An antenna is designed to transmit or receive electromagnetic waves. An electromagnetic wave is created when an electric field and a magnetic field oscillate together. Antennas are typically known to be used for radio and television, but are also used with radar, wireless devices and many other types of communication.
Thomas Edison patented his antenna in 1891, but was using them since 1885. In 1889, Heinrich Hertz published his works to prove the existence of electromagnetic waves (based upon Maxwell's theory of electromagnetics) in Annalen der Physik und Chemie (Annals of Physics and Chemistry).
Most antennas are a single rod ¼ wavelengths long. There are two types of antennas, omni-directional (car radio antenna) and directional (a satellite dish). An antenna array is two or more antennas that are partnered to serve a single purpose.
There are several aspects that affect an antenna's performance: impedance, polarization, bandwidth, resonant frequency, gain, aperture and radiation pattern are all such parameters. Most of these are equally important for both transmission and receiving antennas, but there are some variances that we will not discuss here.
Electromagnetic waves can be hindered greatly by ground. The higher the antenna is away from the ground the better chance there is of the antenna creating a direct ray away from the ground. Otherwise, a reflected ray is created by bouncing the electromagnetic wave off the ground. Most of the electromagnetic wave is absorbed into the ground by doing this, but can be greatly reduced if the angle is less than 60 degrees of incidence by creating grazing angles. Formulas for determining actual signal strength must calculate the polarization, impedance, and combination of joined reflected rays and direct rays.
When the power hits the antenna the signal radiated is a mere 1%. The antenna then reflects the signal back adding a 1% boost to the signal. This reflection continues back and forth until the signal reaches near 100%. Each reflection must be in the exact phase as the other reflections, or the signal will not be as strong or may even be non-existent. Each reflection creates a watt of power, charging the antenna and aiding in the boosting of the signal. When fully charged the antenna could have hundreds of watts of power maintained constantly.
There are full wave, ½ wave and ¼ wave antennas. A full wave antenna is never used by amateurs merely because of its size. Most commonly used is the ½ wave and ¼ wave antennas. A ½ wave antenna has a large amount of volts surging through it, but it is not connected to anything; it is driven by a transmitter. The length of this antenna is calculation as 150/fMHz meters long. A ¼ wave antenna is typically connected to a metal source (or the Earth), and is known as a Ground Plane antenna.