EMG4066:Antennas and Propagation Exp 1:ANTENNAS MMU:FOE. To study the radiation pattern characteristics of various types of antennas.

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1 OBJECTIVES To study the radiation pattern characteristics of various types of antennas. APPARATUS Microwave Source Rotating Antenna Platform Measurement Interface Transmitting Horn Antenna Dipole and Yagi Antenna Set Coax Cables Microstrip Antenna INTRODUCTION Antennas are often designed to accentuate the radiation energy in some directions and suppress it in others. In order to meet the particular requirements, it may take various forms ranging from a piece of conducting wire, an aperture, an array of elements, a reflector, a lens etc. The straight wire dipole antenna is the most familiar one because it is seen virtually everywhere. Its radiation properties are dependent on the ratio of dipole length l to the wavelength λ. In practice, the antenna length is normally between (1/3) λ and (5/4) λ, and only rarely exceeds 2λ. The spacing between the two-equal length segments of the dipole is assumed to be infinitely small. The radiated waves are linearly polarized. If the dipole wire runs along the z-axis, the radiation is maximum in the radial direction on the x-y plane. The magnetic field vectors lay on this plane of propagation and thus referred to as the H-plane. The radiation pattern is often described in terms of its principle E- and H- plane patterns, instead of a 3D spatial distribution of radiated energy as a function of the observer s position along a constant radius sphere. The E-plane pattern is defined as the plane containing the electric field vector and the direction of maximum radiation, and the H- plane as the plane containing the magnetic-field vector and the direction of maximum radiation. For the well-known donut-shaped radiation pattern of a half-wave dipole, the H- plane pattern is a circle while the E-plane patterns is a figure 8, as illustrated in Figure 1. The patterns will be completely different if the dipole length is longer than λ. For a wavelength of 3 m or less, the construction of a half-wave dipole is simple to realize. In the case of medium and long waves (λ = m), only relatively short monopole antennas can be constructed due to technical and economic reasons. For reasons of space, aesthetics or concealment, the antennas need to be short. The solutions are inductive and capacitive loadings of the antennas, as shown in Figure 2. The inductive AP1: Applied EM lab BR4025 1

2 loading inserts a coil at the middle of the rod length to distort the current distribution along the conductor. To add a capacitive load, a circular conductive plate is attached onto the top of the monopole. These coil and plate electrically lengthen the antenna, and therefore, a physically shorter structure can be used to obtain the required radiation properties. Figure 1: Radiation pattern of a half-wave dipole. Figure 2: Current distribution of half-wave dipole with inductive loading and capacitive loading Figure 3: Yagi-Uda antenna and its radiation pattern. AP1: Applied EM lab BR4025 2

3 Yagi-Uda antenna (Figure 3), named after its developers, is widely used for TV reception. A dipole (or folded dipole) is used as the only active element to intercept radio waves and transfer the electromagnetic energy to a transmission line in the form of electric current and voltage. All other elements are considered as parasitic radiators without any feedline or matching network; thus making the realization considerably cheaper. The parasitic elements influence both the input impedance of the active element as well as the radiation pattern of the overall antenna system. An element longer than λ/2 behind the active element will act as a reflector, which reflects the approaching waves in the major lobe toward the dipole. Conversely, a shorter element in front of the active element will act as a director, which concentrates the received waves in the major lobe and reradiates toward the dipole. The directivity of the antenna system is greater with an increased number of parasitic elements, particularly the directors. Practically, there is a limit beyond which very little gain is obtained by the addition of more directors. The length of the directors (0.30λ 0.45λ) and the spacing between them (0.30λ 0.4λ) must be properly selected to optimise the front-to-back ratio of the antenna. On the other hand, it has been concluded numerically and experimentally that the reflector spacing and size have negligible effects on the forward gain, but large effects on the backward gain (front-to-back ratio) and input impedance. The major role of the reflector is played by the first element next to the active one (~0.25λ) away). Increasing the number of reflectors will only contribute slightly in the performance of Yagi-Uda antenna. The input impedance of the dipole, which acts as the active element, is lower compared to an isolated dipole. In order to make the matching easier, a 2-element folded dipole is often used to replace the single dipole. This will give impedance step-up by a factor of 4. Figure 4: Microstrip antenna and the various feeding networks. AP1: Applied EM lab BR4025 3

4 A 2D or planar array antenna is produced by arranging linear array antennas in a parallel configuration. One example of the planar array is the microstrip antenna (Figure 4). Each rectangular patch of conductors on the dielectric substrate acts as a radiator. With l = w = λ/2, portion of electric-field lines cross over from one edge to another above the patch. These fringing fields are the source of radiation. If every patch in the planar array is excited with equal amplitude and phase, the beamwidth will be constricted in both x- and y- direction. The feeding networks must be dimensioned accurately in order to obtain the desired efficiency of the antenna system. These feeding networks can be designed in one of the following methods: a) A corporate feeder in which all of the radiators are connected on electrically equal-length line segments, as in Figure 4(b). b) Series feeder in which the radiators are connected in series. The time delay of the signal from one radiator to the subsequent radiator introduces phase difference in the excitation of each element. This may result in a tilt of the beam sometimes called squinting effect of the main lobe. Changing the signal frequency will cause a variation in the squint angle. c) A combination of the two, as in Figure 4(c). PROCEDURE Initial setting 1) The antenna measurement system is set-up according to Figure 5. 2) Switch on the rotating antenna platform. 3) Switch on the computer and run the antenna measurement software. 4) Rotate the transmitting horn antenna to produce the required wave polarization for E- plane and H-plane pattern measurements. Figure 5: The measurement setup. AP1: Applied EM lab BR4025 4

5 Dipole antenna 5) A set of dipole antenna is provided with the experiment set. 6) Determine the arrangement of the antenna for E-plane measurement. 7) Measure the E-plane pattern for the λ/2 dipole. Note that the transmitted waves must be horizontally polarized. 8) Determine the arrangement of the antenna for H-plane measurement. 9) Determine the direction of transmitting horn antenna to produce the vertically polarized wave. Measure the H-plane patterns for the λ/2 dipole. 10) Analyse the E- and H-plane radiation pattern of the dipole antenna. Deduce the 3-dB beamwidth for each of the plane patterns. Yagi-Uda antenna 11) Construct a Yagi antenna using the dipole antenna kit provided with the experiment set. 12) Investigate the E-plane patterns for the configuration with a reflector, a director, a director plus a reflector, and 4 directors plus a reflector. The transmitted waves must be horizontally polarized. 13) Arrange the set to produce vertically polarized waves. Measure the H-plane patterns for the configuration with a reflector, a director, a director plus a reflector, and 4 directors plus a reflector. 14) Analyse the E- and H-plane radiation pattern of the all the Yagi antenna. Deduce the 3-dB beamwidth for each of the plane patterns. Microstrip antenna 15) Set up the microstrip antenna on the rotating platform. 16) Determine the E-plane and H-plane patterns of the microstrip antenna. 17) Analyse the E- and H-plane radiation pattern of the all the Yagi antenna. Deduce the 3-dB beamwidth for each of the plane patterns. QUESTIONS: Q. 1 What is the difference between infinitesimal dipole and small dipole? Q. 2 What is the radiation resistance of a half-wave dipole, and what about its directivity? Q. 3 What do you understand by the radiation intensity? Q. 4 What do you mean by the isotropic radiator? Q. 5 What do you mean by the antenna impedance? Q. 6 What is the concept of effective aperture of an antenna? How is it different from the physical aperture? Q. 7 What do you mean by Hertzian dipole? What about the current distribution on such a dipole? Q. 8 What are the different elements in a Yagi-Uda antenna? Mention the use of each. AP1: Applied EM lab BR4025 5

6 MARKING SCHEME: Results of each radiation patterns: (2 mark for each pattern, total of 12 patterns) 24 marks Measurement of 3dB beamwidth: (1 mark for each pattern, total of 12 patterns) 12 marks Conclusion & Discussion: 8 marks Answer to questions: 16 marks TOTAL: 60 marks AP1: Applied EM lab BR4025 6

Antenna Fundamentals Basics antenna theory and concepts

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