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Module 4 provides information on some of the technical aspects of telecommunications and the resulting operational implications. Students will acquire a basic knowledge of technical facts needed for the selection of the most appropriate tools when preparedness plans are developed; such information will also help them to make the best use of the equipment and the networks possibly available in an emergency situation. Students with some background in elementary physics will probably complete work on this module very quickly, and might be interested mostly in the links to information on more advanced aspects of telecommunication technology. Some basic guidelines for the operation of private radio communication networks are given in an annex to this module.
On the fixed line telephone network, a voice signal is transported as an electric current changing in the rhythm of the audio waves. This form of transmission is called analogue transmission, as the information content maintains its original form (sound waves in air) even while being carried by another medium (electric current on wires).
Information can also be transformed into digital form. One example of this mode is actually older than the fixed line telephone service: Morse code, used already on the earliest telegraph systems, converts letters into signals consisting of a sequence of pulses, and a telegraph link distinguishes only between "current or no current". Expressed in mathematical terms: The numbers one and zero represent all information.
Digital modes have the advantage of being more suitable for electronic handling of the information, but they require the transformation or conversion of the content at the transmitting and the receiving end.
The transmission of text is carried out almost exclusively in digital mode. In many cases, such as on cellular phones, even voice is converted into digital signals. The additional complication of the equipment is more than compensated by the increased efficiency of a digital network, and by the additional features it makes possible.
When Albert Einstein was asked to explain how wireless communication worked, he answered: "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."
In reality, things are somewhat more complicated, but in order to understand what radio equipment can, and cannot do in emergency telecommunications we need to understand some basic principles.
Sound waves are carried as movements of the molecules of the air. Electric currents travel through conducting materials, such as metal wires. Sound waves cannot leave the atmosphere or travel through a vacuum, but electric waves can, under certain circumstances, leave the wires carrying an electric current. They do so only if the direction of the current changes very rapidly; such a current is called an alternating current. The speed of the current's direction change is called the frequency of a radio wave. The equipment producing such a very rapidly changing current is called the transmitter; and the transmitting antenna is the wire, from which the radio waves depart. On the other side of a radio link, the waves transmitted produce a very weak electric current in a similar wire, the receiving antenna. Another piece of equipment, the receiver, amplifies this current and converts it into a signal, which can then be processed.
So far, so good. The only information available at the receiving end will be, whether or not the transmitter is switched on. In order to use the radio waves as carriers of information, we need to modulate them: we have to influence their characteristics in such a way that the information "written" on them at the transmitting side can be "read" at the receiving end. In telecommunication, radio waves are like the paper a letter is written on, and the information can be in different languages or codes. The easiest way to impose the message on the radio wave: By switching the transmitter on and off, we transmit pulses, a code, the meaning of which is known on both sides of the link. What we have now, is what used to be called "wireless telegraphy", and the Morse code is a "language". [illustration 4.1]
We can however go beyond this method, by varying the characteristics of a radio wave for example in the rhythm of audio waves. We now have what used to be called "wireless telephony". Varying the strength or amplitude of a radio signal results in "Amplitude Modulation" or AM, the mode which is so typical for HF radio broadcast that the term "AM" is commonly used as a name for the short- medium-and longwave bands used by it.
Changing the frequency of a radio wave in the rhythm of the modulation results in what is called "Frequency Modulation" or FM. We all use this term for VHF broadcast, simply because Frequency Modulation is the most common type of modulation used for high fidelity or "Hi-Fi" broadcasting. Most of the communication equipment used on VHF and UHF networks also uses FM, and the sound quality is therefore often better than that of the shortwave networks using AM or derivates thereof. "Single Side Band (SSB)" is one such mode and used in most voice communication links on shortwave.
Digital modes are a very efficient also when carried on radio waves. Rapid changes between the two states of a link, the presence or absence of radio waves, or changes between two characteristics such as between a high and a low amplitude or frequency, are what digital radio links use.
The frequency of a radio wave can also be expressed by its wavelength. Radio waves travel with the speed of light, and their wavelength is the distance they travel per unit of time, divided by the number of changes over the same period. Frequencies are measured in changes or cycles per second; one change per second is named "one Hertz", after the German scientist Heinrich Hertz who first described the characteristics of electric waves. More practical units are being used when talking about radio waves, where very large numbers of cycles occur per second: Like for other units of the metric system, the prefix "kilo" is used to denominate 1000, the prefix "mega" one million. The terms Kilohertz (kHz) and Megahertz (MHz) are the most commonly used units. [illustration 4.2]
The way radio waves travel depends on their frequency or wavelength. The higher the frequency, the more their propagation characteristics approach those of visible light. VHF, and even more so UHF waves travel in a straight line and are reflected only by objects with specific physical characteristics. Obstacles such as hills or buildings cast a "shadow", a region within which they cannot be received.
Waves in the shortwave range of the radio spectrum are to some extent following the contours of the terrain. In addition, higher layers of the earth's atmosphere are reflecting them. The degree of reflection depends on the angle in which they reach the reflecting layers, but also on the physical characteristics of the latter. These in turn change with the time of day, the seasons and solar activity (such as sunspots). Efficient use of the shortwave spectrum for long distance communications requires basic knowledge of the factors influencing the propagation of radio waves and the experience of skilled operators. Under favourable conditions, global shortwave communications are possible with no more power than that of a flashlight. [illustration 4.3]
Even longer waves, such as used by the AM or "medium wave" and "long wave" broadcast services, are not reflected in the atmosphere. The distance the "ground wave" travels until its energy is absorbed therefore limits its range. This absorption again depends on certain conditions in the atmosphere, during the night the range of the longer radio waves typically increases.
Using a different frequency for each radio link allows the receiver to distinguish between the signals arriving from different transmitters. Not only the sensitivity, but also the selectivity of a receiver is a main criterion determining its performance. Of equal importance is the quality of the antenna, the importance of which we shall consider in the following chapter.
In module 3 we have considered different systems and their application. With the technical explanations in the above chapter we will now better understand the reasons for the use of those different systems for different purposes.
Appropriate antennas are indispensable for all radio communication. As we have seen earlier, VHF equipment is usually smaller and lighter than shortwave equipment. In particular this applies to the antennas. An antenna must have specific dimensions in order to effectively transmit or receive waves of a specific wavelength, an antenna must have specific dimensions. An optimal size for a vertical antenna is a length of one quarter of a wavelength. For VHF transceiver, the fulfilment of this requirement results in a physical length in the dimension of 50 cm and even less for UHF. For a portable or mobile shortwave station, such a quarter wavelength vertical antenna can become unpractical, as its physical dimensions would be between 2.5 and 25 meters.
There are ways to reduce the physical dimensions of an antenna, but they inevitably reduce its efficiency. The short, rubber-coated antennas commonly used hand-held VHF transceivers are an example of such compromise: The actual antenna wire still has the length of a quarter wavelength, but it is would in a spiral around a length of rubber rod or pipe. Mobile phones use similar techniques to shorten the antenna even to the extent that it fits inside the actual phone, but due to the high density of cellular base stations the resulting loss in antenna efficiency is usually acceptable. On a hand-held VHF transceiver, a straight, full length antenna may be less convenient, but will in any case result in a much improved communication range.
On the other hand, using antennas concentrating the radio waves into one direction provides an actual gain just like the one that could be achieved by increasing transmitter power. For the wavelengths used in VHF communications, such directional antennas are not practical for portable or mobile use, but can be installed at a base stations for traffic in a specific direction or with a rotator to change the direction in which they point.. Directional antennas are equally useful for transmission and reception, and their best-known form is that of the common television antenna. Antenna gain can also be achieved by concentrating the radiation of the antenna in vertical direction; such antennas radiate less energy up into the sky but focus it on a horizontal angle while still maintaining their omnidirectional character. They are most commonly found at VHF base and repeater stations. On very high frequencies, even a parabolic reflector might be used; such a "dish" concentrates the radio waves just like the optical reflector or mirror concentrates the light in a flashlight.
On shortwave, the same principles apply. An antenna, which is physically shorter than at least a quarter of a wavelength, will always have a lower efficiency than one of full size, and directional antennas can extend the range of a station considerably. Among the most efficient shortwave antennas are the so-called beam antennas, which look like giant TV antennas: Their elements are typically between 5 and 20 meters long. A special variety of such antennas is what is called a logarithmic-periodic antenna, working with high efficiency on a very wide range of frequencies. Shortwave antennas can also be made from wires suspended between masts, trees or buildings.
Two basic rules apply for all antennas:
To be efficient an antenna must be resonant, i.e. have the proper dimension for the operating frequency, and it should be installed as high above ground as possible.
As for all rules there are exceptions, there are broadband antennas and in some situations a lower position of a shortwave antenna might improve communication over a specific distance. Broadband antennas however are always a compromise between operational convenience and optimal performance, and deviations from "the higher the better" need to be analysed by a trained technician. Exceptions to the second rule are antennas used for communication via satellites: Height above ground is not a relevant factor, as long as the path to the satellite is not obstructed by obstacles such as buildings or terrain formations.
Power sources for telecommunication equipment need particular considerations when the use of equipment during emergency situations is intended. The infrastructure of the AC power network is vulnerable to the physical impact a disaster might have on elements such as masts, wires and cables. Automatic safety systems monitor all functions and will, under certain conditions, also shut down sectors not directly affected by an event. A sudden dramatic reduction in consumption caused by disruptions or a disconnection of parts of the network may in turn result in an automatic shut-down of generators, thus affecting also parts of a network not directly affected by the impact of the event.
In all these cases, telecommunication systems will be affected unless they have access to other power sources. Batteries are the most obvious alternative, but their capacity and thus the time for which they can supply current is limited. The capacity of a battery is calculated in "Ampere-hours", the product of the available current and the time during which it can be maintained. "Ah" is the measure for the electric size of a battery.
Two fundamentally different types of batteries have each their advantages and disadvantages for use in emergency situations. The type commonly known as "flashlight batteries" is non-rechargeable. A chemical reaction in such batteries creates electrical current only until the substances between which the reaction takes place have been used up. In the rechargeable batteries, best known as car batteries but also powering most mobile phones and more correctly called "accumulators", the chemical process is reversible: Using or discharging an accumulator reverses the chemical process of charging without affecting the reacting substances permanently. Only after a large number of charge and discharge cycles, an accumulator will gradually use its ability to store energy.
For very occasional use or as spares, non-rechargeable batteries are most suitable. They have a shelf life of usually several years, and can therefore be put to use immediately. For more frequent use, such as typical for starter batteries of cars, accumulators are far preferable. Over their whole life span they can be re-used thousands of times, provided they are re-charged regularly. During longer storage, an accumulator will however slowly lose its capacity unless re-charged regularly. For use in case of emergency only, accumulators therefore need regular maintenance.
The size and capacity of non-rechargeable batteries are limited. They are suitable for small telecommunication equipment only, typically for portable, hand-held VHF or UHF transceivers, in case such equipment is to be used exclusively in case of emergency. For use of the equipment over a time of more than a few hours, sufficient spare batteries need to be kept available. [example 4.3]
In most cases, also equipment with low power requirements uses accumulators. For emergency use, regular maintenance is required, and, with re-charging, they can cover power needs over extended periods. Without re-charging, they are of no more use than one-way, non-rechargeable batteries. For this reason, but also due to the limited capacity even of very big and heavy accumulators, other emergency power sources need to be considered.
Generators, powered by petrol engines, can cover higher demand over longer periods. They exist in all sizes, from small, portable generators delivering just enough energy to run some lights and other small appliances, to stationary equipment covering all power requirements of a large building or the needs encountered at the site of an emergency. When choosing the type of engine, the availability of fuel must be considered; transport and handling of Diesel fuel are safer than those of gasoline, and for other than very small generators, Diesel engines should be the preferred.
Alternative sources of electric energy have potential applications in emergency telecommunications only on longer term. The installation of solar panels or wind generators requires expertise and manpower not usually available in an emergency situation. These sources of free and renewable energy are however excellent choices for equipment operating in remote locations over a longer period. Typical examples are VHF and UHF repeater stations. Their accumulators will be re-charged periodically, whenever the meteorological conditions allow this. Other alternatives such as small generators, driven by hand or by bicycle-like pedals, have very limited power. Hand-driven generators have found recent applications only in the "wind up radio", a small transistor broadcast receiver receiving its power from a generator driven by a kind of manually wound clockwork, and in small emergency flashlights. A small hand-generator to re-charge the accumulator of a mobile phone is also available.
New technologies for power generation include fuel cells, producing electric energy from hydrogen and oxygen and even generators using nuclear processes. Such solutions can be expected to become available for use in emergency telecommunications within the next few years. For the time being, they are still far too complex to be suitable for rapid deployment in an emergency situation.
The technical basics explained above should mainly help in the selection of appropriate equipment. For the user of such equipment, they may be of limited interest. Reliable communication is all he or she wants.
Emergency Telecommunication Equipment needs to be User-Friendly. Personal, mobile communication equipment makes everyone a potential user. Training is therefore indispensable. This concerns less the technical aspects that operational procedures: The driver of a car does not need to know much about the technical details of his vehicle, but needs to be familiar with its operation and with traffic rules. Only specialized machinery requires professionally trained operators, and maintaining the functionality of a vehicle is the task of a professional mechanic. The user of emergency telecommunications equipment must be trained in its operation and in the rules governing the traffic on the network. Only special equipment, such as data or satellite networks, need the know-how of a skilled operator, and to maintain the equipment and the network infrastructure is the task of the telecommunications specialist.
The most important rules for voice communication networks are included in the annex to this module. [annex 4.4]
Traffic rules are useful only, if everyone applies them in the same way. Practical exercises allow the users to acquire the routine, which will make communications a tool rather than a burden, when he or she is called upon to fulfil a task in emergency or disaster response.
User-friendliness can also be achieved by making an emergency telecommunication system work in the same way as equipment we use every day. A VHF, UHF and HF (shortwave) data network, based on standard Internet hardware and software allows the user to apply the same procedures as those he or she is familiar with from accessing the Internet from any PC at home or at the office. [illustration 4.4] [illustration 4.5]
In the following module, number 5, we shall look at the laws and regulations governing all telecommunications, and at the consequences these have for their application in different situations. We shall also review the subjects covered in earlier modules, and try to develop some guidelines for the practical application of what we have learned.
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