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An Antenna is also known as aerial . It is transducer designed to transmit or receive electromagnetic waves .antennas convert electromagnetic waves into electrical currents and vice versa.
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. They can also be operated under water or even through soil and rock at certain frequencies for short distances.
An antenna, physically is simply an arrangement of one or more conductors ( usually known as 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 electro magnetic field. The inverse occurs in reception the electro magnetic field from another source induces an alternating current in the elements and a corresponding voltage at the antenna's terminals.
Antennas such as parabolic and horn types incorporate shaped reflective surface to collect EM waves from free space and direct or focus them onto the actual conductive elements.
Heinrich Hertz(1857-1894) has used antennas in 1888 to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell.
There are two fundamental types of antenna directional patterns with reference to a specific two dimensional plane.
1. Omni-directional, such as a vertical rod in the horizontal plane i.e. parallel to ground. It radiates equally in all directions.
2. Directional, radiates more in one direction than in the other.
Parameters Affecting Antenna's Performance:
There are several critical parameters affecting an antenna's performance. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. They can be adjusted during the design process. We can measure these parameters through various means. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.
Resonant Frequency and Electrical resonance is related to the electrical length of an antenna. The electrical length is defined as the physical length of the wire divided by its velocity factor E.g. The ratio of the speed of wave propagation in the wire to the speed of light in a vacuum. The antenna is usually tuned to specific frequency. Other properties of antennas such as radiation pattern and impedance change with frequency.
Gain is usually achieved by modification of its directionality, It measures the efficiency of a given antenna with respect to given norm. The antennas with a low gain emits radiation with about same power in all directions, whereas a high gain antenna will radiate only in particular directions. The Gain, Directive gain or Power gain of an antenna is defined as ratio of the intensity i.e. power per unit surface radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by the hypothetical isotropic antenna.
High range antenna have an advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. For example a dish antenna on a space craft. Low gain antennas have shorter range but the orientation of antenna is relatively inconsequential. For example Wi-Fi antenna in a laptop computer.
The radiation pattern of an antenna is the geometric pattern of the relative field strengths of the field emitted by the antenna. For the ideal isotropic antenna, this would be a sphere. It would be toroid if it is typical dipole. The radiation pattern of an antenna is typically represented by a three dimensional graph, or polar plots of the horizontal and vertical cross sections. The graph should show sidelobes and backlobes, where the antenna's gain is at a minima or maxima.
An electro magnetic wave travels through the different parts of the antenna system i.e. radio, feed line, antenna, free space, it may encounter differences in impedance ( E/H, V/I, etc). At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ration(SWR). SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.
Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have an SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and reduces efficiency. Mathematically, efficiency is calculated as radiation resistance divided by total resistance
The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered on the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components like in feed horn, and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size efficiency.
The polarization of an antenna is the orientation of the electric field of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. Polarization has nothing common with directionality terms: horizontal, vertical and circular. " Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electro magnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.
Basic antenna models:
1. Isotropic radiator: It is purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. this antenna cannot physically exist, but it is useful as a theoretical model for comparison.
2. Dipole antenna : It is a antenna which is simply pointed two wires in opposite directions arranged in vertical or horizontal with one end of each wire connected to the radio and other end hanging free in space. It is also used as reference model for the antennas .
3. Yagi-Udi antenna: This antenna is a directional variation of the dipole with parasitic elements .it is similar to adding a reflector and lenses to focus a filament light bulb.
4. Random wire antenna: It is a very long wire connected with one end connected to the radio and the other in free space .It also require an antenna tuner .
5. Parabolic antenna: It consists of an active element at the focus of a parabolic reflector to reflect the waves into a plane wave. it is used for high gain ,micro wave applications such as satellite dishes.
6. Patch antenna: It is a popular antenna .It consists of a single metal patch suspended over a ground plane. the assembly is usually contained inside a plastic Radom which protects the antenna structure from damage patch antenna is generally constructed on a dielectric substrate ,usually employing the same sort of lithographic patterning used to fabricate printed circuit boards.
7. Loop Antenna:http://radiology.rsna.org/content/231/1/143.full
It is a continuous conducting path leading from one conductor of a two-wire transmission line to the conductor.
It has three types of loops:
1. S mall loops: It is less than ¼ of a wave length in circumference, that loop is called as small loops. Most directional receiving loops are about 1/10 of a wave length. The small loop is also called as magnetic loop because it is more sensitive to the magnetic component of the electromagnetic wave. It is less sensitive. The received voltage of a small loop can be greatly increased by bringing the loop into resonance with a tuning capacitor.
2. Medium Loops: There are two special cases of loop antennas which are neither short nor long and have particular characteristics.
(i) Half-wavelength loop: A half wave dipole curved into a circle, can be mounted in the horizontal planes as horizontal polarized omni directional antenna.
(ii) Full wavelength loop: An element of the quad antenna , which radiates on its axis and is polarized according to the position of the feed point
3. Large loops: The large loop antenna is similar to a dipole ,except that the ends of the are connected to form a circle ,triangle or square .a circular large loop gets higher gain than the other forms .the large loop usually has its strongest signal in the plane of the loop unless it is very large .
1. In loop antennas providing precise targeting of a specific area for thermal destruction while minimizing collateral damage.
2. It has the ability to precisely control the area of ablation . these areas are relatively protected from RF ablation as a result of vessel-mediated cooling. This advantage is the major advantage for loop antenna.
1. The most disadvantage is more complicated targeting of tumors.
2. The loop antennas delivered through a single needle compared with use of microwave antennas delivered through a single needle compared with use of straight microwave antennas or RF arrays.