A Report on Types of Antennas
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A REPORT ON TYPES OF ANTENNAS A REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE COURSE EEE C472- SATELLITE COMMUNICATION Prof. M.K.Deshmukh (Instructor-in-charge) Dept. of EEE/ENI Submitted By:- Deepak Uppal 2010A3PS216G CONTENTS 1. Introduction 1 2. Different Types of Antennas used in Satellite Communications 1 2.1 Horn Antenna 1 2.2 Phased Array Antenna 2 2.3 Helical Antenna 2 2.4 Reflector Array Antenna 2 2.5 Patch Antenna 3 2.6 Parabolic Antenna 4 2.7 Lens Antenna 4 3. Different Parameters of Antenna Systems 5 3.1 Gain 5 3.2 Beam width 5 3.3 Bandwidth 5 3.4 Polarization 6 3.5 Effective Area or Aperture 6 4. Comparison between Parabolic Reflector and Horn Antennas 7 References 9 1. Introduction An antenna (or aerial) is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. high frequency AC) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals that is applied to a receiver to be amplified. Antennas may also include reflective or directive elements or surfaces not connected to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. Antennas can be designed to transmit or receive radio waves in all directions equally (omnidirectional antennas), or transmit them in a beam in a particular direction, and receive from that one direction only (directional or high gain antennas). The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving. 2. Different Types of Antennas used in Satellite Communications 2.1 Horn Antenna A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz They are used as feeders (called feed horns) for larger antenna structures such as parabolic antennas, as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers. Their advantages are moderate directivity (gain), low standing wave ratio (SWR), broad bandwidth, and simple construction and adjustment. An advantage of horn antennas is that since they have no resonant elements, they can operate over a wide range of frequencies, a wide bandwidth. The usable bandwidth of horn antennas is typically of the order of 10:1, and can be up to 20:1. 1 2.2 Phased Array Antenna A phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. An antenna array is a group of multiple active antennas coupled to a common source or load to produce a directive radiation pattern. Usually, the spatial relationship of the individual antennas also contributes to the directivity of the antenna array. Use of the term "active antennas" is intended to describe elements whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input. One common application of this is with a standard multiband television antenna, which has multiple elements coupled together. 2.3 Helical Antenna A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes: normal mode or axial mode. In the normal mode or broadside helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarised parallel to the helix axis. In the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarised radio waves. 2 2.4 Reflector Array Antenna In telecommunication and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are often used in the VHF frequency band, and these versions often resemble a highway billboard, so they are sometimes called "billboard antennas". The curtain array is a larger version used by shortwave radio stations. Reflective array antennas usually have a number of identical driven elements, fed in phase, in front of a flat, electrically large reflecting surface to produce a unidirectional beam, increasing antenna gain and reducing radiation in unwanted directions. The individual elements are most commonly half wave dipoles, although they sometimes contain parasitic elements as well as driven elements. The reflector may be a metal sheet or more commonly a wire screen. A metal screen reflects radio waves as well as a solid metal sheet as long as the holes in the screen are smaller than about one-tenth of a wavelength, so screens are often used to reduce weight and wind loads on the antenna. They usually consist of a grill of parallel wires or rods, oriented parallel to the axis of the dipole elements. The driven elements are fed by a network of transmission lines, which divide the power from the RF source equally between the elements. This often has the circuit geometry of a tree structure. 2.5 Patch Antenna A patch antenna (also known as a rectangular micro strip antenna) is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane. The assembly is usually contained inside a plastic Radom, which protects the antenna structure from damage. Patch antennas are simple to fabricate and easy to modify and customize. They are the original type of micro strip antenna described by Howell in 1972; the two metal sheets together form a resonant piece of micro strip transmission line with a length of approximately one-half wavelength of the radio waves. The radiation mechanism arises from discontinuities at each truncated edge of the micro strip transmission line. The radiation at the edges causes the antenna to act slightly larger electrically than its physical dimensions, so in order for the antenna to be resonant, a length of micro strip transmission line slightly shorter than one-half a wavelength at the frequency is used. A patch antenna is usually constructed on a dielectric substrate, using the same materials and lithography processes used to make printed circuit boards. 3 2.6 Parabolic Antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, that is, they can produce the narrowest beam-widths, of any antenna type. In order to achieve narrow beam- widths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used. Parabolic antennas are used as high-gain antennas for point-to-point communications, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite communications and spacecraft communication antennas. They are also used in radio telescopes. The other large use of parabolic antennas is for radar antennas, in which, there is a need to transmit a narrow beam of radio waves to locate objects like ships, airplanes, and guided missiles. With the advent of home satellite television receivers, parabolic antennas have become a common feature of the landscapes of modern countries. 2.7 Lens Antenna Lens Antenna is an antenna whose directivity pattern is a result of the difference between the phase velocity of propagation of an electromagnetic wave in air and that in the lens material.