== Characteristics of an Antenna ==

=== Field Regions <ref>http://www.giangrandi.ch/electronics/anttool/regions.shtml</ref>===

There are three types of field regions:

* '''Reactive near field:''' It is in immediate vicinity of antenna. The E-fields and H-fields here are out of phase and there distribution is very complex and it requires large computations to find that distribution. The distance from antenna is deciding factor in this region. It's boundary is given by [[File:Equation68.png]].

These plots are very helpful as they give insight to many properties of antennas such as gain, directivity and also assist in visualization of power distribution in the real scenario. <br \>

Example: The radiation pattern for dipole antenna is as follows.

=== Frequency ===

=== Half Power Beamwidth ===

As we have directional antennas, one obvious question we can get for what angle we can get good radiation power? Although the answer to this question also depends on receiving setup, it is generally defined as the point where the power output drops to half its maximum value. When converted to dB it amounts to approx 3 dB loss, hence it is also called 3 dB point. In one plane, there are going to be two such points. The angular separation between the two points is half power beamwidth or 3 dB beamwidth.

Polarization is very easily visualised as pattern traced out by the E-field in any electromagnetic wave.

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[[File:LHCP1.gif|frame|right|Left Hand Circularly Polarized Signal. Image reproduced from [https://commons.wikimedia.org/wiki/File:Circular.Polarization.Circularly.Polarized.Light_Left.Hand.Animation.305x190.255Colors.gif here]]] <br \>

*All the antennas we have looked at so far were linearly polarised antennas. But in actuality circular polarised can be made quite easily from all the linearly polarised antennas. As we mentioned in polarisation, circular polarization is constructed simply from two equal magnitude linearly polarized orthogonal signals which are 90° out of phase. Antennas with same geometry will have same output radiation for same input. Hence two linearly polarized antennas with same geometry placed in such a way such that their output is orthogonal to one another and a 90° phase shifted signal given to one of them will result in circular polarisation antenna. The antennas have to placed in near field region of each other.

* In a crossed Yagi antenna, two yagi antennas are used to achieve circular polarization. Both the antennas are placed geometrically orthogonal to one another and 90° phase shifted signals are given to their feed. Hence it acts as a circularly polarised antenna. Usually this 90° phase shift is added by introducing an additional cable of length about 0.25*wavelength (actually the length also depend upon the kind of wire used. The phase difference due to transmission cable is its path difference times velocity factor which depends on cable). Weather it's RHCP or LHCP can actually be changed by changing its connections. <ref>http://www.qsl.net/dk7zb/Cross-Yagi/crossyagi.htm</ref>

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