5.3 Transmission Theory5.3.1 Transmitters5.3.1.1 Transmitter DefinitionA transmitter is a device which generates high frequency power, which, by means of a suitable antenna, is radiated (or transmitted) through space, or through conductors or through obstructive media, such as walls and glass. It may be modulated with information, (noise, voice, pictures (TV)). This modulation or information may be received and demodulated in a receiver, where the combination of a transmitter and receiver is known as a radio system. Transmitter is often abbreviated to XMTR or TX, and receiver to RCVR or RX. 5.3.1.2 CW TransmitterContinuous wave transmission is used primarily for radiotelegraphy. This is the transmission of short or long pulses of RF energy to form dots and dashes that will correspond to some code such as the Morse Code, sometimes referred to as interrupted continuous wave (ICW). This was the first type of radio communication and is still in use today. Advantages of this system are simplicity, narrow bandwidth, and a high degree of intelligibility, CW is often used in long rang applications.Operation is very simple, as may be seen by reference to the diagram of a very basic CW transmitter  The oscillator generates the frequency to be transmitted. The buffer amplifier provides isolation between the oscillator and the power amplifier. The power amplifier is connected to the antenna. 5.3.1.3 Modulated TransmittersIntelligence or data is conveyed by the CW transmitter above by switching it on and off, and represents the simplest form of a transmission, known as class A1 transmissions, where AO is an unmodulated "carrier".5.3.1.4 Amplitude Modulated TransmittersThis may be achieved by feeding the output from an audio amplifier to a transformer whose secondary is in series with the supply line feeding the RF oscillator (low level modulation) or Power Amplifier (high level modulation) of the transmitter. This adds to, or subtracts from, the DC power feeding the transmitter, thus varying the amplitude of the output as a function of the input to the audio amplifier. The transformer secondary voltage can be such that on peaks it reduces the DC to the Power Amplifier to Zero. It can be seen that under this condition, when the AF reverses in phase, this will add to the DC supply voltage, up to twice the DC voltage. This condition is known as 100% modulation, which is the maximum possible - any more will cause distortion and cause the transmitter to produce and radiate harmonics.The simplest form of Amplitude Modulation is MCW or Modulated Carrier Wave. This consists of keying the modulator with a fixed AF tone, say 400 Hertz, with for instance Morse Code. This is known as class A2 transmission. Modulating a transmitter with voice or other frequencies, in amplitude, is known as Class A3 transmission. FSK or frequency shift keying may be applied to an AM transmitter so that, for instance, binary 0 = 1 KHz and binary 1 = 2 KHz modulation. Anything from Morse to RS 232 serial computer data may be sent by this means. Of course, the carrier itself may be "frequency shifted", this being a form of FM transmission. 5.3.1.5 Frequency Modulated TransmittersTransmitters are usually controlled by a low level oscillator, the oscillators themselves being controlled by an LC (Inductor-capacitor) circuit resonant at the oscillator frequency, or by a quartz crystal.The first FM transmitters employed motor driven capacitors in the resonant circuit which caused the frequency to vary according to the capacitor RPM. Nowadays, modulation is normally achieved by the use of a varactor diode, one which varies its capacity as a function of the applied voltage. If an audio voltage is applied, its capacity will follow the AF variations. Thus, such a capacitor is used to form part of an LC oscillator, or a quartz crystal oscillator. The output frequency of quartz crystal oscillators varies to a small degree according to any paralleled capacity in circuit. A variation of frequency modulation is phase modulation, normally both are present in FM transmissions. 5.3.1.6 Line TransmitterThis transmitter is similar to those previously discussed except that the output is connected to a pair of wires as opposed to an antenna. If the line consists of the AC power line, these may be referred to as Carrier Current transmitters. Many baby alarms and intercoms employ this principle.5.3.2 Antennas5.3.2.1 DefinitionAn antenna is a conductor or system of conductors (or, in some instances, partial or semi-conductors) used to either radiate electromagnetic waves (EM) into space or to receive EM waves from space. In effect, electrical energy is converted into EM energy by the transmitter antenna while the receive antenna converts EM radiation into electrical energy. One antenna may serve as a dual purpose device for both receiving and transmitting purposes.5.3.2.2 WavelengthThe wavelength of a signal is the physical distance (normally in free air) it takes an AC signal to complete one cycle. The formula used to calculate wavelength is as follows:lambda = c/f The Greek letter lambda stands for wavelength in meters. The letter c represents the velocity of light (approximately 300 million meters per second) and f denotes frequency. So the wavelength of a 550 KHz AM signal is 545.5 meters, almost 6 football fields. The wavelength of a 104,7 MHz FM signal is 2.9 meters. 5.3.2.3 Basic AntennasIdeally, an antenna should be one-half the wavelength of the transmitting frequency. Maximum current flow at the center of the half wave, maximum voltage at the ends. The impedance at the center happens to work out at about 75 ohms, which matches standard 75 ohm coaxial cable very nicely. Thus, the half wave antenna is most usually broken into two equal quarter waves and fed by coaxial cable at the center. This type of antenna is known as a half wave dipole, and is the fundamental type by which the performance of other types of antenna are judged.The next most fundamental antenna is the "quarter wave ground plane". Imagine one leg of a half wave dipole is replaced by a sheet of metal at right angles. If this was a mirror, an observer standing to one side would see the reflection of the remaining quarter wave element as if it was the cut-off element. The roof or trunk of a car, or body of a walkie talkie acts as a good reflector. The feed impedance of a quarter wave ground plane is around 40 ohms, sufficiently close to 50 ohm coaxial cable to form a potential match. To radiate efficiently, a transmitting antenna has to be resonant, thus the approximate frequency may be ascertained by measuring the length of a dipole in meters and multiplying the figure by two. 5.3.3 Propagation5.3.3.1 DefinitionPropagation is the traveling of EM waves through space and how certain conditions may affect them.5.3.3.2 Ground WaveThe part of the EM wave that leaves the antenna in contact with the ground. There are two components of ground waves: surface and direct waves. The surface wave is a low frequency radio wave that tends to follow the curvature of the earth. The transmitter that broadcasts a surface wave is usually very low powered. Some of the more common uses for these waves is in LORAN, DECCA, OMEGA, and SLF communications. The direct wave is a high frequency, low power transmission that travels in a straight line. Beyond the horizon communication is useless. This is called line-of-sight transmission. 5.3.3.3 Sky waveA sky wave is one which, having left the antenna, is refracted/reflected by the ionosphere. Note that waves above about 30 MHz are not refracted or reflected by the ionosphere to any great extent, and pass through to outer space. For different frequencies, there is a critical angle (just as with visible light and glass prisms). Thus, is an observer travels away from the transmitter on, say, 20 MHz, he may well receive it out to 20 miles, then lose it in the "skip effect" until he starts receiving it again by skywave reflection at around 200 miles onward.In this manner, medium and high frequency transmissions can reach quite a distance if there is enough energy in the radiated wave. This is one reason why commercial AM stations have a greater area of coverage than commercial FM stations. High frequency stations often reach halfway round the world by multiple ionospheric reflections. 5.3.3.4 Atmospheric ConditionsLightning produces noise at all radio frequencies, however, the greatest amount of noise occurs at the lower frequencies. This is one of the reasons why commercial AM signals will appear noisier in a lightning storm than an FM station. The other main reason is that a lightning strike varies in amplitude, and an FM receiver is only responsive to frequency modulation: it has a limiter to eliminate all AM components.5.3.4 Modulation5.3.4.1 DefinitionModulation causes variations in an AC (carrier) signal. There are three basic ways to modulate a signal: alter the frequency, the amplitude, or the phase. We will be concentrating on the first two. By introducing some intelligence to an AC signal, we will change the characteristic to correspond with the input signal.5.3.4.2 Amplitude Modulation (AM)5.3.4.2.1 Standard AMThe intelligence (or modulating signal) is introduced to the carrier frequency in the modulator of a transmitter.  If we were to view this on a frequency spectrum chart (one that maps frequency versus amplitude), notice that the carrier is present at the proper frequency with two sidebands. The power in the carrier always remains the same while the power in the sidebands depends upon the percentage of modulation. Percentage of modulation is calculated by a very simple formula:  At 100% modulation, half of the total radiated power is in the carrier (which contains no modulation or intelligence), while the remainder of the power is divided between the two equal sidebands (a quarter in each sideband). The bandwidth depends solely upon the frequency of the modulating tone. If an audio tone of 3 KHz is applied to he modulator then each sideband will be 3 KHz above and below the carrier and the total bandwidth will be 6 KHz. Commercial AM stations are limited to a bandwidth of 9 KHz, or modulating frequencies up to 4 1/2 KHz, thus resulting in restricted fidelity. Fidelity is the reproduction of all frequencies within the audio bandwidth (20 Hz to 20 KHz). The frequency band for commercial AM is 540 KHz to 1600 KHz. 5.3.4.2.2 Double Sideband Suppressed Carrier (DSSC or DSB)Double sideband suppressed carrier or simply double sideband is the transmission of both sidebands by an AM transmitter, but without the power consuming carrier. This is not a common method of modulation. It is, however, used in modulating subcarriers such as the FM stereo subcarrier. More on both of these later.  The modulating signal and the carrier are both applied to a balanced modulator. This circuit cancels out the carrier when the audio level is at zero. The resulting waveform will have a 180 degree phase shift that corresponds to wherever the original modulating tone crosses the zero reference line. 5.3.4.2.3 Single Sideband Suppressed Carrier (SSSC or SSB)Single sideband suppressed carrier or simply single sideband is the transmission of either sideband without the carrier. The advantages of SSB over standard AM and DSB are two fold:(1) Less power (approximately 1/4) to transmit same information over the same distance.   (2) Bandwidth requirements are halved.  In a SSB system, the modulating signal and the carrier are applied to a balanced modulator just as the DSB did. The result is DSB. Then this signal is fed into either an upper sideband (USB) or a lower sideband (LSB) filter that will eliminate one of the sidebands. The resulting signal is then amplified and transmitted out the antenna. 5.3.4.3 Frequency Modulation (FM)5.3.4.3.1 General Aspects of FMThe major reason for the development of FM is due to its inherent capability for noise reduction. Noise appears as pulses of energy in amplitude so in an AM receiver, these pulses will be demodulated as pulses of sound. With an FM transmission, amplitude remains constant, so noise is reduced by the use of circuits called limiters. There are three areas where FM is used extensively: commercial Band 2 VHF FM stations, communications (such as police or two way), and television audio.In the FM modulator, the carrier frequency will alter as the modulating tone increases or decreases.  This change in frequency is called the deviation. Deviation can best be described as the magnitude of the frequency change away from the assigned center frequency. It is this amount of deviation that will establish the bandwidth requirements. Note that there may be an infinite number of sidebands produced by the modulating signal. Our major concern is the deviation which will correspond to the amplitude of audio. 5.3.4.3.2 Wideband FMStandard wideband FM has a deviation of plus or minus 75 KHz, and thus occupies a bandwidth of 150 KHz, which is why FM stations are spaced 200 KHz apart. FM stations make use of subcarriers for various purposes. Stereo transmissions make use of a 19 KHz pilot and a 38 KHz DSSC signal. The Subsidiary Communication Authority (SCA) FM signal may be found in some of them. This signal is used to transmit weather, music, or low speed digital information to private subscribers. Commercial FM is found between 88 and 108 MHz. Each station begins on an odd decimal (88.1, 93.7, etc.) and has a bandwidth of 200 KHz. 5.3.4.3.3 Narrowband FMNarrowband FM is used primarily for voice communications and has a deviation limit of plus or minus 5 KHz.The majority of the users are emergency services, two way radios and cellular phones. 5.3.5 TelevisionOne of the most advanced forms of transmission today is television. Although the concept of television transmission is simple and easy to understand, the practical application of these concepts is highly complicated and beyond the scope of this manual. As a result, I will limit the discussion to just the fundamentals. A video camera captures visual images and slices (scans) them into about 495 horizontal lines. Each line varies in intensity along its length> In negative type transmissions, a stronger signal will be transmitted when the lines are dark and a weaker signal will be transmitted when the lines are light. After the video processing, the varying voltages are added to the output of a sync pulse generator. This will enable TV receivers to synchronize the saw-tooth oscillators with the saw-tooth scanning voltages applied at the camera. This is essential for the timing and true reproduction of the video at the receiver. This composite signal is fed to an AM modulator along with the carrier frequency. The resulting AM signal is then amplified and passed through a sideband filter. The carrier and sidebands are fed into a diplexer which prevents them from interfering with the FM audio transmitter. The microphone transforms sound into electrical signals. After processing, the audio is fed into a FM modulator, amplified, and fed into the same diplexer as the AM video signal. The diplexer keeps electrical isolation between the two transmitters. The maximum deviation of the FM audio signal is plus or minus 25 KHz. Remember that wideband FM is 75 KHz and narrowband FM is 4 KHz. When the transmitted signal is demodulated at the receiver, the signal appears as in the diagram.  This signal contains (1) varying DC components that represent changes in illumination for the picture lines (0-4.2 MHz video frequencies); (2) varying frequencies centered around 4.5 MHz (FM audio information); and (3) the horizontal and vertical sync pulses.
This chart shows the frequency allocation of each commercial TV channel. Each channel is 6 MHz wide, and the lowest frequency is shown. Only each tenth UHF station is given. In order to transmit color without increasing the 6 MHz bandwidth of the monochrome station, an ingenious method was evolved. The sidebands produced by monochrome modulation are found to cluster around the frequencies that are harmonics of the line frequency of 15.75 KHz. The spaces between these clusters were empty.  The line frequency was changed just slightly lower for color transmission. This allowed the color transmission to utilize the space between the harmonics. The adjustment was so slight that a TV receiver could receive both monochrome and color without any sync adjustment.  5.3.6 SubcarriersSubcarriers are modulated frequencies that are added together with other information and used to modulate the main carrier and is sometimes called multiplexing. Probably the most common example of subcarrier usage is in FM stereo transmission. The main carrier may be either AM or FM. Subcarriers may be either AM or FM, although standard AM is a bit more difficult. The demodulation will be the most difficult aspect as it may not be so easy to discover the presence of any subcarriers.5.3.7 Microwaves5.3.7.1 GeneralMost microwave activity takes place between 1 to 50 GHz. Uses of microwave include point-to-point communications, radar, motion detections, even cooking food. One of the major problems with the transmission of microwaves is the relative ineffectiveness of wire or coaxial cables to carry the AC signal. Microwave energy must be sent through a special metal pipe known as a waveguide.5.4 Receiver Operations (Reserved) | ||||||||||||||||||