In some applications there are performance differences between horizontal and vertical polarization. For example medium wave broadcast stations generally use vertical polarisation because ground wave propagation over the earth is considerably better using vertical polarization, whereas horizontal polarization shows a marginal improvement for long distance communications using the ionosphere. Circular polarisation is sometimes used for satellite radio communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation.
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Answer the following questions.
1.Why is polarization an important factor of RF antennas?
2.How can one determine the polarization? 3. Why is it important to mach polarization? 4. What polarisation categories do you know? 5. How are different types of polarization used?
6.What are performance differences between horizontal and vertical polarization?
Task 1. Explain the difference between:
a.vertical antenna and polarization antenna;
b.magnetic field component and electric field component;
c.circular polarization and elliptical polarization.
Task 2. Find sentences which give the main idea of each paragraph.
Task 3. Propose your own plan of the text in logical consistency and put it down.
Memorize the following basic vocabulary and terminology to text 1C:
antenna feeder — антенный фидер; питатель feed impedance — сопротивление в точке питания
inductance n — индукция, самоиндукция, собственная индукция
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capacitance cancel — отмена, подавление, емкости radiation resistance — сопротивление излучения dissipate v — рассеивать.
Read and translate the following text.
Text 1C
Antenna feed impedance
When a signal source is applied to an RF antenna at its feed point, it is found that it presents a load impedance to the source. This is known as the antenna “feed impedance” and it is a complex impedance made up from resistance, capacitance and inductance. In order to ensure the optimum efficiency for any RF antenna design it is necessary to maximise the transfer of energy by matching the feed impedance of the RF antenna design to the load. This requires some understanding of the operation of antenna design in this respect.
The feed impedance of the antenna results from a number of factors including the size and shape of the RF antenna, the frequency of operation and its environment. The impedance seen is normally complex, i.e. consisting of resistive elements as well as reactive ones.
Antenna feed impedance resistive elements
The resistive elements are made up from two constituents. These add together to form the sum of the total resistive elements.
Loss resistance. The loss resistance arises from the actual resistance of the elements in the RF antenna, and power dissipated in this manner is lost as heat. Although it may appear that the DC resistance is low, at higher frequencies the skin effect is in evidence and only the surface areas of the conductor are used. As a result the effective resistance is higher than would be measured at DC. It is proportional to the circumference of the conductor and to the square root of the frequency.
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The resistance can become particularly significant in high current sections of an RF antenna where the effective resistance is low. Accordingly to reduce the effect of the loss resistance it is necessary to ensure the use of very low resistance conductors.
Radiation resistance. The other resistive element of the impedance is the “radiation resistance”. This can be thought of as virtual resistor. It arises from the fact that power is “dissipated” when it is radiated from the RF antenna. The aim is to “dissipate” as much power in this way as possible. The actual value for the radiation resistance varies from one type of antenna to another, and from one design to another. It is dependent upon a variety of factors. However a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.
Antenna reactive elements
There are also reactive elements to the feed impedance. These arise from the fact that the antenna elements act as tuned circuits that possess inductance and capacitance. At resonance where most antennas are operated the inductance and capacitance cancel one another out to leave only the resistance of the combined radiation resistance and loss resistance. However either side of resonance the feed impedance quickly becomes either inductive (if operated above the resonant frequency) or capacitive (if operated below the resonant frequency).
Efficiency
It is naturally important to ensure that the proportion of the power dissipated in the loss resistance is as low as possible, leaving the highest proportion to be dissipated in the radiation resistance as a radiated signal. The proportion of the power dissipated in the radiation resistance divided by the power applied to the antenna is the efficiency.
A variety of means can be employed to ensure that the efficiency remains as high as possible. These include the use of
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optimum materials for the conductors to ensure low values of resistance, large circumference conductors to ensure large surface area to overcome the skin effect, and not using designs where very high currents and low feed impedance values are present. Other constraints may require that not all these requirements can be met, but by using engineering judgement it is normally possible to obtain a suitable compromise.
It can be seen that the antenna feed impedance is particularly important when considering any RF antenna design. However by maximizing the energy transfer by matching the feeder to the antenna feed impedance the antenna design can be optimized and the best performance obtained.
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Task 1. Put questions to the text. Discuss the questions with the group.
Task 2. Refer the text to your notes and give a spoken summary.
Grammar exercise № 1
Point out Complex object Infinitive constructions in the following sentences and translate them accordingly.
1.In the late 1990s the public expected broadcasting stations to give up their analog frequencies by 2006. But that plan was a failure.
2.TV viewers supposed conventional analog broadcasting to end in 2007. They believed all broadcasts to be digital. Subscribers (абоненты) assumed digital broadcasts to offer many advantages such as crystal — clear pictures and new information services.
3.We know the first high — definition broadcasts in the U.S. to have started in 1998.
4.Consumers believe frequencies no longer used for analog broadcasting to be available for emergency communication.
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5.The public found the idea of switching the country to digital TV to have been approved by U.S. Congress in the 1980s.
6.Operators believe cable systems to be able to handle 750mhz of bandwidth with hundreds of channels, telephone services and broadband Internet access.
7.Some people don t think that analog TV broadcasting to be widely used in future.
8.Operators know digital signals to travel to televisions in homes using 3 different routes — satellite, antenna or cable.
9.Consumers expect cable operators to provide them with digital tuners.
10.Subscribers of TV channels found cable operators to have begun employing a modified system architecture called switched broadcasting.
11.Specialists expect a hybrid fiber coaxial cable system to provide subscribers with limitless bandwidth.
12.Not everybody knows conventional signal distribution to carry every channel broadcasts in parallel into each home.
13.Scientists expect satellite system to be changing permanently. Consumers expect these systems to accommodate twice as many channels.
14.Students believed adaptive antenna arrays to have considerably improved wireless communications.
15.We know each of arrays to consist of up to a dozen antennas and a powerful digital processor.
16.Millions of call-phone users consider adaptive antenna arrays to have provided them with many benefits, among them better quality of wireless communications.
17.Not only communication experts know the Global Positioning System (GPS) to be a space-based navigation satellite system (GNSS) that provides reliable information about the time the message was transmitted.
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