Antenna Antenna Radiation, Antenna Gain, Resistance, Bandwidth, Beam width. Antenna: It is a structure that is generally a metallic object, often a wire or group of wires, used to convert high frequency current into electromagnetic waves or vice versa. Transmitting and receiving antennas have similar characteristics, which means that their behaviour is reciprocal. The spacing, length and shape of the device is related to the wavelength of the desired transmitter frequency. i.e mechanical length is inversely proportional to the numerical value of the frequency. - Physically, an antenna is simply an arrangement of one or more conductors, usually called elements. - In transmission, an alternating current is created in the elements by applying a voltage at the antenna terminals, causing the elements to radiate an electromagnetic field. - In reception, the inverse occurs: an electromagnetic field from another source induces an alternating current in the elements and a corresponding voltage at the antenna's terminals. Electromagnetic Radiation • When RF energy is fed into a mismatched transmission line, standing waves occur. • Here energy is lost or radiated into the space surrounding the line. This process is considered unwanted in the transfer of energy to the radiation device. • when the two ends of the transmission line are separated, more surface area of the wire is exposed to the atmosphere. This enhances the radiation process. (fig 9-2a) • When the two wires are bent at 90 degree to each other, the radiation efficiency improves. (fig 9-2c) • The electric and magnetic fields are fully coupled to the surrounding space instead of being confined between the two wires.. Here the maximum radiation exists. This type of radiator is called dipole. • When the total length of the two wires is a half wavelength, the antenna is called a half-wave dipole. • This configuration has similar characteristics to its equivalent length transmission line(1/4 λ) •. It results in high impedance (Hi Z) at the far ends reflected as low impedance (Lo Z) at the end connected to the transmission line. (fig 9-2c) • This causes the antenna to have large current node at the centre and large voltage nodes at the ends, resulting in maximum radiation. A Hertzian antenna is a set of terminals that does not require the presence of a ground for its operation. Basic dipole facts : The name dipole means two poles and the antenna does in fact consist of two "poles" or sections. These are normally equal in length, making the antenna what is termed a centre fed antenna. The power is applied to the dipole antenna itself through a feeder. Conversely if the dipole antenna is used for receiving, the received signals are taken away to the receiver through a feeder. The feeder serves to transfer the power to or from the antenna with as little loss as possible. The basic dipole antenna configuration The most common form of dipole has an electrical length of half a wavelength. As a result this antenna is called a half wave dipole. As the total length of the dipole is a half wavelength, this makes each section or leg of the dipole a quarter wavelength long. The basic half wave dipole antenna Antenna Gain and effected radiation pattern • Certain antennas can be Omni directional (isotropic) which radiates in all directions, and certain types focus their radiation pattern in a specific direction. • Antennas have gain which is measured in decibels. Directive Gain • Directive gain is defined as the ratio of power density in a particular direction of one antenna to the power density that would be radiated by an omni directional antenna. • The power density of both types of antenna is measured at a specified distance and a comparative ratio Is established. Half wave dipole Gain = 2.15 dB – resonant antennas in free space Gain = 5.05 dB – non resonant antennas n free space Two important point here are: Resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). Power Gain Power gain is a comparison of the output power of an antenna in a certain direction to that of an isotropic antenna. Gain of an antenna is a power ratio comparison between an omni directional and unidirectional radiator. This ratio can be expressed as • Effective radiated power (ERP) applies to the field gain of the antenna and the efficiency of the transmitter. Radiation measurement and field intensity • The voltage induced in a receiving antenna are very small, generally in the microvolt range. • Field strength measurements are given in micro volts per meter. • Field intensity: • Field strength of an antenna’s radiation, at a given point in space, is equal to the amount of voltage induced in a wire antenna 1 m long , located at that given point. • Field strength or induced voltage is affected by a number of conditions such as the time of the day, atmospheric conditions and distance. Antenna Resistance • Radiation Resistance is the ratio of the power radiated by the antenna to the square of the current at the feed point. Antenna losses and efficiency: • In addition to the energy radiated by an antenna, the power losses must be considered. • Antenna losses can be caused by ground resistance, corona effects, imperfect dielectric near the antenna, energy loss due to eddy currents induced into nearby metallic objects, and I*I*R losses in the antenna itself. • The losses can be combined and represented as: • Low and medium frequency antennas are least efficient because of difficulty in achieving the proper physical (resonant) length. These antennas can approach efficiencies of only 75 to 95 percent. • Antennas at higher frequencies can easily achieve values approaching 100 percent. • Radiation resistance values may vary from a few ohms to several hundred ohms depending upon the choice of feed point and physical and electrical characteristics. Bandwidth, beam width and polarization • These are important terms dealing respectively with the operating frequency range, the degree of concentration of the radiation pattern and the space orientation of the radiated waves. Bandwidth: • Bandwidth refers to the range of frequencies the antenna will radiate effectively i.e. the antenna will perform satisfactorily throughout this range of frequencies. • When the antenna power drops to ½ (3 dB), upper and lower extremities of these frequencies have been reached and the antenna no longer performs satisfactorily. • Beam width • Beam width of an antenna is described as the angles created by comparing half power points (3 dB) on the main radiation lobe to its maximum power point. • In fig 9-9, the beam angle is 30 degree, which is the sum of two angles created at the points where the field strength drops to 0.707 ( field strength is measured in microvolts/m) of the maximum voltage at the centre of the lobe (these points are knows as half-power points) • Polarization • Polarization of an antenna refers to the direction on space of the E field (electric vector) portion of the electromagnetic wave being radiated (fig 9-10) by the transmitting system. • Low frequency antennas are usually vertically polarized because of ground effects (reflected waves etc) and physical construction methods. • High frequency antennas are generally horizontally polarized. • High frequency antennas are more desired because of its rejection to noise made by people which is mostly vertically polarized. • Corona Effects The presence of corona can reduce the reliability of a system by degrading insulation. While corona is a low energy process, over long periods of time, it can substantially degrade insulators, causing a system to fail due to dielectric breakdown. The effects of corona are cumulative and permanent, and failure can occur without warning. An eddy current (also known as Foucault current) is an electrical phenomenon caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body of the conductor. The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as a reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, the whip, and the J-pole. The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added which are functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb. http://www.electronics- radio.com/articles/radio/antennas/dipol e/dipole-antenna.php Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and space exploration. Antennas are most commonly employed in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.
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