Antennas and Wave Propagation The interface between the transmitter and free space and between free space and the receiver is the antenna. At the transmitting end the antenna converts the transmitter RF power into electromagnetic signals; at the receiving end the antenna picks up the electromagnetic signals and converts them into signals for the receiver.
Topics Covered in Chapter 14 Antenna Fundamentals Common Antenna Types Radio-Wave Propagation
Antenna Fundamentals A radio signal is called an electromagnetic wave because it is made up of both electric and magnetic fields. Whenever voltage is applied to the antenna, an electric field is set up. This voltage causes current to flow in the antenna, producing a magnetic field. These fields are emitted from the antenna and propagate through space at the speed of light.
Magnetic Fields A magnetic field is an invisible force field created by a magnet. An antenna is a type of electromagnet. A magnetic field is generated around a conductor when current flows through it. The SI unit for magnetic field strength is ampere- turns per meter.
Electric Field An electric field is an invisible force field produced by the presence of a potential difference between two conductors. An electric field is produced between the plates of a charged capacitor. An electric field exists between any two points across which a potential difference exists. The SI unit for electric field strength is volts per meter.
Magnetic and Electric Fields in a Transmission Line A transmission line is made up of a conductor or conductors. Transmission lines do not radiate signals efficiently. The closeness of the conductors keeps the electric field primarily concentrated in the transmission-line dielectric. The magnetic fields mostly cancel one another. The electric and magnetic fields do extend outward from the transmission line, but the small amount of radiation that does occur is extremely inefficient.
The Nature of an Antenna If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end of the line and radiate into space. This radiation is inefficient and unsuitable for reliable transmission or reception. The radiation from a transmission line can be greatly improved by bending the transmission-line conductors so they are at a right angle to the transmission line.
The Nature of an Antenna (Continued) The magnetic fields no longer cancel, and in fact, aid one another. The electric field spreads out from conductor to conductor. Optimum radiation occurs if the segment of transmission wire converted into an antenna is a quarter wavelength long at the operating frequency. This makes an antenna that is one-half wavelength long.
By Definition… The ratio of the electric field strength of a radiated wave to the magnetic field strength is a constant and is called the impedance of space, or the wave impedance. The electric and magnetic fields produced by the antenna are at right angles to one another, and are both perpendicular to the direction of propagation of the wave. Polarization refers to the orientation of magnetic and electric fields with respect to the earth.
Antenna Reciprocity Antenna reciprocity means that the characteristics and performance of an antenna are the same whether the antenna is radiating or intercepting an electromagnetic signal. A transmitting antenna takes a voltage from the transmitter and converts it into an electromagnetic signal. A receiving antenna has a voltage induced into it by the electromagnetic signal that passes across it.
Basic Antenna An antenna can be a length of wire, a metal rod, or a piece of tubing. Antennas radiate most effectively when their length is directly related to the wavelength of the transmitted signal. Most antennas have a length that is some fraction of a wavelength. One-half and one-quarter wavelengths are most common.
Dipole Antenna One of the most widely used antenna types is the half-wave dipole. The half-wave dipole, also called a doublet, is formally known as the Hertz antenna. A dipole antenna is two pieces of wire, rod, or tubing that are one-quarter wavelength long at the operating resonant frequency. Wire dipoles are supported with glass, ceramic, or plastic insulators at the ends and middle.
By Definition… The dipole has an impedance of 73 Ω at its center, which is the radiation resistance. An antenna is a frequency-sensitive device. End effect is a phenomenon caused by any support insulators used at the ends of the wire antenna and has the effect of adding capacitance to the end of each wire. If a dipole is used at a frequency different from its design frequency, the SWR rises and power is lost.
Conical Antenna A common way to increase bandwidth is to use a version of the dipole antenna known as the conical antenna. The center radiation resistance of a conical antenna is much higher than the 73 Ω usually found when straight-wire or tubing conductors are used. The primary advantage of conical antennas is their tremendous bandwidth. They can maintain a constant impedance and gain over a 4:1 frequency range.
Dipole Polarization Most half-wave dipole antennas are mounted horizontally to the earth. This makes the electric field horizontal to the earth and the antenna is horizontally polarized. Horizontal mounting is preferred at the lower frequencies because the physical construction, mounting, and support are easier. This mounting makes it easier to attach the transmission line and route it to the transmitter or receiver.
Radiation Pattern and Directivity The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna. Most antennas have directional characteristics that cause them to radiate or receive energy in a specific direction. Typically that radiation is concentrated in a pattern that has a recognizable geometric shape.
Three-Dimensional Pattern of a Half-Wave Dipole
By Definition… The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna. The measure of an antenna’s directivity is beamwidth, the angle of the radiation pattern over which a transmitter’s energy is directed or received. The power radiated by an antenna with directivity and therefore gain is called the effective radiated power (ERP).
Folded Dipole A popular variation of the half-wave dipole is the folded dipole. The folded dipole is also one-half wavelength long. It consists of two parallel conductors connected at the ends with one side open at the center for connection to the transmission line. The impedance of this antenna is 300 Ω The folded dipole is an effective, low-cost antenna that can be used for transmitting and receiving.
Marconi or Ground-Plane Vertical Antenna The quarter-wavelength vertical antenna, also called a Marconi antenna is widely used. It is similar in operation to a vertically mounted dipole antenna. The Marconi antenna offers major advantages because it is half the length of a dipole antenna.
Radiation Pattern Vertical polarization and omnidirectional characteristics can be achieved using a quarter- wavelength vertical radiator. It is usually fed with coaxial cable; the center conductor is connected to the vertical radiator and the shield is connected to earth ground. With this arrangement, the earth acts as a type of electrical “mirror,” effectively providing the other quarter wavelength making it equivalent to a vertical dipole.
Directivity Directivity refers to the ability of an antenna to send or receive signals over a narrow horizontal directional range. The physical orientation of the antenna gives it a highly directional response or directivity curve. A directional antenna eliminates interference from other signals being received from all directions other than the desired signal. A highly directional antenna acts as a type of filter to provide selectivity. Directional antennas provide greater efficiency of power transmission.
Parasitic Array A parasitic array consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line. These extra conductors are referred to as parasitic elements and the antenna is called a driven element. A Yagi antenna is made up of a driven element and one or more parasitic elements.
Driven Arrays A driven array is an antenna that has two or more driven elements. Each element receives RF energy from the transmission line, and different arrangements of the elements produce different degrees of directivity and gain. The three basic types of driven arrays are the collinear, the broadside, and the end-fire.
Collinear Antenna Collinear antennas usually consist of two or more half-wave dipoles mounted end to end. Collinear antennas use half-wave sections separated by shorted quarter-wave matching stubs which ensure that the signals radiated by each half-wave section are in phase. Collinear antennas are generally used only on VHF and UHF bands because their length becomes prohibited at the lower frequencies.
Radiation Pattern of a Four- Element Collinear Antenna
Broadside Antenna A broadside array is, essentially, a stacked collinear antenna consisting of half-wave dipoles spaced from one another by one-half wavelengths. This antenna produces a highly directional radiation pattern which is broadside or perpendicular to the plane of the array. The broasdside antenna is bidirectional, but the radiation pattern has a very narrow beamwidth and high gain.
End-Fire Antenna The end-fire array uses two half-wave dipoles spaced one-half wavelength apart. The end-fire array has a bidirectional radiation pattern, but with a narrower bandwidth and lower gain. The radiation is in the plane of the driven elements. By careful selection of the optimal number of elements with the appropriately related spacing, a highly unidirectional antenna is created.
Impedance Matching One of the most critical aspects of any antenna system is ensuring maximum power transfer from the transmitter to the antenna. When the characteristic impedance of the transmission line matches the output impedance of the transmitter and the impedance of the antenna, the SWR will be 1:1. When SWR is 1:1 maximum power transfer will take place.
By Definition… A Q section, or matching stub, is a quarter wavelength of coaxial or balanced transmission line of a specific impedance that is connected between a load and source and is used to match impedances. A balun is a transformer used to match impedances. An antenna tuner is a variable inductor, one or more variable capacitors, or a combination of these components connected in various configurations.
Radio-Wave Propagation Once a radio signal has been radiated by an antenna, it travels or propagates through space and ultimately reaches the receiving antenna. The energy level of the signal decreases rapidly with distance from the transmitting antenna. The electromagnetic wave is affected by objects that it encounters along the way such as trees, buildings, and other large structures.
Optical Characteristics of Radio Waves Radio waves act much like light waves. Light waves can be reflected, refracted, diffracted, and focused by other objects. The focusing of waves by antennas to make them more concentrated in a desired direction is comparable to a lens focusing light waves into a narrower beam.
Reflection Any conducting surface looks like a mirror to a radio wave, and so radio waves are reflected by any conducting surface they encounter. Radio-wave reflection follows the principles of light- wave reflection which states the angle of reflection is equal to the angle of incidence. The direction of the electric field approaching the reflecting surface is reversed from that leaving the surface. This is equivalent to a 180 degree phase shift.
Refraction Refraction is the bending of a wave due to the physical makeup of the medium through which the wave passes. Index of refraction is obtained by dividing the speed of a light (or radio) wave in a vacuum and the speed of a light (or radio) wave in the medium that causes the wave to be bent. The relationship between the angles and the indices of refraction are given by a formula known as Snell’s law.
Diffraction Diffraction is the bending of waves around an object. Diffraction is explained by what is known as Huygen’s principle. Huygens’ principle is based on the assumption that all electromagnetic waves, light as well as radio waves, radiate as spherical waveforms from a source.
Radio-Wave Propagation Through Space The three basic paths that a radio signal can take through space are: Ground wave Sky wave Space wave
Ground Waves Ground or surface waves leave an antenna and remain close to the earth. Ground waves actually follow the curvature of the earth and can travel at distances beyond the horizon. Ground waves must have vertical polarization to be propagated from an antenna. Ground-wave propagation is strongest at the low- and medium-frequency ranges. AM broadcast signals are propagated primarily by ground waves during the day and sky waves at night.
Sky Waves Sky-wave signals are radiated by the antenna into the upper atmosphere, where they are bent back to earth. When a radio signal goes into the ionosphere, the different levels of ionization cause the radio waves to be gradually bent. The smaller the angle with respect to the earth, the more likely it is that the waves will be refracted and sent back to earth. The higher the frequency, the smaller the radiation angle required for refraction to occur.
Space Waves A direct, or space wave travels in a straight line directly from the transmitting antenna to the receiving antenna. Direct-wave radio signaling is often referred to as lin- of-sight communication. Direct or space waves are not refracted, nor do they follow the curvature of the earth. Line-of-sight communication is characteristic of most radio signals with a frequency above 30 MHz, particularly VHF, UHF, and microwave signals.
Fading Fading is the variation in signal amplitude at the receiver caused by the characteristics of the signal path and changes in it. Fading causes the received signal to vary in amplitude, typically making it smaller. Fading is caused by four factors: variation in distance between transmitter and receiver, changes in the environmental characteristics of the signal path, the presence of multiple signal paths, and relative motion between the transmitter and receiver.
Diversity A diversity system uses multiple transmitters, receivers, or antennas to mitigate the problems caused by multipath signals. With frequency diversity, two separate sets of transmitters and receivers operating on different frequencies are used to transmit the same information simultaneously. Space or spatial diversity uses two receive antennas spaced as far apart as possible to receive the signals.