SLOT ANTENNA

SLOT ANTENNABHAVIN V KAKANIInstitute of Technology, Nirma UniversityIntroduction Aslot antennaconsists of ametalsurface, usually a flat plate, with a hole or slot cut out. When the plate isdrivenas anantennaby a driving frequency, the slot radiateselectromagnetic wavesin similar way to adipole antenna The shape and size of the slot, as well as the driving frequency, determine theradiation distribution pattern. Slot antennas are often used atUHFandmicrowavefrequencies instead of line antennas when greater control of theradiation patternis required. Slot antennas are widely used inradarantennas, for thesector antennas used forcell phone base stations.

Slot antennasare used typically at frequencies between 300 MHz and 24 GHz. The slot antenna is popular because they can be cut out of whatever surface they are to be mounted on, and have radiation patternsthat are roughly omnidirectional. The polarization of the slot antenna is linear. The slot size, shape and what is behind it (the cavity) offer design variables that can be used to tune performance. A properly designed slot is efficient, radiating upto 90% of the power in transmission line & with low sidelobes. Slot array antennas have relatively narrow bandwidths. Whether the slot is directly fed or excited by the incident EF, the voltage is clearly zero at the ends and maximum in the middle. This is similar to half wave dipole, the voltage distribution has the minimum at the centre and maxima at the ends- the reverse of slot antenna. The electric vector is parallel to the length of the dipole instead of transverse.

Babinets Principle (Babinays Principle) or Complementary Antenna

This principle relates the radiated fields and impedance of an aperture or slot antenna to that of the field of its dual antenna. The dual of a slot antenna would be if the conductive material and air were interchanged - that is, the slot antenna became a metal slab in space. Whenever a high frequency field exists across a very narrow slot in an infinite conducting sheet, the energy is radiating through a slot. This is the working principle of a slot antenna.

a voltage source is applied across the short end of the slot antenna. This induces an E-field distribution within the slot, and currents that travel around the slot perimeter, both contributed to radiation. The dual antenna is similar to adipole antenna. The voltage source is applied at the center of the dipole, so that the voltage source is rotated.

Babinet's principle relates these two antennas. The first result states that theimpedanceof the slot antenna (Zs)is related to the impedance of its dual antenna (Zc) by the relation:

SEE THE DERIVATION OF ABOVE FORMULA FROM THE BOOK.eta is intrinsic impedance of free space. A single half wavelength slot in a conducting sheet is analogous to half wave dipole interms of gain and directivity with only difference in terms of polarization. The horizontal slot produces vertical polarization in the direction normal to the slot, while the vertical slot produces horizontal polarization.

slot impedance is proportional to the admittance of the dipole. If the slot requires inductance for impedance match, the complementary dipole requires capacitance. SEE NUMERICALS OF HORN ANTENNA FROM JD KRAUSS AND K D PRASAD.Horn AntennaIntroduction It may be regarded as a flared out or opened out waveguide. A waveguide is capable of radiating radiation into open space provided it is excited at one end and opened at other end. They are mostly used in microwave band (above 1 GHz). They provide High Gain, Low VSWR, relatively high bandwidth. Types of Horn Antenna

If flaring is done in only one direction then sectorial horn is produced. E-plane sectorial and H-plane sectorial is the example. If the flaring is done along both the walls(E&H) of the rectangular waveguide then Pyramidal horn is obtained. By flaring the walls of a circular waveguide a conical horn is formed. The horn acts as a gradual transition from a waveguide mode to a free-space mode of the EM wave.