Файл: Учебнометодическое пособие по английскому языку для специалистов и бакалавров 2 курса института ртс. Москва 2019.docx

Antenna array
An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas (called elements) are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together (interfering constructively) to enhance the power radiated in desired directions, and cancelling interfering destructively) to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements. An antenna array can achieve higher gain (directivity), that is a narrower beam of radio waves, than could be achieved by a single element. In general, the larger the number of individual antenna elements used, the higher the gain and the narrower the beam. Some antenna arrays (such as military phased array radars) are composed of thousands of individual antennas. Arrays can be used to achieve higher gain, to give path diversity (also called MIMO) which increases communication reliability, to cancel interference from specific directions, to steer the radio beam electronically to point in different directions, and for radio direction finding (RDF). The term antenna array most commonly means a driven array consisting of multiple identical driven elements all connected to the receiver or transmitter. A parasitic array consists of a single driven element connected to the feedline, and other elements which are not, called parasitic elements. It is usually another name for a Yagi-Uda antenna. A phased array usually means an electronically scanned array; a driven array antenna in which each individual element is connected to the transmitter or receiver through a phase shifter controlled by a computer. The beam of radio waves can be steered electronically to point instantly in any direction over a wide angle, without moving the antennas. However the term "phased array" is sometimes used to mean an ordinary array antenna.
Description

Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles, don't have much directivity (gain); they are omnidirectional antennas which radiate radio waves over a wide angle. To create a directional antenna (high gain antenna), which radiates radio waves in a narrow beam, two general techniques can be used. One technique is to use reflection by large metal surfaces such as parabolic reflectors or horns, or refraction by dielectric lenses to change the direction of the radio waves, to focus the radio waves from a single low gain antenna into a beam. This type is called an aperture antenna. A parabolic dish is an example of this type of antenna. A second technique is to use multiple antennas which are fed from the same transmitter or receiver; this is called an array antenna, or antenna array. If the currents are fed to the antennas with the proper phase, due to the phenomenon of interference the spherical waves from the individual antennas combine (superpose) in front of the array to create plane waves, a beam of radio waves traveling in a specific direction. In directions in which the waves from the individual antennas arrive in phase, the waves add together (constructive interference) to enhance the power radiated. In directions in which the individual waves arrive out of phase, with the peak of one wave coinciding with the valley of another, the waves cancel (destructive interference) reducing the power radiated in that direction. Similarly, when receiving, the oscillating currents received by the separate antennas from radio waves received from desired directions are in phase and when combined in the receiver reinforce each other, while currents from radio waves received from other directions are out of phase and when combined in the receiver cancel each other. The radiation pattern of such an antenna consists of a strong beam in one direction, the main lobe, plus a series of weaker beams at different angles called sidelobes, usually representing residual radiation in unwanted directions. The larger the width of the antenna and the greater the number of component antenna elements, the narrower the main lobe, and the higher the gain which can be achieved, and the smaller the sidelobes will be. Arrays in which the antenna elements are fed in phase are broadside arrays; the main lobe is emitted perpendicular to the plane of the elements. The largest array antennas are radio interferometers used in the field of radio astronomy, in which multiple radio telescopes consisting of large parabolic antennas are linked together into an antenna array, to achieve higher resolution. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a diameter equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry (VLBI) dishes on separate continents have been linked, creating "array antennas" thousands of miles in size.

Task 1. Read and translate text E.

Task 2. Write the abstract of this text, divide text into parts, express the main idea of each part.

Unit 2

Text A

Radio Waves
Can you count how many devices you use every day thanks to radio waves? It can be a bit overwhelming to think about just how impacted our lives are by the utilization of this technology. From smartphones to laptops, GPS to baby monitors and more, we’ve come to harness this form of electromagnetic energy to create some amazing things. But while we use these devices each and every day, do we truly understand how they work?

Radio waves are but one type of wave in what’s called the electromagnetic spectrum, which consists of a variety of waves that all serve a specific function, like infrared, x-ray, gamma rays, and radio. All of these waves manage to defy physical barriers, hurtling through the vacuum of space at the speed of light.

The organization of this spectrum is categorized by two measurements, frequency, and wavelength. Here’s how they breakdown:

• Frequency. This is basically how many electromagnetic waves will pass through a given point every second. You can measure this by counting the crests of each wave (the highest point in the wave), which provides a value in Hertz.

• Wavelength. This is the actual distance that you can measure between two of the highest points in a wave, or the period. Wavelengths can be shorter than the size of an atom for some waves, and longer than the diameter of our entire planet! On this electromagnetic spectrum, radio waves have both the longest wavelengths and the lowest frequencies, which makes them slow and steady, the long-distance runners of the bunch.

• Speed of a radio signal. Another feature which can be noted about an electromagnetic wave is its speed. As it is the same as a light wave it has the same speed. Normally this is taken to be 3 x 10 8metres a second but a more exact figure is 299 792 500 metres a second in a vacuum.

The speed, frequency and wavelength of a radio wave are all related to one another. As the speed is virtually the same whether the signal is travelling in free space, or in the atmosphere, it is very easy to work out the wavelength of a signal if its frequency is known. Conversely the frequency can be calculated if the wavelength is known. The formula is very simple:

v =  x f

where

v =the velocity of the radio wave in metres per second (normally taken as 3 x 10 8m/s

 =the wavelength in metres

f =the frequency in Hertz

For example a signal with a frequency of 1 MHz will have a wavelength of 300 metres.

The ultra high frequency (UHF) band has a frequency between 300 megahertz (MHz) and 3 gigahertz (GHz). You’ll find the UHF band used for specific technologies like WiFi, Bluetooth, GPS, walkie-talkies, and more. On the flip side, you’ll find very low frequency (VLF) in the 3 – 30 hertz range and this band is reserved exclusively for government radio stations, secure military communications, and submarines.

Now you might be wondering, how exactly those radio waves in their particular frequencies get from place to place? Every radio, whether it’s a traditional AM/FM radio or a radio found in a smartphone, uses the same basic method of transmitting information with the help of both a transmitter and a receiver.

A transmitter, as its name implies, transmits information through the air in the form of a sine wave. This wave goes flying through the air, eventually being caught by a receiver, which decodes the information within the sine wave to extract the stuff we want, like music, a human voice, or some other bit of data.

What’s interesting is that a sine wave alone doesn’t contain any of the data that we need, it’s basically an empty signal. This is why we need to take this sine wave and modulate it, which is the process of adding another layer of useful information. There are three methods of modulation, including:

• Pulse Modulation. In this method, you are turning a sine wave on and off, which will send bits of a signal in separate chunks. Ever heard of Morse Code to send distress signals? It uses pulse modulation.

• Amplitude Modulation. This method is used in both AM radio stations and those old analog TV signals. Here, a sine wave is overlaid with another wave of information, like a person’s voice. Embedding another layer of information in this wave will create a fluctuation in the amplitude of the original sine wave, which can create static.

• Frequency Modulation. This method is used by FM radio stations and virtually every other wireless technology out there. Unlike amplitude modulation which creates some significant fluctuations in a sine wave, frequency modulation changes a sine wave very little, which has the added benefit of resulting in less static.

Once all of those modulated sine waves are sent via a transmitter and received by a receiver, the wave of information that we embedded gets extracted, allowing us to do with it as we please, like play it as audio through a speaker, or view it as video on a television screen. Waves don’t always fly through the air straight from a transmitter to a receiver, and how they travel ultimately depends on what kind of wave frequency you want to send, and when. There are three ways this journey can happen, including:
Line of Sight (Space Wave)

With this method of travel, radio waves are sent as a simple beam of light from point A to point B. This method was commonly used in old-fashioned telephone networks that had to transmit calls over a long distance between two massive communication towers.
Ground Wave (Surface Wave)

You can also send radio waves along the curvature of the earth’s surface in the form of a ground wave. You’ll find AM radio waves traveling in this manner for short to medium distances, which is why you can still hear radio signals even when there isn’t a transmitter and receiver in your line of sight.
Ionosphere (Sky Wave)

Last, you can also send radio waves straight up into the sky, which ends up bouncing off of the earth’s ionosphere, which is an electrically charged part of the atmosphere. When you do this, the radio waves will hit the ionosphere, bounce back down to earth, and bounce back up again. This is the process of mirroring a wave, bouncing it back and forth to its final destination.

At this point we’ve gathered several things about radio waves, namely that they travel at very specific frequencies, they communicate with both a transmitter and receiver, and they can travel in a variety of ways across the earth. But with all of the different radio frequencies floating about, how does your smartphone or car radio know which particular frequency to receive, and which ones to ignore? There is where antennas come into play.

It’s All About Antennas

Antennas come in a bunch of different shapes and sizes, but they’re all designed for the same purpose – to pick up a very specific radio wave frequency. You’ll find antennas ranging from the long metal wires sticking out from an FM radio to something rounder like a satellite dish, or even a tightly tune piece of copper on a PCB. In a transmitter, antennas are used to send radio waves, and in receivers, they’ll be used to pick up on a radio frequency. Antennas have three distinct features that they’re measured by, including:

• Direction. For some antenna types, such as a dipole, the antenna has to be mounted in the proper direction, facing the direction of the radio wave transmission. Some antenna types, like those found in an FM radio, don’t need to be oriented in a specific direction and can capture radio wave signals from any angle.

• Gain. The gain of an antenna describes how much it is going to boost a signal. For example, if you turn on an old analog TV, then you’ll still likely get a picture, just a fuzzy one. This is because of the metal case and components in the TV act as an antenna. But plug an actual directional antenna in, and you’ll be able to boost the signal, and gain a better picture. The bigger the gain, measured in decibels (dB), the better the reception you’ll get.

• Bandwidth. Last, an antenna’s bandwidth is its particular range of useful frequencies. The higher the bandwidth, the more radio waves it can pick up. This is ideal for televisions as it allows them to get more channels. But for things like your smartphone which only need a specific radio wave, a full bandwidth isn’t as necessary.

Radio waves are everywhere! They have indeed shaped our modern lives like nothing else, and without them, we would never get to enjoy such useful inventions as GPS, WiFi, Bluetooth, and more.

Task 1. Read and translate text A.

Task 2. Answer the following questions.

What is frequency of a radio wave?
What is frequency measured in?
What is wavelength?
How do we calculate the speed of electromagnetic waves?
How does a transmitter send information through the air?
What are three main types of modulation? Where are they used?
How are radio waves propagated?
Name the main properties of antennas.

Task 3. Scan the text to mark the statements below true or false.

On the electromagnetic spectrum radio waves have both the longest wavelength and the highest frequency.
A transmitter transmits information through the air on the form of a sine wave.
There are two types of modulation.
Waves always fly through the air straight from a transmitter to a receiver.
Sky waves give line of sight transmission.
Sky waves give line of sight transmission.
You can send radio waves along the curvature of the earth’s surface in the form of a ground wave.
Antennas have three distinct features: direction, gain and bandwidth.

Task 4. Find English equivalents of the following words.

Частота, длина волны, скорость, синусоидальная волна, основной метод передачи информации, значительные изменения (колебания), беспроводная технология, извлекать, поверхностная волна, прямые волны, коэффициент усиления.

Task 5. Read the text and complete the table below.

Propagation of radio waves

Radio waves from a transmitting aerial can travel in one or more of three different ways.

Surface or ground wave

This travels along the ground, following the curvature of the earth’s surface. Its range is limited mainly by the extent to which energy is absorbed from it by the ground. Poor conductors, such as sand, absorb more strongly than water, and the higher the frequency the greater the absorption. The range is about 1500km at low frequencies (long waves).

Sky wave

It travels skywards and, if it is below a certain critical frequency (typically 30MHz), is returned to earth by the ionosphere. This consists of layers of air molecules stretching from about 80km above the earth to 500km. On striking the earth, the sky wave bounces back to the ionosphere where it is again gradually refracted and returned earthwards as if by ‘reflection’. This continues until it is completely attenuated.

The critical frequency varies with the time of day and the seasons. Sky waves of high frequencies can travel thousands of kilometres but at VHF and above they usually pass through the ionosphere into outer space.

Space wave

For VHF, UHF, and microwave signals, only the space wave, giving line of sight transmission, is effective. A range of up to 150km is possible on earth if the transmitting aerial is on high ground and there are no intervening obstacles such as hills, building, or trees. Space waves are also used for satellite communications.