HORN ANTENNA

MEASUREMENTSHORN ANTENNA

Presented By:Ayushi Gagneja

The horn is nothing more than a hollow pipe of different cross sections, which has been tapered (flared) to a larger opening. The type, direction, and amount of taper (flare) can have a profound effect on the overall performance of the element as a radiator.

An electromagnetic horn can take many different forms, four of which are: (a) E-plane (b) H-plane (c) Pyramidal (d) Conical

What is HORN Antenna?

The horn is widely used as a feed element for large radio astronomysatellite trackingcommunication dishes installed throughout the worldfeed for reflectors and lensesit is a common element of phased arraysserves as a universal standard for calibration and gain measurements of other high-gain antennas.

Its widespread applicability stems from its simplicity in construction, ease of excitation, versatility, large gain, and preferred overall performance.

IMPEDANCEHorns input impedance is slowly varying over a wide frequency range.Antenna impedance is represented as ratio of voltage to current at antennas terminal.In order to achieve maximum energy transfer input impedance of antenna must identically match characteristics impedance of transmission line. If the two impedance dont match, reflected wave will be generated and travel back towards energy source. This reflection of energy results in overall antenna efficiency.Impedance of an antenna, with no load attached, is defined as:ZA=RA+jXA

ZA= antenna impedance (ohms)RA= antenna resistance (ohms)XA= antenna reactance (ohms)

Resistive part is RA=Rr+RLwhereRr=radiation resistance of antennaRL=loss resistance of antenna

Input impedance of antenna is function of frequency. Thus antenna will be matched to interconnecting transmission line and other associated equipment only within a bandwidth.In addition, impedance depends on factors like geometry, its method of excitation, and its proximity surrounding objects.

POLARIZATION

Polarization of radiated wave is defined as the property of electromagnetic wave describing time varying direction and relative magnitude of electric field vector. It is described by the geometric figure traced by the electric field vector upon a stationary plane perpendicular to direction of propagation, as waves travel through the plane.Three different types of polarization are shown:

RADIATION FIELD

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The radiation pattern at 2 GHz is shown

For example: Assumea=3.69 inches,b=1.64 inches,A=30 inches, andB=23.8 inches. This horn is somewhat large, and will work well above roughly 2 GHz. Horns made for higher frequencies are smaller. This horn antenna, with a waveguide feed is shown

GAINHorn antennas have very little loss, so thedirectivityof a horn is roughly equal to its gain.For a horn to be realizable, the following must be true:

It can be shown that

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The optimum-gain condition in the E-plane

There is only one physically meaningful solution

Similarly, the maximum-gain condition for the H-plane

Since must be fulfilled

Substituting in the expression for the horns gain,

gives the relation between A, the gain G, and the aperture efficiency