ANTENNAS

Outline

• Introduction
• Types of antenna
• Radiation mechanism
• Fundamental parameters of antenna
• Antenna synthesis
• Wave propagation in antennas
• Antenna measurements

Introduction

An antenna is an array of conductors, electrically connected to the transmitter or receiver. In radio engineering, an antenna is the interface between radio waves propagating through space and electric currents moving in a metallic conductor which is used as a receiver or a transmitter. Radio waves are electromagnetic waves which carry signals through the air or space at the speed of light with minimum transmission loss. Antennas are normally designed to transmit and receive radio waves in either a particular direction or in all horizontal directions. An antenna my comprise of elements not connected to the transmitter which serve to direct the radio waves into a beam or any other desired radiation pattern, example of these elements includesparabolic reflectors, horns and parasitic elements. In other cases, the word antenna is occasionally interchanged by the term aerial which is used to specifically mean an elevated wire antenna[1].

In addition to receiving and transmitting energy, an antenna is also an advanced wireless system which is utilized to optimize or actuate the radiation energy in some specific directions and suppress it in others. Therefore, the antenna serves as a directional device in addition toa a probing device. Antenna can be classified as either omnidirectional or directional. Omnidirectional antennas are known to radiate energy in approximately equally in all directions while directional antennas radiate radio wave energy more along one direction than others. As a guiding device or a transmission line, an antenna may take the form of a coaxial line or a hollow pipe (wave guide) which are used to transport electromagnetic energy from the transmitting source to the antenna or from the antenna to the receiver[2].

Types of antennas

Antennas can be categorized in various types, for instance, according to their operating principles and specific applications. Types of antennas are discussed below:

Isotropic

An isotropic antenna is a hypothetical antenna which radiates equal signal power in all directions and has a mathematical model that is used as the base of comparison to calculate the directionality or gain of real antennas.Isotropic antennas are constructed using multiple small elements that are used as reference antenna for testing other antennas and for field strength measurements. They are also used for backup antennas on satellites which work without being oriented towards a communication station.

Wire antennas

There are various shapes of wire antennas, for example, straight wire (dipole), loop and helix. They normally consist of a long wire suspended above the ground, but their length does bear a relation to the wavelength of the radio wave used, thus making them no as effective as antennas whose lengths are adjusted to resonate at the wavelength used. Wire antennas are widely utilized as receiving antennas on long, medium and short-wave bands in automobiles, buildings, ships, aircrafts, FM radio receivers and many more.

Dipole

The dipole is the prototype antenna on which a large category of antennasis based. A typical dipole antenna comprises of two conductors normally metal rods or wires which are arranged symmetrically, where one side of the feedline is balanced from the transmitter and receiver. The most common type of dipole antenna is the half wave dipole which consists of two resonant elements just under a quarter wavelength long. Examples of dipole antennas include:

• Microstrip antennas- These antennas consist of a metallic patch on a grounded substrate. The microstrip antennas are low profile, conformable to planar and nonplanar surfaces, simple and inexpensive to fabricate using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs, and very versatile in terms of resonant frequency, polarization, pattern, and impedance. These antennas are easily mounted on the surface of high-performance aircraft, spacecraft, satellites, missiles, cars, and even handheld mobile telephones[3].
• Reflector antennas take many geometrical configurations, some of the most popular shapes are the plane, corner, and curved reflectors (especially the paraboloid), The success in the exploration of outer space has resulted in the advancement of antenna theory. Because of the need to communicate over great distances, sophisticated forms of antennas had to be used in order to transmit and receive signals that had to travel millions of miles. A very common antenna form for such an application is a parabolic reflector. A directive antenna with moderate gain of about 8 dB often used at UHF frequencies. Consists of a dipole mounted in front of two reflective metal screens joined at an angle, usually 90°. Used as a rooftop UHF television antenna and for point-to-point data links.

Monopole

A monopole antenna consists of a single conductor which protrudes as a metal rod and it is normally mounted over the ground or an artificial conducting surface. The antenna is designed in such a way that one side of the feedline from the receiver or transmitter is linked to the conductor while the other side is connected to the ground or the artificial ground plane. During operation, the radio waves reflected from the ground place seem to come from a virtual antenna below the ground. Thus, monopole antenna has a radiation pattern similar to the top half of a dipole antenna. This type of antenna hasvertical polarization and an omnidirectional radiation pattern making them suitable to be used for broad coverage of an area. In addition,ground waves utilized for broadcasting at low frequencies ought to be vertically polarized. However, large vertical monopole antennas are used for broadcasting in the medium, low and very low frequencies bands, while on the other hand, small monopoles are utilized as non-directional antennason portable radios in the high, very high and ultra-high frequency bands.

Array antenna

Array antenna comprise of several simple antennas working in coordination to form a single compound antenna. Broadside arrays consist of multiple identical driven elements, usually dipoles, fed in phase, radiating a beam perpendicular to the antenna plane. Many applications require radiation characteristics that may not be achievable by a single element. It may, however, be possible that an aggregate of radiating elements in an electrical and geometrical arrangement (an array) will result in the desired radiation characteristics. The arrangement of the array may be such that the radiation from the elements adds up to give a radiation maximum in a particular direction or directions, minimum in others, or otherwise as desired[3].

Loop antenna

Loop antenna is made up of a loop (or coil) of wire. Loop antennas interact directly with the magnetic field of the radio wave, rather than its electric field, making them relatively insensitive to electrical noise within about a quarter-wavelength of the antenna. There are basically two broad categories of loop antennas: large loops (full-wave loops) and small loops. Full loops have the highest radiation resistance, and hence the highest efficiency of all antennas: Their radiation resistances are several hundreds of Ohms, whereas dipoles and monopoles are tens of Ohms, and small loops are a few ohms, or even fractions of an Ohm. On the other hand small loop antennas have very small , typically much smaller than the loss resistance, making them very inefficient for transmitting. However, small loops are very effective receiving antennas, especially at low frequencies, where all feasible antennas are quite small compared to a wavelength.

Lens antenna

Lenses are primarily used to collimate incident divergent energy to prevent it from spreading in undesired directions. By properly shaping the geometrical configuration and choosing the appropriate material of the lenses, they can transform various forms of divergent energy into plane waves. They can be used in most of the same applications as are the parabolic reflectors, especially at higher frequencies. Their dimensions and weight become exceedingly large at lower frequencies. Lens antennas are classified according to the material from which they are constructed, or according to their geometrical shape.

Radiation mechanism

The functioning of an antenna greatly depends upon the radiation mechanism of the transmission media.In understanding the radiation mechanism of an antenna, it is important to familiarize with the working principle of antennas which is to convert electrical currents into electromagnetic radiation in free space and vice versa. Therefore, in this section we are going to analyze how electromagnetic fields are generated by various sources, contained and guided within the transmission line and antenna, and lastly detached from the antenna to form a free-space wave.

To begin with, it is crucial to examine some basic source of radiation, which are:

Single wire

Conducting wires are material whose prominent characteristic is the motion of electric charges and the creation of current flow. To create radiation, there must be a time-varying current or an acceleration or deceleration of charge. To create charge acceleration or deceleration the wire must be curved, bent, discontinuous, or terminated. Periodic charge acceleration or deceleration or time-varying current is also created when charge is oscillating in a time-harmonic motion.

A qualitative understanding of the radiation mechanism may be obtained by considering a pulse source attached to an open-ended conducting wire, which may be connected to the ground through a discrete load at its open end. When the wire is initially energized, the charges (free electrons) in the wire are set in motion by the electrical lines of force created by the source. When charges are accelerated in the source-end of the wire and decelerated (negative acceleration with respect to original motion) during reflection from its end, it is suggested that radiated fields are produced at each end and along the remaining part of the wire. Stronger radiation with a broader frequency spectrum occurs if the pulses are of shorter or more compact duration while continuous time-harmonic oscillating charge produces, ideally, radiation of single frequency determined by the frequency of oscillation. The acceleration of the charges is accomplished by the external source in which forces set the charges in motion and produce the associated field radiated. The deceleration of the charges at the end of the wire is accomplished by the internal forces associated with the induced field due to the buildup of charge concentration at the ends of the wire. The internal forces receive energy from the charge buildup as its velocity is reduced to zero at the ends of the wire. Therefore, charge acceleration due to an exciting electric field and deceleration due to impedance discontinuities or smooth curves of the wire are mechanisms responsible for electromagnetic radiation. While both current density and charge density appear as source terms in Maxwell’s equation, charge is viewed as a more fundamental quantity, especially for transient fields.

Two-wires

Applying a voltage across the two-conductor transmission line creates an electric field between the conductors. The electric field has associated with its electric lines of force which are tangent to the electric field at each point and their strength is proportional to the electric field intensity. The electric lines of force have a tendency to act on the free electrons (easily detachable from the atoms) associated with each conductor and force them to be displaced. The movement of the charges creates a current that in turn creates magnetic field intensity. Associated with the magnetic field intensity are magnetic lines of force which are tangent to the magnetic field. We have accepted that electric field lines start on positive charges and end on negative charges. They also can start on a positive charge and end at infinity, start at infinity and end on a negative charge, or form closed loops neither starting nor ending on any charge. Magnetic field lines always form closed loops encircling current-carrying conductors because physically there are no magnetic charges. In some mathematical formulations, it is often convenient to introduce equivalent magnetic charges and magnetic currents to draw a parallel between solutions involving electric and magnetic sources.

Radiation pattern

The radiation pattern is usually determined in the field region and it is represented as a function of the directional coordinates.

Figure 1: Example of an antenna radiation pattern

Radian and steradian

The measure of a plane angle is a radian. One radianis defined as the plane angle with its vertex at the center of a circle of radius r that is subtended by an arc whose length is r. Since the circumference of a circle of radius r is C = 2πr, there are 2π rad (2πr/r) in a full circle.

The measure of a solid angle is a steradian. One steradian is defined as the solid angle with its vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r. Since the area of a sphere of radius r is A = 4πr2, there are 4π sr(4πr2/r2) in a closed sphere.

Radiation power density

Electromagnetic waves are used to transport information through a wireless medium or a guiding structure, from one point to the other. It is then natural to assume that power and energy are associated with electromagnetic fields. The quantity used to describe the power associated with an electromagnetic wave is the instantaneous Poynting vector calculated as:

Where W=instantaneous pointing vector, ℰ=instantaneous electric field and H=instantaneous magnetic field intensity.

Since the Poynting vector is a power density, the total power crossing a closed surface can be obtained by integrating the normal component of the Poynting vector over the entire surface:

The average power radiated by an antenna (radiated power) can therefore be calculated as:

Radiation intensity

Radiation intensity in a given direction is defined as “the power radiated from an antenna per unit solid angle.” The radiation intensity is a far-field parameter, and it can be obtained by simply multiplying the radiation density by the square of the distance. In mathematical form it is expressed as:

Where; U=radiation intensity and W_rad=radiation density.

The total power radiated is calculated by integrating the radiation intensity

Beam-width

The beam-widthof a radiation pattern is defined as the angular separation between two identical points on opposite side of the pattern maximum. In an antenna pattern, there are a number of beam-widths. The beam-width of an antenna is a very important figure of merit and often is used as a trade-off between it and the side lobe level; that is, as the beam-width decreases, the side lobe increases and vice versa[5].

Directivity

The directivity of an antennais defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4π. If the direction is not specified, the direction of maximum radiation intensity is implied.Stated more simply, the directivity of a non-isotropic source is equal to the ratio of its radiation intensity in a given direction over that of an isotropic source[5].

If the direction is not specified, it implies the direction of maximum radiation intensity (maximum directivity) expressed as:

Antenna efficiency

The total antenna efficiency e0 is used to take into account losses at the input terminals and within the structure of the antenna. Such losses may be due to:

1. Reflections because of the mismatch between the transmission line and the antenna
2. I₂Rlosses (conduction and dielectric)

Where

Ҽo= total efficiency

Ҽr= reflection efficiency (mismatch) = (1-|Ӷ|²)

Ҽc= conduction efficiency

Ҽd= dielectric efficiency

Ӷ= voltage reflection coefficient at the input of the antenna

Gain

Gain of an antenna (in a given direction) is defined as the ratio of the intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The radiation intensity corresponding to the isotropic radiated power is equal to the power accepted (input) by the antenna divided by 4π. In equation form this can be expressed as:

Bandwidth

The bandwidth of an antenna can be defined as the range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard. The bandwidth can be considered to be the range of frequencies, on either side of a center frequency (usually the resonance frequency for a dipole), where the antenna characteristics (such as input impedance, pattern, beam-width, polarization, side lobe level, gain, beam direction, radiation efficiency) are within an acceptable value of those at the center frequency. For broadband antennas, the bandwidth is usually expressed as the ratio of the upper-to-lower frequencies of acceptable operation. For narrowband antennas, the bandwidth is expressed as a percentage of the frequency difference (upper minus lower) over the center frequency of the bandwidth.

The limiting factor for this antenna is its impedance, and its bandwidth can be formulated in terms of the Q. The Qof antennas or arrays with dimensions large compared to the wavelength, excluding super-directive designs, is near unity. Therefore, the bandwidth is usually formulated in terms of beam-width, side lobe level, and pattern characteristics. For intermediate length antennas, the bandwidth may be limited by either pattern or impedance variations, depending upon the particular application.

Figure 2: A graph showing the bandwidth of an antenna

Polarization

Polarization of a radiated wave is defined as that property of an electromagnetic wave describing the time-varying direction and relative magnitude of the electric-field vector; specifically, the figure traced as a function of time by the extremity of the vector at a fixed location in space, and the sense in which it is traced, as observed along the direction of propagation. Polarization is the curve traced by the end point of the arrow (vector) representing the instantaneous electric field. The field must be observed along the direction of propagation. The polarization of a wave can be defined in terms of a wave radiated (transmitted) or receivedby an antenna in a given direction. The polarization of a wave radiated by an antenna in a specified direction at a point in the far field is defined as the polarization of the (locally) plane wave which is used to represent the radiated wave at that point. At any point in the far field of an antenna the radiated wave can be represented by a plane wave whose electric-field strength is the same as that of the wave and whose direction of propagation is in the radial direction from the antenna. As the radial distance approaches infinity, the radius of curvature of the radiated wave’s phase front also approaches infinity and thus in any specified direction the wave appears locally as a plane wave. The polarization of a wave receivedby an antenna can be defined as the polarization of a plane wave, incident from a given direction and having a given power flux density, which results in maximum available power at the antenna terminals[6].

Polarization may be classified as linear, circular, or elliptical. If the vector that describes the electric field at a point in space as a function of time is always directed along a line, the field is said to be linearly polarized. In general, however, the figure that the electric field traces an ellipse, and the field is said to be elliptically polarized. Linear and circular polarizations are special cases of elliptical, and they can be obtained when the ellipse becomes a straight line or a circle, respectively. The figure of the electric field is traced in a clockwise or counterclockwisesense. Clockwise rotation of the electric-field vector is also designated as right-hand polarizationand counterclockwiseas left-hand polarization.

Figure 3: Linear and circular polarization

Antenna synthesis

In antenna synthesis, an antenna model is chosen and analyzed for its radiation characteristics which include; pattern, directivity, beam-width, efficiency, bandwidth and polarization. This is usually accomplished by initially specifying the current distribution of the antenna, and then analyzing it using standard procedures. If the antenna current is not known, it can usually be determined from integral equation formulations. Antenna pattern synthesis usually requires that first an approximate analytical model is chosen to represent, either exactly or approximately, the desired pattern. The second step is to match the analytical model to a physical antenna model. The synthesis methods will be utilized to design line-sources and linear arrays whose space factors and array factors will yield desired far-field radiation patterns. Discussed below are methods used in synthesis of antennas:

Schelkunoff polynomial method

A method that is conducive to the synthesis of arrays whose patterns possess nulls in desired directions are thus introduced by Schelkunoff. To complete the design, this method requires information on the number of nulls and their locations. The number of elements and their excitation coefficients are then derived[7]. The analytical formulation of the technique follows:

AF=∑a_ne^{j(n-1) (kd cos θ + β)} = ∑a_ne^{j(n-1) ψ}

AF=∑a_nz^n-1 = a₁ + a₂z + …+ a_nz^n-1

AF= a_n(z-z₁) (z-z₂) (z-z₃) … (z-z_n-1)

Fourier transform method

This method can be used to determine, given a complete description of the desired pattern, the excitation distribution of a continuous or a discrete source antenna system. The derived excitation will yield, either exactly or approximately, the desired antenna pattern. The pattern synthesis using this method is referred to as beam shaping. For a continuous line source distribution of antenna length, the normalized space factor can be written as:

SF(θ) = ∫I(z’) e^{j(k cos θ – k)z}dz’

SF(θ) =

Woodward-Lawson method

The synthesis is accomplished by sampling the desired pattern at various discrete locations. Associated with each pattern sample is a harmonic current of uniform amplitude distribution and uniform progressive phase, whose corresponding field is referred to as a composing function. The excitation coefficientof each harmonic current is such that its field strength is equal to the amplitude of the desired pattern at its corresponding sampled point. The total excitation of the source is comprised of a finite summation of space harmonics. The corresponding synthesized pattern is represented by a finite summation of composing functions with each term representing the field of a current harmonic with uniform amplitude distribution and uniform progressive phase[8]. The analytical formulation of this method is similar to the Shannon sampling theorem used in communications which states that if a function g(t) is band-limited, with its highest frequency being f_h the function g(t) can be reconstructed using samples taken at a frequency f_s.

SF(θ) =

Wave propagation in antennas

Radio waves are quite simple to generate and transmit mainly because of their ability to pass through objects, for instance, buildings and also to travel long distances. In radio communication systems, electromagnetic waves are used as channels while the antennas of different specifications are utilized for propagation purposes. Electromagnetic wave propagation in the atmosphere can be classified into three categories; line of sight, ground wave and sky wave propagation[9].

Line of sight propagation (LOS)

In the line of sight communication, the radio wave travels a minimum distance, for example, a distance of sight.

Figure 4: Line of sight propagation

This type of propagation is normally not smooth in cases where there are obstacles in the transmission path. Line of sight propagation is mainly used in transmitting signal over short distances, for instance, infrared and microwave signals.

Ground wave propagation

It is a method of radio wave propagation that uses the earth’s contour for transmission. Ground wave can propagate over a considerable distance on the earth’s surface especially for the low and medium frequencies e.g. frequencies up to 2Mhz. When the direct waves reach the receiver, the lags are cancelled out, the signal is filtered to avoid distortion and amplifies for clear output.

Figure 5: Ground wave propagation

Sky wave propagation

This is type of propagation is preferred when the wave has to travel a longer distance. In sky wave propagation, the radio waves are reflected from the ionized layer of atmosphere back down to earth. Application of sky wave propagation include; amateur radio, CB radio and international broadcasts.

Figure 6: Sky wave propagation

Antenna measurements

Antenna measurements are usually performed either when the antenna is in transmitting or receiving mode. However, it is usually important to perform antenna measurements with the test antenna in its receiving mode. If the test antenna is reciprocal, the receiving mode characteristics such gain, radiation pattern, and many more are identical to those transmitted by the antenna. The most ideal condition for measuring far-field radiation characteristics is the illumination of the test antenna by plane waves, that is, uniform amplitude and phase. Although this ideal condition is not achievable, it can be approximated by separating the test antenna from the illumination source by a large distance on an outdoor range. At large radii, the curvature of the spherical phase front produced by the source antenna is small over the test antenna aperture.

Instrumentation

There is the need to design antenna range instrument to perform the following tasks; transmitting, receiving, positioning, recording and data processing. A technique that is utilized to carry out antenna measurements which are associated with large structures is the geometrical scale modelling. This method is essential for physically accommodating small ranges, providing experimental control over the measurements and minimizing costs associated with large structures. The testing and evaluation of antennas are performed in antenna ranges. Antenna facilities are categorized as outdoorand indoorranges. Outdoor ranges are not protected from environmental conditions while indoor facilities are limited by space restrictions. Other ranges involve in antenna measurements include; reflection, free-space, and compact ranges.

There are two basic system methods utilized in measuring the phase pattern of antennas i.e. short and long distances. In addition, a references signal is coupled from the transmission line and it is used to compare the phase of the received signal. The gain-transfer method is the most common technique used to measure the gain of an antenna. The radiation efficiency can be acquired by the formula below:

Another important antenna measurement is the current distribution along an antenna, and it is normally measured using sampling probes. Polarization of a wave of an antenna is best measured and displayed on the visualized surface of Poincare sphere.

References