Introduction to Antenna and Design of micro strip Dipole Antenna for Wireless Applications

Introduction to Antenna and Design of micro strip Dipole Antenna for Wireless Applications

Sanjay kumar sahu1,Kaibalya Sethi2,Chandra shekhar Mishra3,Ashiwad Dutta4 & Pravanjana Behera5

Department of Electronics and Communication Engineering 1,2,3,4 Gandhi Institute of Technological Advancement, Bhubaneswar

5Gandhi Institute For Technology, Bhubaneswar

Abstract This paper introduces antenna theory and discusses various antenna types and applications. The basic equations and principles of antenna theory are presented. Five different types of antennas are introduced: dipoles and monopoles, loop antennas, microstrip antennas, helical antennas, and horn antennas. The characteristics of each type of antenna are given. This paper has a selection of antenna applications from the low-frequency ELF system to high frequency wireless applications. This paper is not a comprehensive study of antennas, but serves as an introduction to antenna types and their applications.

Keywords Antennas, Antenna Applications, Antenna

current (I) flowing into the antenna. The equation for radiation resistance is given in (1). As can be seen, the greater the radiation resistance, the more energy is radiated or received by the antenna. When the radiation resistance of the antenna matches the resistance of the transmitter or receiver, the system is optimized. Antennas have also ohmic or loss resistance which decreases efficiency. It can be shown that an efficient antenna must be comparable to a wavelength in size.

P

Theory R rad (1)

rad I 2

1. INTRODUCTION

   The broadest definition of an antenna is that it is a transducerit changes energy from one form into another. A receiving antenna changes electromagnetic energy into electric or magnetic energy. A transmitting antenna changes the energy from electric or magnetic into electromagnetic energy. Current flowing in the antenna induces the electric and magnetic fields. Antennas have been used for over a century in a variety of applications. They can transmit over a massive range of frequencies, from a fraction of a kilohertz to over one hundred gigahertz. This paper will give a brief introduction to antenna principles, and then discuss various antenna types and their applications.

2. BASIC ANTENNA PRINCIPLES

   1. Antenna Parameters

      There are five basic parameters that one must understand to determine how an antenna will operate and perform. The first is the characteristic of radiation resistance. The radiation resistance (Rrad) of an antenna relates the power supplied (Prad) to the antenna and the

      A second parameter of antennas is the antenna pattern. This gives a distribution of radiated power as a function of direction in space. Generally, planar sections of the radiation pattern will be shown instead of the complete three-dimensional surface. The most important views are those of the principal E-plane and H-plane patterns. The E-plane pattern contains the plane in which the electric field lies. Similarly, the H- plane pattern is a sectional view in which the magnetic field lies. An example of each type is shown in Figure 1, which gives the antenna pattern for a half-wave dipole orientated in the z-direction.

      1. (b)

         Fig. 1 (a) Dipole Principal E-plane Pattern (b) Dipole Principal H-plane Pattern

         Most antennas do not radiate uniformly, as seen in Figure 1a. This implies that there is some directivity, which is the third parameter. Closely related to this is the gain of the antenna. The directivity is a measure of the ability of an antenna to concentrate the radiated power in a given direction. The gain is similarly defined. Usually given in decibels, it is the ratio of power radiated to input power. Thus, using an antenna with a higher gain would require less input power. Gain is a more significant parameter in practice than the directivity.

         Bandwidth is another important antenna parameter. This refers to the inclusive frequencies available outside the center frequency. For example, a 10 MHz transmitter with a 10% bandwidth could send 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 send data.

         The final antenna parameter is the signal-to-noise

         Despite the complexity of these equations, some observations can be made. The exponential factor in each equation is the same. This indicates wave propagation outward from the origin in the positive r direction. The wavelength is equal to , and d is the length of the current filament. Also as expected, the strength of the fields is related directly to the peak current, I0. The factor is equal to 120 in free space.

         At radial distances 10 or more wavelengths away from the oscillating current element, these equations can be simplified. All terms except the 1/r term may be neglected, and the radiation fields are given in (5), (6), and (7). Assuming a vertical orientation of the current element, plotting E s against for a constant r will result in a graph similar to Figure 1a, called the vertical pattern. Plotting H s shows the variation of field intensity with , often referred to as the horizontal pattern, such as in Figure 1b.

         ratio. This is the relationship between the desired I d r

         information signal and the noise. If this ratio does not exceed unity, information will not be transferred. Noise can be caused by obstructions, large distances between antennas, and environmental RF noise, such as power supplies and digital switching devices.

   2. Electromagnetic Energy Calculations

   An antennas 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 (2), (3), and (4), given in spherical coordinates. Because of the complexity of these equations, most problems involving antennas are

   H j 0 sin e- j 2

   s 2r

   E j I 0 d sin e- j2 r

   s 2r

   Ers 0

3. TYPES OF ANTENNAS

   1. Diploes and Monopoles

      (5)

      (6)

      (7)

      solved by experimental rather than theoretical methods.

      The worlds most popular antenna is the half-wave

      dipole. Shown in Figure 2, the total length of the

      I 0 d

      – j 2 r 2 1

      antenna is equal to half of the wavelength. The

      H s sin e j

      2 (2) relationship between wavelength and frequency is

      4 r r

      f = c / , where c 3 108 m/s in free space. Dipoles

      E I 0 d cose- j 2 r 1

      (3)

      may be shorter or longer than half of the wavelength, but this fraction provides the best antenna efficiency.

      rs 2

      j2 r3 r 2

      The radiation resistance can be calculated as 73.1 .

      E I 0 d sin e- j 2 r

      s 4

      j 2 1

      r r 2 j2 r3

      (4)

      Fig. 2 Dipole Antenna, 1/2 Wave

      The dipole antenna is fed by a two-wire line, where the two currents in the conductors are equal in amplitude but opposite in direction. Since the antenna ends are essentially an open circuit, the current distribution along the length of the half-wave dipole is sinusoidal, shown in Figure 3. This produces the antenna pattern shown in Figure 1. This pattern shows that when the antenna is vertical, it radiates the most in the horizontal direction and very little out the ends of the antenna. A typical gain for a dipole antenna is 2dB, and the bandwidth is generally around 10%.

      Fig. 3 Current and Voltage Distributions for a Half-Wave Dipole Antenna

      A monopole antenna is one-half a dipole plus a perfectly conductin plane. It behaves in a similar way to the dipole, but most of its parameters are halved. Figure 4 shows a quarter-wave monopole, also known as the vertical whip antenna.

      Fig. 4 Monopole Antenna, 1/4 Wave

      The radiation resistance is 36.5 , half that of a dipole. The total power radiated is also half that of a dipole, and the radiation pattern is shown in Figure 5. A typical gain for a monopole antenna is 2 to 6dB, and the bandwidth is also around 10%.

      1. (b)

      Fig. 5 (a) Monopole Principal E-plane Pattern (b) Monopole Principal H- plane Pattern

   2. Loop Antennas

      The loop antenna is a conductor bent into the shape of a closed curve, such as a circle or square, with a gap in the conductor to form the terminals. Figure 6 shows a circular and a square loop. These antennas may also be found as multi-turn loops or coils, designed with a series connection of overlaying turns. There are two sizes of loop antennas: electrically small and electrically large. If the total conductor length is small compared with a wavelength, it is considered small. An electrically large loop typically has a circumference approaching one wavelength.

      1. (b)

         Fig. 6 (a) Circular Loop Antenna (b) Square Loop Antenna

         The current distribution on a small loop antenna is assumed to be uniform. This allows it to be simply analyzed as a radiating inductor. Used as transmitters, loop antennas have a pattern that follows Figure 7. Loop antennas can have a gain from 2dB to 3dB and a bandwidth of around 10%.

         R 120

         (8)

         rad W

         (a) (b)

         Fig. 7 (a) Loop Antenna Principal E-plane Pattern (b) Loop Antenna Principal H-plane Pattern

         Loop antennas have found to be very useful as receivers. For low frequencieswhere dipoles would become very largeloop antennas can be used. While the efficiency of a small loop antenna 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.

   3. Microstrip Antennas

      The microstrip or patch antenna is often manufactured directly on a printed circuit board, where the patch is a rectangular element that is photoetched from one side of the board (Figure 8). Most microstrip elements are fed by a coaxial conductor which is soldered to the back of the ground plane. Typically the upper plate conductor is smaller than the ground plane to allow fringing of the electric field. The dielectric substrate between the microstrip and the ground plane is simply the printed-circuit substrate.

      Fig. 8 Rectangular Microstrip-Antenna Element

      Despite its low profile, the microstrip antenna has an efficient radiation resistance. The source of this radiation is the electric field that is excited between the edges of the microstrip element and the ground plane. The equation for the radiation resistance is a function of the desired wavelength ( ) and the width (W) of the microstrip:

      Microstrip antennas are generally built for devices that require small antennas, which lead to high frequencies, typically in the Gigahertz. Most microstrip elements are very efficient, anywhere from

      80 to 99 percent. Factors that affect 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 photoetched antennas.

   4. Helical Antennas

      The basic helical antenna consists of a single conductor wound into a helical shape, shown in Figure

      9. 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.

      Fig. 9 Helical Antenna

      The antenna shown in Figure 9 has a gain of about 12dB. It operates in the 100 to 500 MHz range and is fed with a coax where the center conductor is fed through the center of the ground plane. The spacing of the turns is 1/4-wave and the diameter of the turns is

      1/3-wave. This is just one example of a helical antenna; they can be scaled to other frequencies of operation as well. Other modifications can be made; nonuniform-diameter helical structures can widen the bandwidth and improve radiation performance.

   5. Horn Antennas

   Horn antennas are made for the purpose of controlling one or more of the fundamental antenna properties: gain, antenna pattern, and radiation resistance. A horn antenna works in conjunction with a waveguidea tube that channels energy from one location to another. Horn antennas can have several shapes, depending on their function. Several are shown

   Figure-11 Proposed antenna

   Name X Y m10.00 2.1000 -19.9348

   -2.50

   -5.00

   dB(S(1,1))

   dB(S(1,1))

   -7.50

   -10.00

   -12.50

   -15.00

   -17.50

   XY Plot 1 HFSSDesign1 ANSOFT

   Curve Info dB(S(1,1))

   Setup1 : Sw eep

   Fig. 10 Rectangular Waveguide Horns (a) Pyramidal (b) Sectoral H-plane

   1. Sectoral E-plane (d) Diagonal

      The pyramidal horn in Figure 10a is used to maximize the gain, since the antenna is flared in both the H-plane and E-plane. This obviously gives the antenna a fixed directivity, and it will radiate principally in the direction of the horns axis. Figures 10b and 10c are special cases of the pyramidal horn, where either the H-plane or E-plane is flared thus and maximized.

4. ANTENNA SIMULATION AND

   RESULT

   -20.00

   m1

   1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

   Freq [GHz]

   Figure12 S11 Result of proposed structure

   Substrate width=160mm Substrate height=1.6mm Patch width=3.5mm Patch length=20mm

   Gap between two patch=4mm

   figure13-Current distribution of the proposed structure

   Each type of antenna has certain characteristics that are important to determining its behavior: radiation resistance, antenna pattern, directivity and gain, bandwidth, and the signal-to-noise ratio. Antennas are used in numerous applications. A few were mentioned in this paper, such as the Navys ELF system, VHF and UHF antennas, TV and FM receiving antennas, and some wireless applications.

   Figure -14 orientation of E field vector

5. CONCLUSIONS

   Antennas are devices that change electromagnetic energy into electric or magnetic energy or vice versa. The equations describing antenna behavior are complicated, and engineers use experimental measurements and data in most antenna design work. There are many different types of antennas, and this paper introduced just a few of the basic shapes: dipoles, monopoles, loops, microstrips, helicals, and horns.

6. REFERENCES

1. William H. Hayt, Jr. and John A. Buck, Engineering Electromagnetics, Sixth Edition, McGraw-Hill: 2001.

2. Leo Setian, Practical Communication Antennas with Wireless Applications, Prentice Hall: 1998.

3. Robert E. Collin, Antennas and Radiowave 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 Frequency Transmitter Site Clam Lake, Wisconsin, http://enterprise.spawar.navy.mil/spawarpublicsite/docs/ fs_clam_lake_elf.pdf

6. Rick Warnett, Stations, ITU Licenses And Services Below 22kHz, http://web.tiscali.it/vlfradio/itulist/itulist.htm

7. Richard C. Johnson and Henry Jasik, Antenna Engineering Handbook, Second Edition, Mc-Graw Hill: 198

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