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IJEE Volume 5 • Number 1 • January-June 2013 pp. 21-25 Serials Publications, ISSN : 0973-7383

Various Antennas and Its Applications in Domain: A Review Paper

P.A. Ambresh1, P.M. Hadalgi2 and P.V. Hunagund3 1,2,3Microwave Electronics Research Laboratory, Department of P.G. Studies & Research in Applied Electronics, Gulbarga University, Gulbarga-585106.Karnataka, (INDIA). 1E-mail: [email protected]

Abstract: This paper presents some details on theory and discusses various antenna types and applications in wireless domain. The fundamental equations and principles of antenna theory are also discussed. Five different types of antennas are presented: dipoles and monopoles, loop antennas, patch antennas, helical antennas and horn antennas. The distinctiveness of each type of antenna is studied. This paper has a selection of antenna applications from the extremely low-frequency system to wireless applications. Keywords: Wireless domain, antenna, electromagnetism, gigahertz.

1. INTRODUCTION resistance, the more energy is radiated or received by the Antennas are a very important component of communication antenna. When the of the antenna systems. The definition of an antenna according to IEEE matches the resistance of the transmitter or receiver, the standard 145-1983 is that it is a means for radiating and system is optimized. Antennas also have ohmic (or) loss receiving the waves (or) in a broad way it is also defined resistance which decreases efficiency. It can be shown that as a transducer which converts voltage and current on a an efficient antenna must be comparable to a in transmission line into electromagnetic field in space. A size. receiving antenna changes electromagnetic energy into Prad electric or magnetic energy. A transmitting antenna changes Rrad = 2 (1) the energy from electric or magnetic into electromagnetic I energy. Current flowing in the antenna induces the electric The second parameter of antenna is the and magnetic fields. Antennas demonstrate a property of the antenna. The radiation or antenna pattern describes known as reciprocity, which means that an antenna will the relative strength of the radiated field in various directions maintain the same characteristics regardless if it is transmitting from the antenna, at a constant distance. In practical, the or receiving. Most antennas are resonant devices, which planar sections of the radiation pattern are shown instead of operate efficiently over a relatively narrow frequency band. the complete three-dimensional surface. The most important Antennas have been used for over a century in a variety of analysis are those of the principal E-plane and H-plane applications. They can transmit over a massive range of patterns. The E-plane pattern contains the plane in which frequencies, from a fraction of a kilohertz to over one hundred the electric field lies. Similarly, the H-plane pattern is a gigahertz. This paper will give a brief introduction to antenna sectional view in which the lies as shown in principles and then discusses various antenna types and Fig. 1, which gives the antenna pattern for a half-wave dipole their applications. orientated in the z-direction.

2. ANTENNA PRINCIPLE

2.1. Antenna Parameters In general, there are five basic parameters that one must understand to determine how an antenna will operate and perform. The first parameter is radiation resistance. The radiation resistance (Rrad) of an antenna relates the power supplied (Prad) to the antenna and the current (I) flowing into the antenna. The equation for radiation resistance is given in (1) [1]. As can be seen, the greater the radiation Figure 1: (a) Dipole Principal E-Plane & H-Plane Pattern 22

Most antennas do not radiate uniformly. This implies At radial distances 10 or more away from that there is some , which is the third important the oscillating current element, these equations can be parameter. Closely related to this is the gain of the antenna. simplified. All terms except the 1/r term may be neglected, Directivity is the ability of an antenna to focus energy in a and the radiation fields are given in equations 5, 6 and 7 [5-7]. particular direction when transmitting, or to receive energy Assuming a vertical orientation of the current element, better from a particular direction when receiving. The gain of plotting E against � for a constant r will result in a graph an antenna in a given direction is the amount of energy �s similar to Fig. 1, called the vertical pattern. Plotting H�s shows radiated in that direction compared to the energy an isotropic the variation of field intensity with �, often referred to as the antenna would radiate in the same direction when driven horizontal pattern, such as in Fig. 1. with the same input power. Usually given in decibels, it is the ratio of power radiated to input power. Thus, using an I d H = j0 sinθ e�j2 ��r � (5) antenna with a higher gain would require less input power. �s 2λr Bandwidth is another important antenna parameter. The I dη bandwidth of an antenna refers to the range of frequencies 0 �j2 �� r � E�s = jsinθ e (6) over which the antenna can operate correctly. For example, a 2λr

10 MHz transmitter with a 10% bandwidth could send Ers = 0 (7) information on frequencies from 9 MHz to 11 MHz. This is important to Frequency Modulated (FM) signals, as they require modulation about the carrier frequency in order to 3. ANTENNA TYPES send data. The final antenna parameter is the signal-to-noise ratio. 3.1. This is the relationship between the desired information The world’s most popular antenna is the half-wave dipole. signal and the noise. If this ratio does not exceed unity, As shown in Fig. 2, the total length of the antenna is equal to information will not be transferred. Noise can be caused by half of the wavelength. The relationship between wavelength obstructions, large distances between antennas, and and frequency is f = c/�, where c = 3 × 108 m/s in free space. environmental RF noise, such as power supplies and digital Dipoles may be shorter or longer than half of the wavelength, switching devices. but this fraction provides the best . The radiation resistance can be calculated as 73.1�. 2.2. Electromagnetic Energy Calculations An antenna’s electric and magnetic fields can be calculated at any point, but the equations are not simple. A short current filament generates the fields shown in equations 2, 3 and 4 [2-4] given in spherical coordinates. Because of the complexity of these equations, most problems involving antennas are solved by experimental rather than theoretical methods.

I0 d �j2 �� r � � 2� 1 � H�s = sin θe� j � 2 � (2) 4� � λr r �

I0 dη �j2 �� r � � � 1 � Ers = cosθe � 3� 2 � (3) 2�� j 2 � � r r �

Figure 2: Dipole Antenna, 1/2 Wave I0 dη�j2 �� r � � 2� 1 � � E�s = sinθe� j �2 � 3 � (4) The dipole antenna is fed by a two-wire line, where the 4� � λr r j2�� r � two currents in the conductors are equal in amplitude but Despite the complexity of these equations, some opposite in direction. Since the antenna ends are essentially observations can be made. The exponential factor in each an open circuit, the current distribution along the length of equation is the same. This indicates wave propagation the half-wave dipole is sinusoidal, shown in Fig. 3. This outward from the origin in the positive r direction. The produces the antenna pattern shown in Fig. 1. This pattern wavelength is equal to �, and d is the length of the current shows that when the antenna is vertical, it radiates the most filament. Also as expected, the strength of the fields is related in the horizontal direction and very little out the ends of the directly to the peak current, I0. The factor ��is equal to 120�� antenna. A typical gain for a dipole antenna is 2dB, and the in free space. bandwidth is generally around 10%. 23

is not good, a high signal-to-noise ratio makes up for it. A common method to increase loop antennas’ performance is to fill the core with a ferrite. This has the effect of increasing the magnetic flux through the loop and increasing radiation resistance.

Figure 5: (a) Loop Antenna Principal E-plane Pattern (b) Loop Antenna Principal H-Plane Pattern Figure 3: Current and Voltage Distributions for a Half-Wave Dipole Antenna 3.3. The microstrip or patch antenna is often manufactured 3.2. Loop Antenna directly on a , where the patch is a The loop antenna is a conductor bent into the shape of a rectangular element that is photo etched on one side of the closed curve, such as a circle or square, with a gap in the board (Fig. 6). Most microstrip elements are fed by a coaxial conductor to form the terminals. Fig. 4 shows a circular and conductor which is soldered to the back of the plane. a square loop. These antennas may also be found as multiturn Typically the upper plate conductor is smaller than the loops or coils, designed with a series connection of overlaying to allow fringing of the electric field. The turns. There are two sizes of loop antennas: electrically small substrate between the microstrip and the ground and electrically large. If the total conductor length is small plane is simply the printed-circuit substrate. compared with a wavelength, it is considered small. An electrically large loop typically has a circumference approaching one wavelength.

Figure 6: Rectangular Microstrip-Antenna Element Despite its low profile, the has an efficient radiation resistance. The source of this radiation is Figure 4: (a) Circular Loop Antenna (b) Square the electric field that is excited between the edges of the Loop Antenna microstrip element and the ground plane. The equation for The current distribution on a small loop antenna is the radiation resistance is a function of the desired assumed to be uniform. This allows it to be simply analyzed wavelength (�) and the width (W) of the microstrip: as a radiating . Used as transmitters, loop antennas 120� have a pattern as shown in Fig. 5. Loop antennas can have a Rrad = (8) gain from -2dB to 3dB and a bandwidth of around 10% W Microstrip antennas are generally built for devices that Loop antennas are found to be very useful as receivers. require small antennas, which lead to high frequencies, For low frequencies-where dipoles would become very large- typically in the Gigahertz. Most microstrip elements are very loop antennas can be used. While the efficiency of a small efficient, anywhere from 80 to 99 percent. Factors that affect 24 efficiency are dielectric loss, conductor loss, reflected power, and the power dissipated in any loads involved in the elements. Using air as a substrate leads to very high efficiencies, but is not practical for photo etched antennas.

3.4. The basic helical antenna consists of a single conductor wound into a helical shape, shown in Fig. 7. Helical antennas are circularly polarized, that is, the radiated electromagnetic wave contains both vertical and horizontal components. This is unlike the dipole, which only radiates normal to its axis. Like the monopole, a ground plane must be present. Figure 8: Rectangular - Horns (a) Pyramidal (b) Sectoral H-Plane (c) Sectoral E-Plane (d) Diagonal

4. APPLICATIONS OF ANTENNAS IN WIRELESS DOMAIN Only a handful of antennas have been presented in the previous section. There are numerous variations on these basic antennas, as well as many different shapes. Antennas also have countless applications-many more than can be covered in this paper. This section will discuss some of the more familiar antenna applications. When frequencies are low, the earth and ionosphere Figure 7: Helical Antenna behave like conducting mediums-like two spherical shells. The locations of the transmitters were chosen based on The antenna shown in Fig. 7 has a gain of about 12dB. It geological factors. In each site, the bedrock present has a operates in the 100 to 500 MHz range and is fed with a coax low conductivity. where the center conductor is fed through the center of the ground plane. The spacing of the turns is 1/4-wavelength The eight-watt extremely (ELF) signal and the diameter of the turns is 1/3- wavelength. This is just radiates from the dual-site system and travels around the one example of a helical antenna; they can be scaled to other world. As these electromagnetic waves pass over the oceans frequencies of operation as well. Other modifications can be surface, some of their energy passes into the ocean. This made; nonuniform-diameter helical structures can widen the signal reaches submarines almost worldwide at depths of bandwidth and improve radiation performance. hundreds of feet and traveling at operational speeds. All Navy submarines are equipped with ELF receivers that can 3.5. decode ELF transmissions. ELF broadcast signals provide a one-way message system to submarines that is slow, but Horn antennas are made for the purpose of controlling one reliable. The submarines can receive ELF messages but they or more of the fundamental antenna properties: gain, antenna cannot transmit ELF signals because of the large power pattern, and radiation resistance. A horn antenna works in requirements, the large transmitter size and the large antenna conjunction with a waveguide-a tube that channels energy required to transmit ELF. Submarines can communicate on from one location to another. Horn antennas can have several or near the ocean's surface with higher data rate systems shapes, depending on their function. Several are shown in such as satellite communications systems. Figure 8 The pyramidal horn in Fig. 8a is used to maximize the 4.1. HF and UHF Antennas gain, since the antenna is flared in both the H-plane and As shown in Fig. 9, the very-high-frequency (VHF) and ultra- E-plane. This obviously gives the antenna a fixed directivity, high-frequency (UHF) antenna and its bands are used for and it will radiate principally in the direction of the horn's private and public-access services carrying speech and data. axis. Figures 8b and 8c are special cases of the pyramidal There are a wide variety of applications and many different horn, where either the H-plane or E-plane is flared thus and types of antennas. For a typical base-station antenna, such maximized. as for a television or radio station, the transmitter must be 25 large to achieve the desired frequency and provide a large The most common antenna type for cellular telephones range of coverage. The VHF and UHF bands cover is the monopole, often referred to as a . The ¼- frequencies from 3 MHz to 3000 MHz and include television wavelength whip is the simplest type available and is used and FM . in the 400 to 500 MHz range. Other similar types are the 3/8- wavelength whip antenna and the ½-wavelength whip antenna. These are larger than the ¼-wavelength antenna but have improved performance.

5. CONCLUSIONS

Figure 9: HF and UHF RFID Antenna In this paper, the basics on antennas are covered. Devices that change electromagnetic energy into electric or magnetic 4.2. TV and FM Receiving Antennas energy or vice versa is an antenna. There are many different types of antennas, and this paper introduced just a few of The most common type of VHF and UHF receiving antenna is a Yagi array antenna. Fig. 10 shows a combination UHF/ the basic shapes: dipoles, monopoles, loops, , VHF/FM receiving antenna. The array of different size helicals and horns. Each type of antenna has certain conductors is necessary to receive different frequencies. characteristics that are important in determining its behavior: Yagi arrays are highly directional, so the antenna should be radiation resistance, antenna pattern, directivity and gain, pointed towards the transmitting antenna. bandwidth and the signal-to-noise ratio. Antennas are used in numerous applications. A few were mentioned in this paper, such as the Navy's ELF system, VHF and UHF antennas, TV and FM receiving antennas, and some wireless applications.

References [1] William H. Hayt, Jr. and John A. Buck, Engineering Electromagnetic, Sixth Edition, McGraw-Hill: 2001. [2] Leo Setian, Practical Communication Antennas with Wireless Applications, Prentice Hall: 1998. [3] Robert E. Collin, “Antennas and Radio wave Propagation”, McGraw-Hill: 1985. [4] RF Café, “Antenna Patterns”, 2000. http://www.rfcafe.com/ references/electrical/antenna_patterns.htm [5] The United States Navy Fact File, “Extremely Low Figure 10: UHF/VHF/FM Antenna Frequency Transmitter Site Clam Lake, Wisconsin,” http:/ /enterprise.spawar.navy.mil/spawarpublicsite/docs/ 4.3. Wireless Communications Applications fs_clam_lake_elf.pdf Mobile communication, wireless interconnects, and cellular [6] Rick Warnett, “Stations, ITU Licenses and Services Below phone technologies are growing rapidly. Naturally, mobile 22kHz,” http://web.tiscali.it/vlfradio/itulist/itulist.htm and cellular technologies need antennas. Having the right antenna for the right device improves transmission and [7] Richard C. Johnson and Henry Jasik, “Antenna Engineering reception, reduces power consumption, and lasts longer. Handbook”, Second Edition, Mc-Graw Hill: 1984.