line on the correspondent side is loaded with a resistor Ru, equal in value to the characteristic impedance of the line, the second end is connected through a reduction and the feeder to the receiving device. 
The collecting line allows you to bring energy from all vibrators to the receiving device, which is why it is called the collecting line. The number of vibrators and their length are selected depending on the length of the radio communication line and the frequency range covered by the antenna. To improve the directional properties, two, or less often three, parallel-connected traveling wave antenna sheets are used.
Let's consider the principle of operation of the antenna.
Electromagnetic waves coming from the side of the correspondent, crossing symmetrical vibrators 1, 2, 3, ..., 20, 21, induce in each of them, respectively, EMF e1, e2, e 3, ..., e20, e21. These EMFs through coupling resistors R St create two current waves in the collecting line, one of which is directed to the load resistor R n, the other towards the receiving device.
Let us assume that the speed of propagation of electromagnetic oscillations in space from vibrator 1 to vibrator 21 is equal to the speed of propagation of the current wave in the collecting line from vibrator 1 to vibrator 21.
Initially, when the current i1 from the emf e1 induced in vibrator 1 by electromagnetic oscillations reaches vibrator 2, then the emf e2 will be induced in the second vibrator and current i1 + i2 + "V will flow towards vibrator 3. When this sum of currents reaches vibrator 3, then the emf e3 will be induced in it. Next, a current i1 + i2 + i3 will flow towards the vibrator 4, etc. Thus, the currents directed towards the receiving device from the vibrators are summed up and create a signal voltage in the input circuit of this device. along the collecting line towards the load resistance, are absorbed by this resistance.
In real conditions, the speed of current waves passing along the collecting line is lower than the speed of radio waves in free space. The presence of a difference in the speed of propagation of electromagnetic waves and the passage of currents in the collecting line causes phase shifts of currents and a decrease in the signal level in the input circuit of the receiving device, and also affects the directional properties of the antenna. The speed of current passage in the collecting line also depends on the load that the vibrators introduce into this line.
To weaken the shunting effect of the vibrators, the latter are connected to the collecting line through coupling elements - resistors R CB.
The use of active rather than reactive coupling elements facilitates the operation of a traveling wave antenna in a wide frequency range.
The antenna radiation pattern in the horizontal plane has low levels of side lobes, and the reception of almost all the energy of electromagnetic waves is concentrated in the main lobe.
Two typical antennas are used at receiving radio stations:
used on radio communication lines with a length of 400 ...
(Fig. 2.19, 2.20) is used on radio communication lines with a length of 2000... 4000 km and operates in the range of 12.5. . . 100 m. It consists of two parallel-connected canvases BS /, each of which contains 21 vibrators 7 with an arm length of 8 m. Collecting lines 6 have a wave impedance of 170 Ohms and, on the correspondent side, through reductions 5, are loaded onto resistances of 170 Ohms with a power of 10 W.
The vibrators are connected to the collecting line through resistors with a resistance of 200 Ohms.
The BS canvases are suspended on six masts 8. The ends of the collecting lines of the antenna facing the receiving device through reductions made in the form of two matching transformers 170/200 Ohm (vertical 2) and
two 200/400 Ohms (horizontal 3), connected in parallel and connected to feeder 4 with a characteristic impedance of 208 Ohms.
In Fig. Figure 2.21 shows the unit for attaching the vibrators to the collecting line.
The suspension height of the canvas, 11.17 and 25 m, is selected depending on the angle of inclination of the main lobe of the radiation pattern in the vertical plane.
In order to learn about the operation of modern cellular communications in the 3G/4G format, I invited myself to visit the new federal operator Tele2 and spent the whole day with their engineers, who explained to me all the intricacies of data transmission through our mobile phones.
But first I’ll tell you a little about the history of cellular communications.
The principles of wireless communication were tested almost 70 years ago - the first public mobile radiotelephone appeared in 1946 in St. Louis, USA. In the Soviet Union, a prototype of a mobile radiotelephone was created in 1957, then scientists in other countries created similar devices with different characteristics, and only in the 70s of the last century in America were the modern principles of cellular communication determined, after which its development began.
Martin Cooper is the inventor of the Motorola DynaTAC portable cell phone prototype, weighing 1.15 kg and measuring 22.5 x 12.5 x 3.75 cm
If in Western countries by the mid-90s of the last century, cellular communications were widespread and used by most of the population, then in Russia it just began to appear, and became available to everyone a little over 10 years ago.
Bulky, brick-shaped mobile phones that worked in the first and second generation formats have become history, giving way to smartphones with 3G and 4G, better voice communications and high Internet speeds.
Why is the connection called cellular? Because the territory in which communication is provided is divided into separate cells or cells, in the center of which base stations (BS) are located. In each “cell” the subscriber receives the same set of services within certain territorial boundaries. This means that moving from one cell to another, the subscriber does not feel territorial attachment and can freely use communication services.
It is very important that there is continuity of connection when moving. This is ensured thanks to the so-called handover, in which the connection established by the subscriber is, as it were, picked up by neighboring cells in a relay race, and the subscriber continues to talk or delve into social networks.
The entire network is divided into two subsystems: the base station subsystem and the switching subsystem. Schematically it looks like this:
In the middle of the "cell", as mentioned above, there is a base station, which usually serves three "cells". The radio signal from the base station is emitted through 3 sector antennas, each of which is aimed at its own “cell”. It happens that several antennas of one base station are directed at one “cell”. This is due to the fact that the cellular network operates in several bands (900 and 1800 MHz). In addition, a given base station may contain equipment from several generations of communications (2G and 3G).
But Tele2 BS towers only have third and fourth generation equipment - 3G/4G, since the company decided to abandon old formats in favor of new ones, which help avoid interruptions in voice communication and provide a more stable Internet. Regulars of social networks will support me in the fact that nowadays Internet speed is very important, 100-200 kb/s is no longer enough, as it was a couple of years ago.
The most common location for a BS is a tower or mast built specifically for it. Surely you could see red and white BS towers somewhere far from residential buildings (in a field, on a hill), or where there are no tall buildings nearby. Like this one, which is visible from my window.
However, in urban areas it is difficult to find a place to place a massive structure. Therefore, in large cities, base stations are located on buildings. Each station picks up signals from mobile phones at a distance of up to 35 km.
These are antennas, the BS equipment itself is located in the attic, or in a container on the roof, which is a pair of iron cabinets.
Some base stations are located in places you wouldn't even guess. Like, for example, on the roof of this parking lot.
The BS antenna consists of several sectors, each of which receives/sends a signal in its own direction. If the vertical antenna communicates with phones, then the round antenna connects the BS to the controller.
Depending on the characteristics, each sector can handle up to 72 calls simultaneously. A BS can consist of 6 sectors and serve up to 432 calls, but usually fewer transmitters and sectors are installed at stations. Cellular operators such as Tele2 prefer to install more BS to improve the quality of communication. As I was told, the most modern equipment is used here: Ericsson base stations, transport network - Alcatel Lucent.
From the base station subsystem, the signal is transmitted towards the switching subsystem, where a connection is established in the direction desired by the subscriber. The switching subsystem has a number of databases that store subscriber information. In addition, this subsystem is responsible for security. To put it simply, the switch is complete It has the same functions as the female operators who used to connect you with the subscriber with their hands, only now all this happens automatically.
The equipment for this base station is hidden in this iron cabinet.
In addition to conventional towers, there are also mobile versions of base stations located on trucks. They are very convenient to use during natural disasters or in crowded places (football stadiums, central squares) during holidays, concerts and various events. But, unfortunately, due to problems in legislation, they have not yet found wide application.
To ensure optimal radio signal coverage at ground level, base stations are designed in a special way, therefore, despite the range of 35 km. the signal does not extend to aircraft flight altitude. However, some airlines have already begun installing small base stations on their boards that provide cellular communications inside the aircraft. Such a BS is connected to a terrestrial cellular network using a satellite channel. The system is complemented by a control panel that allows the crew to turn the system on and off, as well as certain types of services, for example, turning off the voice on night flights.
I also looked into the Tele2 office to see how specialists monitor the quality of cellular communications. If a few years ago such a room would have been hung to the ceiling with monitors showing network data (load, network failures, etc.), then over time the need for so many monitors has disappeared.
Technologies have developed greatly over time, and such a small room with several specialists is enough to monitor the work of the entire network in Moscow.
Some views from the Tele2 office.
At a meeting of company employees, plans to capture the capital are discussed) From the beginning of construction until today, Tele2 has managed to cover all of Moscow with its network, and is gradually conquering the Moscow region, launching more than 100 base stations weekly. Since I now live in the region, it is very important to me. so that this network comes to my town as quickly as possible.
The company's plans for 2016 include providing high-speed communications in the metro at all stations; at the beginning of 2016, Tele2 communications are present at 11 stations: 3G/4G communications at the Borisovo, Delovoy Tsentr, Kotelniki, and Lermontovsky Prospekt metro stations. , “Troparevo”, “Shipilovskaya”, “Zyablikovo”, 3G: “Belorusskaya” (Ring), “Spartak”, “Pyatnitskoye Shosse”, “Zhulebino”.
As I said above, Tele2 abandoned the GSM format in favor of third and fourth generation standards - 3G/4G. This allows you to install 3G/4G base stations with a higher frequency (for example, inside the Moscow Ring Road, the BSs are located at a distance of about 500 meters from each other) to provide more stable communications and high speed mobile Internet, which was not the case in networks of previous formats.
From the company’s office, I, in the company of engineers Nikifor and Vladimir, go to one of the points where they need to measure the communication speed. Nikifor stands in front of one of the masts on which communication equipment is installed. If you look closely, you will notice a little further to the left another such mast, with equipment from other cellular operators.
Oddly enough, cellular operators often allow their competitors to use their tower structures to place antennas (naturally on mutually beneficial terms). This is because building a tower or mast is an expensive proposition, and such an exchange can save a lot of money!
While we were measuring the communication speed, Nikifor was asked several times by passing grandmothers and uncles if he was a spy)) “Yes, we are jamming Radio Liberty!”
The equipment actually looks unusual; from its appearance one can assume anything.
The company’s specialists have a lot of work to do, considering that the company has more than 7 thousand in Moscow and the region. base stations: about 5 thousand of them. 3G and about 2 thousand. LTE base stations, and recently the number of base stations has increased by about a thousand.
In just three months, 55% of the total number of new operator base stations in the region were put on air in the Moscow region. Currently, the company provides high-quality coverage of the territory where more than 90% of the population of Moscow and the Moscow region lives.
By the way, in December, Tele2’s 3G network was recognized as the best in quality among all capital operators.
But I decided to personally check how good Tele2’s connection is, so I bought a SIM card in the shopping center closest to me on Voykovskaya metro station, with the simplest tariff “Very Black” for 299 rubles (400 SMS/minutes and 4 GB). By the way, I had a similar Beeline tariff, which was 100 rubles more expensive.
I checked the speed without going far from the cash register. Reception - 6.13 Mbps, transmission - 2.57 Mbps. Considering that I am standing in the center of a shopping center, this is a good result; Tele2 communication penetrates well through the walls of a large shopping center.
At metro Tretyakovskaya. Signal reception - 5.82 Mbps, transmission - 3.22 Mbps.
And on metro station Krasnogvardeyskaya. Reception - 6.22 Mbps, transmission - 3.77 Mbps. I stopped at the subway exit. If you take into account that this is the outskirts of Moscow, it’s very decent. I think that the connection is quite acceptable, we can confidently say that it is stable, considering that Tele2 appeared in Moscow just a couple of months ago.
There is a stable Tele2 connection in the capital, which is good. I really hope that they will come to the region as soon as possible and I will be able to take full advantage of their connection.
Now you know how cellular communication works!
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An antenna of type OB-2, in particular, with the help of insulators, can be installed on the rigging of an antenna of type BS-2 (Fig. 7.45) with very little effort and money. In this case, the antennas can operate independently of each other, being a mutual reserve. The antennas have linear mutually orthogonal polarizations, so the wires of the fabric and rigging of the BS antenna have almost no effect on the characteristics of the OB antenna. The antennas allow dual reception of radio signals using the polarization diversity method.
Antennas of the OB type have a relatively wide lobe of the radiation pattern, which reduces their noise immunity. The OB-E antenna does not have this drawback.
Figure 7.45. Layout of the OB-2 antenna on the rigging
antennas type BS-2
Antenna type OB-E
During development, the OB-E antenna was intended to be used at receiving radio centers for mainline radio communications to replace antennas of the BS-2, 2 BS-2, 3 BS-2 types, which are the best available in terms of efficiency, but are bulky, expensive, unreliable in operation and labor-intensive to maintain. The OB-E antenna has a high efficiency/cost ratio C.
The OB-E antenna diagram is shown in Fig. 7.46. It is marked OB-E, where A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.– length of the antenna web; h– height of the antenna web suspension. In Fig. 7.46 is indicated: 1 – surface of the “ground”; 2, 8 – counterweight conductors; 3 – EMF source (radio transmitter, GSS); 5 – conductor with a traveling wave; 7 – resistor – load.
The antenna was marked OB-E (single-wire, traveling wave), where the letter E indicates the presence of another wave on the conductor, similar in structure to the wave E 0 in a round waveguide, if you look at the end of the conductor.
The OB-E antenna had a length A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.= 300 m; equivalent diameter of traveling wave conductor d eq = 280 mm; load resistor rating R n = 200 Ohm; suspension height h= 3 m. The operating frequency range of the OB-E antenna is Δ f= 3 ÷ 30 MHz.
Rice. 7.46. Antenna OB-E
Research has revealed fundamental differences in the operating principles of the OB and OB-E antennas. They suggest that a redistribution of radiation energy occurs in the near-wire space of the OB-E antenna, which led to the creation of a new, simple in design and very compact in diameter antenna for long-distance radio communications, which is a “horn antenna without visible walls.”
And again, some general educational material. This time we will talk about base stations. Let's look at various technical aspects of their placement, design and range, and also look inside the antenna unit itself.
Base stations. General information
This is what cellular antennas look like installed on the roofs of buildings. These antennas are an element of a base station (BS), and specifically a device for receiving and transmitting a radio signal from one subscriber to another, and then through an amplifier to the base station controller and other devices. Being the most visible part of the BS, they are installed on antenna masts, roofs of residential and industrial buildings, and even chimneys. Today you can find more exotic options for their installation; in Russia they are already installed on lighting poles, and in Egypt they are even “disguised” as palm trees.
The connection of the base station to the telecom operator’s network can be done via radio relay communication, so next to the “rectangular” antennas of the BS units you can see a radio relay dish:
With the transition to more modern standards of the fourth and fifth generations, to meet their requirements, stations will need to be connected exclusively via fiber optics. In modern BS designs, optical fiber becomes an integral medium for transmitting information even between nodes and blocks of the BS itself. For example, the figure below shows the design of a modern base station, where fiber optic cable is used to transmit data from the RRU (remote controlled units) antenna to the base station itself (shown in orange).
The base station equipment is located in non-residential premises of the building, or installed in specialized containers (attached to walls or poles), because modern equipment is quite compact and can easily fit into the system unit of a server computer. Often the radio module is installed next to the antenna unit, this helps reduce losses and dissipation of power transmitted to the antenna. This is what the three installed radio modules of the Flexi Multiradio base station equipment look like, mounted directly on the mast:
Base station service area
To begin with, it should be noted that there are different types of base stations: macro, micro, pico and femtocells. Let's start small. And, in short, a femtocell is not a base station. It is rather an Access Point. This equipment is initially aimed at a home or office user and the owner of such equipment is a private or legal entity. a person other than the operator. The main difference between such equipment is that it has a fully automatic configuration, from assessing radio parameters to connecting to the operator’s network. Femtocell has the dimensions of a home router:
A picocell is a low-power BS owned by an operator and using IP/Ethernet as a transport network. Usually installed in places where there is a possible local concentration of users. The device is comparable in size to a small laptop:
A microcell is an approximate version of the implementation of a base station in a compact form, very common in operator networks. It is distinguished from a “large” base station by its reduced capacity supported by the subscriber and lower radiating power. Weight, as a rule, is up to 50 kg and radio coverage radius is up to 5 km. This solution is used where high network capacities and power are not needed, or where it is not possible to install a large station:
And finally, a macro cell is a standard base station on the basis of which mobile networks are built. It is characterized by powers of the order of 50 W and a coverage radius of up to 100 km (in the limit). The weight of the stand can reach 300 kg.
The coverage area of each BS depends on the height of the antenna section, the terrain and the number of obstacles on the way to the subscriber. When installing a base station, the coverage radius is not always at the forefront. As the subscriber base grows, the maximum throughput of the BS may not be enough, in which case the message “network busy” appears on the phone screen. Then, over time, the operator in this area can deliberately reduce the range of the base station and install several additional stations in areas of greatest load.
When you need to increase network capacity and reduce the load on individual base stations, then microcells come to the rescue. In a megacity, the radio coverage area of one microcell can be only 500 meters.
In a city environment, oddly enough, there are places where the operator needs to locally connect an area with a lot of traffic (metro station areas, large central streets, etc.). In this case, low-power microcells and picocells are used, the antenna units of which can be placed on low buildings and on street lighting poles. When the question arises of organizing high-quality radio coverage inside closed buildings (shopping and business centers, hypermarkets, etc.), then picocell base stations come to the rescue.
Outside cities, the operating range of individual base stations comes to the fore, so the installation of each base station away from the city is becoming an increasingly expensive enterprise due to the need to build power lines, roads and towers in difficult climatic and technological conditions. To increase the coverage area, it is advisable to install the BS on higher masts, use directional sector emitters, and lower frequencies that are less susceptible to attenuation.
So, for example, in the 1800 MHz band, the range of the BS does not exceed 6-7 kilometers, and in the case of using the 900 MHz band, the coverage area can reach 32 kilometers, all other things being equal.
Base station antennas. Let's take a look inside
In cellular communications, sector panel antennas are most often used, which have a radiation pattern with a width of 120, 90, 60 and 30 degrees. Accordingly, to organize communication in all directions (from 0 to 360), 3 (pattern width 120 degrees) or 6 (pattern width 60 degrees) antenna units may be required. An example of organizing uniform coverage in all directions is shown in the figure below:
And below is a view of typical radiation patterns on a logarithmic scale.
Most base station antennas are broadband, allowing operation in one, two or three frequency bands. Starting with UMTS networks, unlike GSM, base station antennas are able to change the radio coverage area depending on the load on the network. One of the most effective methods of controlling radiated power is to control the angle of the antenna, in this way the irradiation area of the radiation pattern changes.
Antennas can have a fixed tilt angle, or can be remotely adjusted using special software located in the BS control unit and built-in phase shifters. There are also solutions that allow you to change the service area from the general data network management system. In this way, it is possible to regulate the service area of the entire sector of the base station.
Base station antennas use both mechanical and electrical pattern control. Mechanical control is easier to implement, but often leads to distortion of the radiation pattern due to the influence of structural parts. Most BS antennas have an electrical tilt angle adjustment system.
A modern antenna unit is a group of radiating elements of an antenna array. The distance between the array elements is selected in such a way as to obtain the lowest level of side lobes of the radiation pattern. The most common panel antenna lengths are from 0.7 to 2.6 meters (for multi-band antenna panels). The gain varies from 12 to 20 dBi.
The figure below (left) shows the design of one of the most common (but already outdated) antenna panels.
Here, the antenna panel emitters are half-wave symmetrical electric vibrators above the conductive screen, located at an angle of 45 degrees. This design allows you to create a diagram with a main lobe width of 65 or 90 degrees. In this design, dual- and even tri-band antenna units are produced (though quite large). For example, a tri-band antenna panel of this design (900, 1800, 2100 MHz) differs from a single-band one, being approximately twice as large in size and weight, which, of course, makes it difficult to maintain.
An alternative manufacturing technology for such antennas involves making strip antenna radiators (square-shaped metal plates), in the figure above on the right.
And here is another option, when half-wave slot magnetic vibrators are used as a radiator. The power line, slots and screen are made on one printed circuit board with double-sided foil fiberglass:
Taking into account the modern realities of the development of wireless technologies, base stations must support 2G, 3G and LTE networks. And if the control units of base stations of networks of different generations can be placed in one switching cabinet without increasing the overall size, then significant difficulties arise with the antenna part.
For example, in multi-band antenna panels the number of coaxial connecting lines reaches 100 meters! Such a significant cable length and the number of soldered connections inevitably leads to line losses and a decrease in gain:
In order to reduce electrical losses and reduce solder points, microstrip lines are often made; this makes it possible to create dipoles and the power supply system for the entire antenna using a single printed technology. This technology is easy to manufacture and ensures high repeatability of antenna characteristics during serial production.
Multiband antennas
With the development of third and fourth generation communication networks, modernization of the antenna part of both base stations and cell phones is required. Antennas must operate in new additional bands exceeding 2.2 GHz. Moreover, work in two and even three ranges must be carried out simultaneously. As a result, the antenna part includes rather complex electromechanical circuits, which must ensure proper functioning in difficult climatic conditions.
As an example, consider the design of the emitters of a dual-band antenna of a Powerwave cellular communication base station operating in the ranges 824-960 MHz and 1710-2170 MHz. Its appearance is shown in the figure below:
This dual-band irradiator consists of two metal plates. The larger one operates in the lower 900 MHz range; above it there is a plate with a smaller slot emitter. Both antennas are excited by slot emitters and thus have a single power line.
If dipole antennas are used as emitters, then it is necessary to install a separate dipole for each wave range. Individual dipoles must have their own power supply line, which, of course, reduces the overall reliability of the system and increases power consumption. An example of such a design is the Kathrein antenna for the same frequency range as discussed above:
Thus, the dipoles for the lower frequency range are, as it were, inside the dipoles of the upper range.
To implement three- (or more) band operating modes, printed multilayer antennas have the greatest technological effectiveness. In such antennas, each new layer operates in a rather narrow frequency range. This “multi-story” design is made of printed antennas with individual emitters, each antenna is tuned to individual frequencies in the operating range. The design is illustrated in the figure below:
As in any other multi-element antennas, in this design there is interaction between elements operating in different frequency ranges. Of course, this interaction affects the directivity and matching of the antennas, but this interaction can be eliminated by methods used in phased array antennas (phased array antennas). For example, one of the most effective methods is to change the design parameters of the elements by displacing the exciting device, as well as changing the dimensions of the feed itself and the thickness of the dielectric separating layer.
An important point is that all modern wireless technologies are broadband, and the operating frequency bandwidth is at least 0.2 GHz. Antennas based on complementary structures, a typical example of which are “bow-tie” antennas, have a wide operating frequency band. Coordination of such an antenna with the transmission line is carried out by selecting the excitation point and optimizing its configuration. To expand the operating frequency band, by agreement, the “butterfly” is supplemented with a capacitive input impedance.
Modeling and calculation of such antennas are carried out in specialized CAD software packages. Modern programs allow you to simulate an antenna in a translucent housing in the presence of the influence of various structural elements of the antenna system and thereby allow you to perform a fairly accurate engineering analysis.
The design of a multi-band antenna is carried out in stages. First, a microstrip printed antenna with a wide bandwidth is calculated and designed for each operating frequency range separately. Next, printed antennas of different ranges are combined (overlapping each other) and their joint operation is examined, eliminating, if possible, the causes of mutual influence.
A broadband butterfly antenna can be successfully used as the basis for a tri-band printed antenna. The figure below shows four different configuration options.
The above antenna designs differ in the shape of the reactive element, which is used to expand the operating frequency band by agreement. Each layer of such a tri-band antenna is a microstrip emitter of given geometric dimensions. The lower the frequencies, the larger the relative size of such an emitter. Each layer of the printed circuit board is separated from the other by a dielectric. The above design can operate in the GSM 1900 band (1850-1990 MHz) - accepts the bottom layer; WiMAX (2.5 - 2.69 GHz) - receives the middle layer; WiMAX (3.3 - 3.5 GHz) - receives the upper layer. This design of the antenna system will make it possible to receive and transmit radio signals without the use of additional active equipment, thereby not increasing the overall dimensions of the antenna unit.
And in conclusion, a little about the dangers of BS
Sometimes, base stations of cellular operators are installed directly on the roofs of residential buildings, which actually demoralizes some of their inhabitants. Apartment owners stop having cats, and gray hair begins to appear faster on grandma's head. Meanwhile, the residents of this house receive almost no electromagnetic field from the installed base station, because the base station does not radiate “downward.” And, by the way, SaNPiN standards for electromagnetic radiation in the Russian Federation are an order of magnitude lower than in “developed” Western countries, and therefore base stations within the city never operate at full capacity. Thus, there is no harm from BS, unless you sunbathe on the roof a couple of meters away from them. Often, a dozen access points installed in residents' apartments, as well as microwave ovens and cell phones (pressed to the head) have a much greater impact on you than a base station installed 100 meters outside the building.
Design characteristics of antennas 3bs-2 and ob-2
An antenna of type OB-2, in particular, with the help of insulators, can be installed on the rigging of an antenna of type BS-2 (Fig. 7.45) with very little effort and money. In this case, the antennas can operate independently of each other, being a mutual reserve. The antennas have linear mutually orthogonal polarizations, so the wires of the fabric and rigging of the BS antenna have almost no effect on the characteristics of the OB antenna. The antennas allow dual reception of radio signals using the polarization diversity method.
Antennas of the OB type have a relatively wide lobe of the radiation pattern, which reduces their noise immunity. The OB-E antenna does not have this drawback.
Figure 7.45. Layout of the OB-2 antenna on the rigging
antennas type BS-2
Antenna type OB-E
During development, the OB-E antenna was intended to be used at receiving radio centers for mainline radio communications to replace antennas of the BS-2, 2 BS-2, 3 BS-2 types, which are the best available in terms of efficiency, but are bulky, expensive, unreliable in operation and labor-intensive to maintain. The OB-E antenna has a high efficiency/cost ratio C.
The OB-E antenna diagram is shown in Fig. 7.46. It is marked OB-E, where A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.– length of the antenna web; h– height of the antenna web suspension. In Fig. 7.46 is indicated: 1 – surface of the “ground”; 2, 8 – counterweight conductors; 3–EMF source (radio transmitter, GSS); 5 – conductor with a traveling wave; 7 – resistor – load.
The antenna was marked OB-E (single-wire, traveling wave), where the letter E indicates the presence of another wave on the conductor, similar in structure to the wave E 0 in a round waveguide, if you look at the end of the conductor.
The OB-E antenna had a length A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.= 300 m; equivalent diameter of traveling wave conductor d eq = 280 mm; load resistor rating R n = 200 Ohm; suspension height h= 3 m. The operating frequency range of the OB-E antenna is Δ f= 3 ÷ 30 MHz.
Rice. 7.46. Antenna OB-E
Research has revealed fundamental differences in the operating principles of the OB and OB-E antennas. They suggest that a redistribution of radiation energy occurs in the near-wire space of the OB-E antenna, which led to the creation of a new, simple in design and very compact in diameter antenna for long-distance radio communications, which is a “horn antenna without visible walls.”
The results of RP calculations in the horizontal and vertical planes and experimental studies obtained using flybys at the same frequencies are shown in Fig. 7.47 and Fig. 7.48. Experimental points are shown with crosses.
Rice. 7.47. Calculated and experimental radiation patterns of the OB-E antenna in the horizontal plane
Rice. 7.48. Calculated and experimental radiation patterns of the OB-E antenna in the vertical plane
From the analysis of the radiation patterns it follows that the OB-E antenna has high noise immunity.
Antenna complex OB-E
To receive signals arriving at different angles in the elevation plane, the OB-E antenna complex was created. It includes three OB-E antennas of different lengths A symmetrical vibrator consists of two equal sections of wire 1 of length I each, at symmetrical points of which the currents are equal and coincide in direction. The height of the suspension is selected taking into account the radiation angle of the radiation pattern in the vertical plane, necessary to ensure the specified field strength at the receiving point. The characteristic impedance of such a vibrator is 1000 Ohms.= 60; 120; 240 m, which are oriented to the terrain in one general azimuth.
The complex is designed to receive radio waves in the range of 10 m λ 100 m, (3 ÷ 30 MHz) with ionospheric propagation on long-distance paths R > 1000 km. Recommendations for choosing receiving antennas are given in Table. 7.22. The parameters of the ionosphere are unstable in time and heterogeneous in space, therefore, at the point of reception of radio waves, instability of the angles pr relative to the horizon and fluctuations in field levels are observed.
Table 7.22
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