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samedi 15 octobre 2016

Info Post
By Carolyn Hughes


Antennas are electrical devices that will convert an electric power into a radio wave and vice versa. These are often used with radio transmitters or radio receivers. Antennas are the essential components for all the equipment that have radios. These are often used in broadcast television, communication receivers, cell phones, radio broadcasting, two way radio, radar, and satellite communication. It can also be used in some other devices such as wireless microphones, wireless computer networks, RFID tags, garage door openers, Bluetooth devices, baby monitors.

One type of this which is being used by the military is mono pole. This is consist of only one conductor like the metal rod which is mounted on the ground or in an artificial surface for conduction. An umbrella is one type of a mono pole which is used for military communications in long range. The production of these by military antenna manufacturers can be very useful for them.

An umbrella is a type which is a large wire that will transmit the antenna being used for the VLF band. It consists of mast radiator tower attached to its top with multiple wires which are extending in mast to ground, like tents and umbrellas. It is insulated in the ends. It also contains narrow bandwidth and requires counterpoise ground in low resistance and large loading coils.

Aside from mono poles, another one is array. Phased array is used by militaries for their jamming and radar systems. It contains antennas which are high gains utilized for microwave frequencies and UHF that are both steered electronically. This has several number of dipoles containing 2 dimensional arrays. Each dipole is being fed into an electric phase shifter being controlled by a computer control system.

One old type of antennas is a radio kite. This is utilized in carrying radios to an aloft, which is higher than a practical mast. Usually, the kite is associated with portable systems for radios and for an increased range of radio in the naval ships. Using this type has limitations because of difficulties for maintaining its height, the wind is unpredictable, and improvements for reception, antenna, and transmission.

The ground is one thing to be given of much importance in the installation of antennas. Its height and also its electrical properties, both the conductivity and permittivity are important. And also, particularly for mono pole antennas, the ground will serve as return connection for the current, thus having an effect.

For those that have a low frequency to high frequency, the grounds will behave as lossy dialectrics. Thus, these are being characterized by conductivity and permittivity. For lower frequencies, it will act as the good conductor.

If the frequency is between 3 to 30 megahertz, a very large portion of energy from the horizontally polarized antennas will reflect on it. While the vertically polarized one will not be well reflected except for the grazing incidence. However, the reflection of grazing angles is in phase with direct waves.

A net quality of a reflection is dependent to surface topography. If irregularities of a surface which are smaller than the wavelength, specular reflection is there. But if these are not big, the reflection is shifted by a random phase.




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