RECONFIGURABLE ANTENAA

RECONFIGURABLE ANTENAA

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  Submitted By: Aditi Agrawal MEC2018003
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   A Reconfigurableantenna is an antenna capable of modifying dynamically its frequency and radiation properties in a controlled and reversible manner.  In order to provide a dynamical response, reconfigurable antennas integrate an inner mechanism (such as RF switches, varactors, mechanical actuators or tunable materials) that enable the intentional redistribution of the RF currents over the antenna surface.
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  1.Frequency reconfiguration  Frequencyreconfigurable antennas can adjust dynamically their frequency of operation. 2.Radiation pattern reconfiguration  Radiation pattern reconfigurability is based on the intentional modification of the spherical distribution of radiation pattern. 3.Polarization reconfiguration  Polarization reconfigurable antennas are capable of switching between different polarization modes 4.Compound reconfiguration  Compound reconfiguration is the capability of simultaneously tuning several antenna parameters, for instance frequency and radiation pattern.
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   Mechanically MovableParts  Reconfigurable Arrays  RF Switches: 1. Semiconductor Switch 2. MEMS Switches  Tunable Materials
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   The firstreconfigurable antennas had mechanically movable parts.  Large reflector antennas moved feeds to steer the main beam, or interchanged feed antennas to change frequency bands.  It is also possible to place nulls in the antenna pattern by altering the reflector’s surface.
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  A phased arraybuilt on a geodesic dome. The yellow regions represent regions of activated subarrays.
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   An RFswitch serves to open or close a current path on a reconfigurable antenna. A popular way to build a reconfigurable antenna is to connect various pieces of the antenna with RF switches. Opening and closing switches guides the current in a desirable path that changes the antenna’s radiation properties, as well as its impedance.  RF switches may be mechanical or semiconductor.  Some important characteristics of a switch are: o Characteristic impedance: if the switch is non-reflective, then it is matched to the transmission path; otherwise, it is reflective. o Bandwidth: some switches are low-pass filters, while others are bandpass filters. o Insertion loss and isolation: the ratio between the switch’s output and input powers when the switch is on and off, respectively.
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  Diagram of afield-effect transistor (FET) switch. Increasing the voltage at the gate increases the conducting channel’s size beneath the gate, and allows current to flow between the source and drain. FET switches come in several varieties.
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  Another widely usedmicrowave switch is the PIN diode. It has heavily doped p-type and n-type regions (used for ohmic contacts), which are separated by a wide, lightly-doped intrinsic region . Forward biasing a PIN diode creates a very low resistance at high frequencies, while reverse biasing results in an open circuit. A diagram of a PIN diode.
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   Some notabledifferences between the two types of switches are : • PIN diodes are current controlled, while FETs are voltage controlled. • PIN diodes have the ability to control large RF signal power while using much smaller levels of control power. • PIN diodes are less susceptible to electrostatic-discharge (ESD) damage.
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  Three types ofMEMS switches in the on and off positions.
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   MEMS switchesare tiny mechanical switches made on a substrate (silicon, quartz, glass). Unlike the PIN-diode and FET switches, a MEMS switch is mechanical.  The cantilever beam in Figures 1 and 2 is anchored to a post on the left, while the other end of the beam is suspended above the drain. An electrostatic force pulls the beamdown when a voltage is applied, and creates an electrical path between the beam and the drain.  MEMS switches have low power consumption, low insertion loss, and high isolation, like mechanical switches, but are small, light weight, and low cost, like semiconductor switches  On the other hand, MEMS switches have high losses at microwave and mm-wave frequencies, limited power- handling capability (~100 mW), and they may need expensive packaging to protect the movable MEMS bridges against the environment
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   Antennas arealso made reconfigurable through a change in the substrate characteristics by using materials such as liquid crystals or ferrites. The change in the material is achieved by a change in the relative electric permittivity or magnetic permeability. A liquid crystal is a nonlinear material whose dielectric constant can Be changed under different voltage levels. For ferrite material, a static applied electric magnetic field can change the relative material permittivity / permeability