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what is Dipole antenna?

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Dipole antenna

Introduction

A dipole antenna is the simplest type of radio antenna, consisting of a conductive wire rod that is half the length of the maximum wavelength the antenna is to generate. This wire rod is split in the middle, and the two sections are separated by an insulator. Each rod is connected to a coaxial cable at the end closest to the middle of the antenna. Radio frequency voltages are applied to dipole antennas at the center, between the two conductors. They are used alone as antennas, especially in rabbit-ear television antennas and as the driven elements in other types of antennas. Dipole means "two  poles." Dipole provides the best performance if it is more than a half-wavelength above the ground, surface of a body of water or horizontal conducting medium such as sheet-metal roofing. The element should also be a certain wavelength away from electrically conducting obstructions such as supporting towers, utility wires and other antennas. Dipole antennas are oriented vertically, horizontally or in slants. Polarization of electromagnetic fields radiated by dipole-transmitting antennas corresponds to element orientation. Radio frequency (RF) current in dipoles is at its maximum at the centers of the dipole and at its minimum at ends of the element, and vice versa for RF voltages. Dipole antennas were invented in 1886 by a German physicist named Heinrich Hertz. These antennas are also referred to as a doublet and make up the main RF radiating and receiving element in different sophisticated type of antennas. Dipole antennas are balanced in that they are bilaterally symmetrical, and they are fed with balanced, parallel wire RF transmission lines.


Dipole  antenna basics

As the name suggests the dipole antenna consists of two terminals or "poles" into which radio frequency current flows. This current and the associated voltage causes and electromagnetic or radio signal to be radiated. As seen the antenna consists of a radiating element that is split, normally in the centre to allow a feeder to apply power to it from a transmitter, or to take power from it to a receiver. The length of the radiating element determines many of the properties of the dipole antenna from its impedance, centre operating frequency, etc. As such this is an important feature of the antenna. Often the term dipole antenna tends to indicate a half wave dipole. This is by far the most widely used length for a dipole. It forms a resonant circuit which resonates where the electrical length is half a wavelength long - the electrical length differs from the wavelength of the signal in free space because of a number of the effects of the radiating element on the signal and it is very slightly shorter than the signal e/m wavelength in free space.. Although the half wavelength dipole antenna is the most popular, a variety of other formats are also available.


Dipole  antenna types

The dipole antenna consists of two conductive elements such as metal wires or rods which are fed by a signal source or feed energy that has been picked up to a receiver. The energy may be transferred to and from the dipole antenna either directly straight into a from the electronic instrument, or it may be transferred some distance using a feeder. This leaves considerable room for a variety of different antenna formats.

Although the dipole antenna is often though in its half wave format, there are nevertheless many forms of the antenna that can be used.

  • Half wave dipole antenna: The half wave dipole antenna is the one that is most widely used . Being half a wavelength long it is a resonant antenna.
  • Multiple half wave’s dipole       antenna: It is possible to utilize a dipole antenna or aerial that is an odd multiple of half wavelengths long.
  • Folded dipole antenna: As the name implies this form of the dipole aerial or dipole antenna is folded back on itself. While still retaining the length between the ends of half a wavelength, an additional length of conductor effectively connects the two ends together.
  • Short dipole: A short dipole antenna is one where the length is much shorter than that of half a wavelength. Where a dipole antenna is shorter than half a wavelength, the feed impedance starts to rise and its response is less dependent upon frequency changes. Its length also becomes smaller and this has many advantages. It is found that the current profile of the antenna approximately a triangular distribution.
  • Non-resonant dipole: A dipole antenna may be operated away from its resonant frequency and fed with a high impedance feeder. This enables it to operate over a much wider bandwidth.


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