Antenna Radio

An antenna—also known as an aerial in British English—is a device that converts electric currents into radio waves during transmission and transforms received radio waves back into electric currents during reception. As the critical interface between electromagnetic radiation and electronic circuitry, antennas underpin the functioning of all wireless communication systems, from broadcast radio and television to radar, satellite links and mobile telecommunications.

Terminology

The terms antenna and aerial are often used interchangeably, though aerial may sometimes refer to simple wire-based structures. The modern usage of antenna in wireless engineering is attributed to Guglielmo Marconi, who in the 1890s used the Italian word antenna—originally meaning the central pole of a tent—to describe the tall support structure carrying his radio wires. Due to Marconi’s prominence, the term spread rapidly among experimenters and became standard across the field.
In technical contexts, an antenna may refer not only to the radiating conductive elements but also to an integrated assembly including supports, housings, matching networks or preamplifiers, especially in microwave systems.

Historical Development

The first practical antennas were constructed by Heinrich Hertz in 1886 during experiments demonstrating the existence of electromagnetic waves predicted by Maxwell’s equations. Hertz employed dipole antennas placed at the foci of parabolic reflectors to transmit and receive radio signals.
Beginning in 1895, Marconi developed antennas suited to long-distance wireless telegraphy and established early manufacturing facilities. His work laid the foundations for commercial radio communication and solidified the fundamental principles of antenna design.

Fundamental Principles

Radio waves are electromagnetic waves that propagate through space at the speed of light. To interface electrical circuits with these waves, antennas must allow currents and voltages to oscillate at radio frequencies, creating corresponding electromagnetic fields.
Key concepts include:

• Reciprocity: An antenna exhibits the same directional properties in both transmission and reception.
• Radiation pattern: A spatial representation of the strength of the radiated field.
• Beamwidth and directivity: Measures of how concentrated the radiation is in a preferred direction.
• Gain: A comparison of power radiated in a given direction relative to an isotropic radiator. Although termed “gain”, it does not increase total power but redistributes energy directionally.

Types and Radiation Characteristics

Omnidirectional antennasThese radiate (or receive) energy nearly uniformly in all horizontal directions. They are suitable when communication with multiple stations in different directions is required. The vertical whip antenna is a common example.
Directional antennasThese concentrate energy in specific directions, increasing effective range and signal strength. Examples include:

• Dipole antenna: The simplest directional antenna element and the basis for many designs.
• Yagi–Uda antenna: A linear array of one driven element and multiple parasitic elements, generating strong horizontal directivity.
• Log-periodic antenna: Composed of a sequence of dipole elements of varying lengths, providing broadband directional performance.
• Parabolic reflector and horn antennas: Aperture antennas capable of very high directivity at microwave frequencies.

Highly directional antennas require dimensions comparable to or greater than half the operating wavelength. Consequently, large physical structures are needed at lower frequencies.
Monopole antennasThese use a single vertical conductor with the ground (or an artificial ground plane) acting as the return path. Common in broadcasting and portable equipment, they are half the size of an equivalent dipole.
Antenna arrays and phased arraysAntenna arrays combine multiple elements to shape and steer radiation patterns. By adjusting the relative phase of signals feeding each element, the array can be configured as:

• A broadside array, radiating perpendicular to the line of elements.
• An end-fire array, radiating along the axis of the elements.
• A phased array, capable of electronic beam steering without mechanical movement.

Key Components and Practical Considerations

The antenna feed includes the transmission line and any matching networks required to couple the antenna to a transmitter or receiver. Matching ensures minimal reflection of power and efficient transfer of energy.
For very small antennas, impedance matching becomes a significant design challenge. Tools such as loading coils (for vertical antennas) or variable capacitors (for magnetic loop antennas) are employed to achieve resonance at the desired operating frequency.
Counterpoise systems and ground planes improve the performance of monopole antennas by providing a stable reference conductor. At low radio frequencies, large land areas may be required for directional arrays used in broadcasting.

Applications

Antennas appear in a broad array of modern technologies:

• Broadcasting: Radio and television transmission towers rely on high-power directional or omnidirectional antennas.
• Telecommunications: Mobile networks employ sectorised base-station antennas for efficient coverage.
• Navigation and radar: Precision arrays and aperture antennas form the basis of radar imaging and aircraft navigation systems.
• Satellite communication: Parabolic dishes and horn feeds enable high-gain links over vast distances.
• Consumer devices: Cars, smartphones and Wi-Fi routers contain compact specialised antennas optimised for multiband operation.