Propagation: Antennas and radio waves

Once upon a time aircraft were only operated when the pilot had a clear view of the ground and visual reference was the only means of navigation. Adaptation of the magnetic compass and development of the gyroscope were pivotal components in promotion of maneuvering when outside vision was impaired. Commercial aviation only became a reality after the introduction of radio navigation. The information age is truly upon us and even our aircraft have a need to know what's going on. To facilitate air to ground and ground to air or even air to air voice and data transmission an array of antennas are strategically placed on the airframe. So what is it exactly that these sometimes oddly shaped devices do in the overall big scheme of radio transmission and reception?

All things on the earth, in the water, or even in the air are showered continually with waves of energy. Some of these waves stimulate our senses and can be seen, felt, or heard. For instance, we can see light, hear sound, and feel heat. Radio waves are propagated, which means "moved through a medium." This is most easily observed by light rays. When a light is turned on in a darkened room, light rays travel from the light bulb throughout the room. When a flashlight is turned on, light rays also radiate from its bulb, but are focused into a narrow beam. You can use these examples to picture how radio waves propagate. Like the light in the room, radio waves may spread out in all directions. They can also be focused (concentrated) like the flashlight, depending upon the need. Radio waves are a form of radiant energy, similar to light and heat. Although they can neither be seen nor felt, their presence can be detected through the use of sensitive measuring devices. The speed at which both forms of waves travel is the same; they both travel at the speed of light.

Historical innovators

The first antenna was devised by the German physicist Heinrich Hertz. During the late 1880s he carried out a landmark experiment to test the theory of the British mathematician-physicist James Clerk Maxwell. This would prove that visible light is only one example of a larger class of electromagnetic effects that could pass through air (or empty space) as a succession of waves. Hertz built a transmitter for such waves consisting of two flat, square metallic plates, each attached to a rod, with the rods in turn connected to metal spheres spaced close together. An induction coil connected to the spheres caused a spark to jump across the gap, producing oscillating currents in the rods. The reception of waves at a distant point was indicated by a spark jumping across a gap in a loop of wire.

The Italian physicist Guglielmo Marconi, considered the principal inventor of wireless telegraphy, constructed various antennas for both sending and receiving, and he also discovered the importance of tall antenna structures in transmitting low-frequency signals. With the early antennas built by Marconi and others, operating frequencies were generally determined by antenna size and shape. In later antennas frequency was regulated by an oscillator, which generated the transmitted signal.

Electromagnetic fields

An electromagnetic wave consists of two primary components: an electric field and a magnetic field. The electric field results from the force of voltage, and the magnetic field results from the flow of current. Electromagnetic fields that are radiated are commonly considered to be waves and electromagnetic radiation in space can be interpreted as horizontal and vertical lines of force oriented at right angles to each other. These lines of force are made up of an electric field (E) and a magnetic field (H), which when combined make up the electromagnetic field. The electric and magnetic fields radiated from an antenna form just such an electromagnetic field which is responsible for the transmission and reception of electromagnetic energy through free space.

An antenna is considered part of the electrical circuit of a transmitter or a receiver and has factors including inductance, capacitance, and resistance. Which means the antenna can be expected to display definite voltage and current relationships with respect to a given input. A current through the antenna produces a magnetic field, and a charge on the antenna produces an electric field. These two fields combine to form the inductive field.

The field that exists around every electrically charged object is a force field that can be detected and measured. This force field can cause electric charges to move in the field. When an object is charged electrically, there is either a greater or a smaller concentration of electrons than normal. This results in a difference of potential between a charged object and an uncharged object. An electric field is associated with a difference of potential, or a voltage. This invisible field of force is commonly represented by lines that are drawn to show the paths along which the force acts. The lines representing the electric field are drawn in the direction that a single positive charge would normally move under the influence of that field. A large electric force is shown by a large concentration of lines; a weak force is indicated by a few lines.

Radio waves

An energy wave generated by a transmitter is called a radio wave. The radio wave radiated into space by the transmitting antenna is a very complex form of energy containing both electric and magnetic fields. Because of this combination of fields, radio waves are also referred to as electromagnetic radiation.

The period of a radio wave is simply the amount of time required for the completion of one full cycle. If a sine wave has a frequency of 2 hertz, each cycle has a duration, or period, of one-half second. If the frequency is 10 hertz, the period of each cycle is one-tenth of a second. Since the frequency of a radio wave is the number of cycles that are completed in one second, you should be able to see that as the frequency of a radio wave increases, its period decreases. A wavelength is the space occupied by one full cycle of a radio wave at any given instant. Wavelengths are expressed in meters (1 meter is equal to 3.28 feet). You need to have a good understanding of frequency and wavelength to be able to select the proper antenna.

There are two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna. One way is by ground waves and the other is by sky waves. Ground waves are radio waves that travel near the surface of the Earth and sky waves are radio waves that are reflected back to Earth from the ionosphere.

Natural interference

Natural interference refers to the static that you often hear when listening to a radio and is interference generated by natural phenomena, such as thunderstorms, snowstorms, cosmic sources, and the sun. The energy they release is transmitted to the receiving site in roughly the same manner as radio waves. As a result, when conditions are favorable for the long-distance propagation of radio waves, they are likewise favorable for the transmission of natural interference. This hindrance is very erratic, particularly in the high frequency (HF) band, but generally will decrease as the operating frequency is increased and wider bandwidths are used. There is little natural interference above 30 megahertz.

One of the most notable phenomena on the surface of the sun is the appearance and disappearance of dark, irregularly shaped areas known as sunspots. Their exact nature is not known, but scientists believe these solar flares are caused by violent eruptions on the sun and are characterized by unusually strong magnetic fields. Sunspots are responsible for variations in the ionization level of the ionosphere and can occur unexpectedly with a variable life span. There is however, a regular cycle of sunspot activity that has been documented. This cycle has both a minimum and maximum level of sunspot activity that occurs approximately every 11 years. During periods of maximum sunspot activity, the ionization density of all layers increases. At these times, higher operating frequencies must be used for long-distance communications. The number of sunspots in existence at any one time is subject to change and as some disappear, new ones emerge. As the sun rotates on its own axis, these sunspots are visible at 27-day intervals, the approximate period required for the sun to make one complete rotation. The 27-day sunspot cycle causes variations in the ionization density on a day-to-day basis. Irregular variations in ionospheric conditions have an important effect on radio wave propagation. These variations are irregular and unpredictable so they can drastically affect communications capabilities without any warning.

The most startling of the ionosphere irregularities is known as a sudden ionospheric disturbance (SID). These disturbances may occur without warning and may prevail for any length of time, from a few minutes to several hours. When SID occurs, long distance propagation of HF radio waves is almost totally "blanked out." The immediate effect is that flight crews listening on normal frequencies are inclined to believe their receivers have gone dead.

For successful communications between any two specified locations at any given time of the day, there is a maximum frequency, a lowest frequency, and an optimum frequency that can be used.

Transmission lines

A transmission line is a device designed to guide electrical energy from one point to another. It is used to transfer the output electromagnetic energy of a transmitter to an antenna. This energy will not travel through normal electrical wires without great losses. Although the antenna could be connected directly to the transmitter, in most aircraft this is impractical so generally the antenna is usually remotely located.

The transmission line has a single purpose for both the transmitter and the antenna. This purpose is to transfer the energy output of the transmitter to the antenna with the least possible power loss. How well this is done depends on the special physical and electrical characteristics (impedance) of the transmission line.

A transmission line is electrically long when its physical length is long compared to a quarter-wavelength of the energy it is to carry. The terms "short" and "long" are relative ones; that is, a line that has a physical length of 3 meters (approximately 10 feet) is considered quite short electrically if it transmits a radio frequency of 30 kilohertz. On the other hand, the same transmission line is considered electrically long if it transmits a frequency of 30,000 megahertz.

A transmission line has the properties of inductance, capacitance, and resistance just as most conventional circuits. However, the constants in conventional circuits are more of a constant based on the type of component. A coil of wire is considered an inductor and when a certain amount of inductance is needed in a circuit, a coil of the proper value is inserted. The inductance of the circuit is based on the one component. Two metal plates separated by a small space, can be used to supply the required capacitance for a circuit as a capacitor consists of two conductors separated by an insulator. Similarly, a fixed resistor can be used to supply a certain value of circuit resistance. Transmission lines as well have constants consisting of inductance, capacitance, and resistance combined and the effects of this combination on an electrical energy transmission is referred to as impedance.

Antennas

The design of the antenna system is very important in a transmitting station. The antenna must be able to radiate efficiently so the power supplied by the transmitter is not wasted. An efficient transmitting antenna must have exact dimensions. The dimensions are determined by the transmitting frequencies. The dimensions of the receiving antenna are not critical for relatively low radio frequencies. However, as the frequency of the signal being received increases, the design and installation of the receiving antenna become more critical. An example of this would be the rabbit ears on a television. If you raise it a few more inches from the ground or give a slight turn in direction, you can change a snowy blur into a clear picture.

Most practical transmitting antennas are divided into two basic classifications, Hertz (half-wave) and Marconi (quarter-wave) antennas. Hertz antennas are generally positioned to radiate either vertically or horizontally. Marconi antennas operate with one end grounded and are mounted perpendicular to the surface acting as a ground. Hertz antennas are often used for frequencies above 2 megahertz. Marconi antennas are used for frequencies below 2 megahertz and may be used at higher frequencies in certain applications.

Aircraft present many challenges to antenna system designers. Even the location has to be matched to the capability. One recent event encountered on a helicopter revealed a situation where the communication radio worked well as long as the aircraft was heading away from the station, however going toward a transmitted signal the crew noticed the range was one-half to two-thirds of the opposite direction. As it turned out the Com antenna was located just behind the main rotor and the signal was effectively blocked with the blades operating. By moving the antenna to the bottom of the craft, the problem disappeared.

There are many concerns when it comes to proper antenna care, installation, and fault detection. Sometimes ideas that look good at the time may actually interfere with signal radiation patterns. Using some types of protective tape on antenna leading edge or not having a good bond to the surrounding airframe may result in a buildup of electrostatic charge causing a degraded signal. Adjusting rabbit ears on the fuselage of an aircraft at 40,000 feet is something that will not result in living happily ever after. AMT

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