Quiz 16: Antennas
An antenna is a device used for transmitting and/or receiving radio waves. This process is done by converting the given electric power into electromagnetic (EM) waves or radio waves for transmission purposes and vice versa for reception purpose. An antenna is a passive device as it cannot generate any power on its own; it just radiates the given input power which is fed to it, for the purpose of transmission of the radio signals. All antennas, even the passive ones have gain. For example, take into consideration an isotropic antenna which radiates Radio frequency energy uniformly in all directions. Any antenna that emits Radio Frequency energy or EM waves in just one direction would certainly send the signal further when compared to an isotropic antenna by using same amount of RF energy. These antennas are called directional antennas and since these antennas can send the same amount of RF energy further than the isotropic antenna, it could be considered to have "gain" relative to that of isotropic antenna. So, in general gain of an antenna is said to be the directional gain and is measured relative to that of an isotropic antenna.
Radiation pattern is defined as a graphical representation of the antenna radiation properties as a function of position in spherical coordinate system. Since an isotropic antenna radiates radio frequency energy uniformly in all respective directions, the emitted radiation is assumed to occupy a spherical shape surrounding the antenna in free space. Half wave dipole on the other hand, is formed generally by connecting two conductors which are quarter wavelength. The combination of the two yields a half wave length conductor. This antenna is said to have a well-known toroid shaped (doughnut) radiation pattern. An isotropic antenna exists only in theory. It radiates uniformly in all directions. It is said to have a gain equivalent to 1 dB (unity). Dipole antennas radiate equally well in all horizontal directions when oriented vertically. This provides dipole more gain over isotropic antenna. A vertical half wave dipole is said to have a maximum gain of 2.15dbi.
The radiation resistance is formed because of the process of radiation reaction among the conduction electrons in an antenna. Definition: The resistance as observed from the feed point of the antenna which is formed due to the radiation of radio waves from an antenna is called radiation resistance. Radiation Resistance results in heating up the given antenna. When the antenna is energized, the electrons are forced to accelerate. Therefore, the antenna will emit radiations (or) radio waves that are produced from the energy, which is obtained from the electrons. The total resistance is given as the sum of both the radiation resistance and the loss resistance of the antenna under consideration.