When discussing the transmission of signals, one will have to address the one main component. This component is an electrical device that converts electromagnetic radiation into space and can absorb electromagnetic energy at its terminals.
The device is the antenna.
What’s important about an antenna is that it includes reflective or directive elements which serve to direct the radio waves and produce the desired radiation pattern and gain that will transmit or receive in a particular direction.
When an antenna’s path is altered, it can have an effect on the signal and cause the direction of the signal to bend (undergo refraction), reflect radio signals (undergo reflection) or travel around an obstacle (undergo diffraction). If such phenomenon occurs, it tends to cause the direction of an electromagnetic wave to change its path.
What needs to be kept in mind is the fact that it is the radiation pattern that yields the directivity of the antenna. Directivity is a component of its gain: This sheds light on the measurement parameter of the total energy radiated in a particular direction. The RF radiation beamwidth is usually measured in two angular directions: azimuth (horizontal) and elevation (vertical) degree.
To sum up, gain and directivity relate to directional antennas where beamwidth (ie, the width in degrees of the main lobe) is a measure of its directivity.
As antennas emit more radiation in some directions than in others, it is crucial to know the antenna’s radiation efficiency and how it can avoid energy from unwanted directions. Generally, a higher gain and a lower beamwidth provides a better overall efficiency of the antenna.
An antenna that radiates in all directions is an isotropic antenna. However, an aerial beam which is pointed in a certain direction and often uses a driven element such as dipole or folded dipole to transmit in a particular direction (or directional pattern) and receive in one direction only is called a directional antenna .
This type of antenna has a tendency to radiate its energy in one direction more efficiently than in others. At times, it becomes necessary to having to physically adjust the resonant frequency at the site of the antenna.
Directional antennas are commonly used in point-to-point communications. An example of this type is the Yagi-Uda array used for amateur radios. One can also spot them on rooftops of houses as they are the most common kind of terrestrial TV antenna.
Other common examples of directional antennas and designs (pertaining to shape and size) are patches, panels and parabolic or dishes.
In contrast, there are those antennas that radiate equally and uniformly, more or less, in all directions in one plane (horizontal or vertical) – these are non-directional aerials that radiate and receive with essentially the same efficiency.
Some examples of a “non-directional” antenna include the dipole antenna which is the “rabbit ears” television antenna – this is the simplest and most common type. Another antenna is the vertical whip, which is commonly used for non-directional radio communication.
Those that form an omnidirectional pattern in the horizontal plane are slot antennas – they are popular for RF and microwave sensors and radar. One more type of omnidirectional antenna are those used in mobile devices (eg, cell phones and walkie-talkies).
Considering everything mentioned, when deciding on what is needed, directional antennas are best suited for point-to-point links and where a particular direction is desired. Omnidirectional (non-directional) antennas do not favor any particular direction and are opt for the surface of the Earth, at AM broadcast stations and useful in open environments that may be in mountainous and metropolitan areas.