Half wavelength dipole antenna

Reference:

1. wikipedia

The dipole antenna is the simplest and most widely used class of antenna. It consists of identical conductive elements such as metal wires or rods, which is usually bilaterally symmetrical. Each side of the feedline to the transmitter or receiver was connected to one of the conductors.

Dipoles is resonant antennas, meaning that the elements serve as resonators, with standing waves of radio current flow ing back and forth between their ends. So the length of the dipole elements was determined by the wavelength of the radio waves used. The most common form are the half-wave dipole, in which each of the the the the the the the the the the the the the the the the the and elements is approximately a long The whole antenna is a half-wavelength long. The radiation pattern of a vertical dipole is omnidirectional; It radiates equal power in allazimuthal directions perpendicular to the axis of the antenna. For a half-wave dipole the radiation are maximum, 2.15 dBi perpendicular to the antenna axis, falling monotonically with El Evation angle to zero on the axis, off the ends of the antenna.

Several different variations of the dipole is also used, such as thefolded dipole, short dipole, cage dipole, Bow-tie, an D batwing antenna. Dipoles May is used as standalone antennas themselves, but they is also employed as feed antennas (driven elements) in MA NY more complex antenna types, such as the Yagi antenna, parabolic antenna, reflective array, turnstile antenna, log perio DIC antenna, and phased array.

Dipole characteristics

1. Impedance

The feedpoint impedance of a dipole antenna is very sensitive to its electrical length. Therefore, a dipole would generally only perform optimally over a rather narrow bandwidth, beyond which its impedance would Become a poor match for the transmitter or receiver (and transmission line). The real (resistive) and imaginary (reactive) components of this impedance, as a function of electrical length, are shown In the accompanying graph.

A true Half-wave dipole is one half of the wavelengthλin length, whereλ=c/f in free space. Such a dipole has a feedpoint impedance consisting of 73ωresistance and +43ωreactance, thus presenting a slightly induct Ive reactance. In order to cancel this reactance, and present a pure resistance to the feedline, the element was shortened by the factor K For a net length of:

The adjustment factor K, in order for the reactance to being cancelled, depends on the diameter of the conductor. For thin wires (diameter= 0.00001 wavelengths), k is approximately 0.98; For thick conductors (diameter= 0.008 wavelengths), K drops to about 0.94. This was because the effect of antenna length on reactance are much greater for thinner conductors. For the same reason, antennas with thicker conductors has a wider operating bandwidth over which they attain an acceptabl Estanding wave ratio.

Dipole antennas of lengths approximately equal to any odd multiple ofλ/2 is also resonant, presenting a small reactance (which can cancelled by a small length adjustment). However these is rarely used. One size that's more practical though are a dipole with a length of 5/4 wavelengths. Not being close to 3/2 wavelengths, this antenna ' s impedance have a large (negative) reactance and can only is used with an Impedance matching network (or "Antenna tuner"). It is a desirable length because such an antenna have the highest gain for any dipole which isn ' t a great deal longer.

Half wavelength dipole antenna