Hardware
An antenna is an array of conductors (elements), electrically connected to the receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional, or high-gain, or “beam” antennas). An antenna may include components not connected to the transmitter, parabolic reflectors, horns, or parasitic elements, which serve to direct the radio waves into a beam or other desired radiation pattern. Strong directivity and good efficiency when transmitting are hard to achieve with antennas with dimensions that are much smaller than a half wavelength.
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.
Antenna Gain
In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna’s directivity and radiation efficiency. The term power gain has been deprecated by IEEE. In a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction. In a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power. When no direction is specified, gain is understood to refer to the peak value of the gain, the gain in the direction of the antenna's main lobe. A plot of the gain as a function of direction is called the antenna pattern or radiation pattern. It is not to be confused with directivity, which does not take an antenna's radiation efficiency into account.
Gain or 'absolute gain' is defined as "The ratio of the radiation intensity in a given direction to the radiation intensity that would be produced if the power accepted by the antenna were isotropically radiated". Usually this ratio is expressed in decibels with respect to an isotropic radiator (dBi). An alternative definition compares the received power to the power received by a lossless half-wave dipole antenna, in which case the units are written as dBd.
Since a lossless dipole antenna has a gain of 2.15 dBi, the relation between these units is Gain(dBd) ≈ Gain(dBi) - 2.15. For a given frequency, the antenna's effective area is proportional to the gain. An antenna's effective length is proportional to the square root of the antenna's gain for a particular frequency and radiation resistance. Due to reciprocity, the gain of any antenna when receiving is equal to its gain when transmitting.
High Gain
High-gain antennas are focused antennas with narrow radio beams, allowing for precise targeting of radio signals. This antenna is used in space missions as well as in flat, open areas where the geography won’t disrupt radio waves.
High-gain antennas transmit more power to the receiver, increasing the strength of the signal it receives. As a result of their reciprocity, high-gain antennas can also make transmitted signals 100 times stronger by capturing more energy when used in receiving antenna. As a result of their directivity, directional antennas send fewer signals from a direction other than the main beam. This property reduces interference.
High-gain antennas can also be produced from parabolic antennas, phased arrays and yagi antennas. Antenna gains are defined with respect to hypothetical antennas that radiate equally in all directions – the isotropic radiator. This gain can be measured in decibels (dBi) or, in certain cases, decibels compared to the maximum intensity of the direction of half-wave dipoles (dBd).
Low Gain
A small spacecraft antenna that provides a low amplification of radio frequency signals. An LGA is capable of transmitting and receiving in almost any direction, and does not require highly accurate pointing. Sometimes an LGA is mounted above the spacecraft's high-gain antenna (HGA), as on Galileo and Cassini. LGAs can be used with space probes to the nearer planets, as with Magellan investigating Venus, but probes traveling further, such as the Voyager probes, must use high-gain antennae.
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). A power amplifier is similarly used to deliver output power (AF or RF), controlled by an input signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude (magnitude of the voltage or current) of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.
An amplifier can either be a separate piece of equipment or an electrical circuit contained within another device. Amplification is fundamental to modern electronics, and amplifiers are widely used in almost all electronic equipment. Amplifiers can be categorized in different ways. One is by the frequency of the electronic signal being amplified. For example, audio amplifiers amplify signals in the audio (sound) range of less than 20 kHz, RF amplifiers amplify frequencies in the radio frequency range between 20 kHz and 300 GHz, and servo amplifiers and instrumentation amplifiers may work with very low frequencies down to direct current. Amplifiers can also be categorized by their physical placement in the signal chain; a preamplifier may precede other signal processing stages, for example. The first practical electrical device which could amplify was the triode vacuum tube, invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912. Today most amplifiers use transistors.
Amplification means increasing the amplitude (voltage or current) of a time-varying signal by a given factor, as shown here. The graph shows the input v_i(t) (blue) and output voltage v_o(t) (red) of an ideal linear amplifier with an arbitrary signal applied as input. In this example the amplifier has a voltage gain of 3; that is at any instant v_o(t) = 3v_i(t).