Dipole Antenna

The antenna is used to RF electromagnetic waves 'energy' into components, as the bulbs to electricity become light, or violin string is used to the potential energy (force) into sound waves. A perfect antenna should be at least its design in 1/2 wavelength emission frequency of the waves, just as the truth of the strings.

Dipole antenna due to the structure of most of the two 1/4 wavelength wire or steel, aluminum tubes composed of straight bar, it could be on its abdomen four weeks to produce electromagnetic waves, but the two top of the antenna is almost no waves generated.

Dipole antenna is usually used for the reference antenna, its gain was defined as '1', also known as the '0 dB, if the ingenious arrangements of two dipole antennas, and can not influence each other, and electricity has been reviewed support the antenna, in theory, will make waves within a small angle intensity increased by 2 times, that is, '3dB'.

And so on, arranged in groups of up and down a lot of section dipole antenna, the radio strength has been improved (namely commonly known Yagi), are usually more practical level less than 10 times, which is below l0 dB, but concentrated in the perspective of the needs of the total energy and Mao increase or decrease, the other point of view of the waves will disappear.

Shown in the following diagram Fig.3, the dotted line for the 'point' of radiation sources, but actually not an ideal sphere represents a mathematical launched the formation of a spherical average of its waves in all directions. In the same power, in any direction, the intensity of the waves are 0.6 times of the dipole antenna, '-2.15dB' In other words, if the ideal point emission source is defined as 'l', '0 dB' , the abdomen, the highest point of the electric field strength in the dipole antenna for 1.6 times, namely '2.l5dB the'. To avoid confusion, usually written as '2.15dBi' to launch the 'i' on behalf of the sphere basis.

Therefore, a gain antenna for 2.l5dBi is the antenna gain is 0 dB is the gain of the ordinary antenna.

Another approach to improving the antenna directivity and gain Yagi (YAGI) ', namely the use of a metal close to the ordinary dipole antenna, the electromagnetic waves tend to the desired direction, commonly known as the ribs antenna, electromagnetic energy is to guide to the direction of the need to produce gain. So the gain is only relative, not really the energy amplification.

Ground Plane is usually upright, its electric field perpendicular to the ground, so the waves vertically polarized wave; ribs is needed, horizontally or vertical, horizontal, horizontal polarization wave, straight put the resulting vertical pole of the wave. Usually yield to the car antenna vertically, the ribs are mostly made of vertical polarization.

Another used to satellite communication antenna may be vertical and horizontal overlap, and phase shift of 90 degrees, the time difference between the two energy sum, while the formation of a circularly polarized wave, so regardless of how the tumbling satellite are can receive a good signal, the most special.

Medium wave broadcast antenna wavelength is usually 600 - 200 meters, the half-wavelength of at least 300 - 100 meters, it is difficult to erection, and stand in the ground, groundwater and minerals under the surface to form a conductive layer, this the performance of the conductive layer as side electronic mirror will produce a reflection of an antenna. Up and down two half-wavelength antenna current of opposite polarity, cancel each other out, but no waves issued. If the antenna height is reduced to 1/4 wavelength below the reflection of the formation of just 1/4 wavelength on the formation of a dipole antenna, can play a good vertical wave radiation function.

The antenna is used to RF electromagnetic waves 'energy' into components, as the bulbs to electricity become light, or violin string is used to the potential energy (force) into sound waves. A perfect antenna should be at least its design in 1/2 wavelength emission frequency of the waves, just as the truth of the strings.Many small car use 144/430MHz antenna is to use this method, but difficult for someone willing to drill holes in the roof antenna mounted on the roof above, while its installed in the trunk next to the reflection of how the poor can be desired but Fortunately, the antenna manufacturers quickly design a non-ground network of the antenna out.