Antenna (radio)
In 1886, German physicist Heinrich Hertz placed a spark gap transmitter near a simple loop of wire in his laboratory. This setup proved the existence of electromagnetic waves predicted by James Clerk Maxwell decades earlier. Hertz used dipole antennas at the focal point of parabolic reflectors to both transmit and receive signals. His experiments established the first practical radio antennas for scientific demonstration. Guglielmo Marconi began developing these devices for long-distance wireless telegraphy starting in 1895. He opened a factory in Chelmsford, England, to manufacture his invention in 1898. The word antenna comes from Italian where l'antenna means tent pole. Marconi called his elevated horizontal wire antenna simply l'antenna because it resembled a tent pole on his father's estate near Bologna.
An antenna converts an alternating electric current into radio waves during transmission. It intercepts power from a radio wave to produce an electric current during reception. Radio waves travel through space at the speed of light with almost no transmission loss. A half-wave dipole consists of two elements arranged end-to-end along the same axis. Each element is approximately one quarter wavelength long. Standing waves form within the conductor when the signal frequency matches the design length. Maximum current occurs at the feed point while voltage remains minimal there. This creates minimum impedance magnitude for maximum efficiency. The ordinary half-wave dipole is probably the most widely used antenna design today.
Omnidirectional antennas radiate energy approximately equally in all horizontal directions. A vertical whip antenna found on portable radios serves as a common example. Directional beam antennas concentrate radio waves in specific angles instead. A Yagi-Uda array uses passive elements to greatly increase gain in one direction. These arrays suffer from having rather limited bandwidth compared to other designs. Parabolic reflectors focus incoming waves toward a feed antenna at their focal point. Such systems achieve effective area comparable to the size of the reflector itself. High-gain antennas require careful aiming at the receiving station. Low-gain antennas allow orientation to be relatively unimportant for operation.
Impedance matching ensures efficient transfer of received signals into transmission lines. Mismatched impedance causes some signal to reflect backwards into the antenna body. A standing wave ratio measures this mismatch on the feedline. An antenna designed for 1 meter wavelength would be approximately 50 cm from tip to tip. If that element has a length-to-diameter ratio of 1000, it will have inherent impedance of about 63 ohms resistive. Feeding that antenna with 1 Ampere requires 63 Volts and radiates 63 Watts of power. Changing the signal frequency alters the phase relationship between current and voltage. The antenna then appears to have very high impedance if not matched properly. Antenna tuners transform resistance to match standard 50 Ohm coaxial cables used commercially.
A Yagi-Uda array uses passive elements to greatly increase gain in one direction. Only one element connects electrically to the transmitter while others remain passive. These passive elements interact with electromagnetic fields to realize highly directional performance. Log-periodic dipole arrays maintain similar characteristics over very large bandwidth ranges. Phased arrays consist of two or more simple antennas connected through electrical networks. Changing relative phases allows radiation patterns to shift without physically moving elements. Parabolic dishes use curved reflecting surfaces to focus waves toward feed antennas. Arrays of vertical towers achieve directionality at low frequencies but occupy large land areas. Amateur radio operators often connect resonant elements for different frequencies in parallel configurations.
The ground acts as a lossy dielectric for frequencies below 30 MHz. Soil conductivity and permittivity influence how much energy reflects from the surface. Vertically polarized radiation reflects in phase at grazing angles providing up to 6 dB boost. Horizontally polarized emission reverses phase upon reflection creating cancellation near the ground. An antenna height of half wavelength changes the phase of reflected wave by 180 degrees. This completely alters the antenna's impedance compared to its theoretical value. Buildings reflect electromagnetic waves creating ghost images due to multipath propagation. Urban areas benefit from horizontal polarization because side reflections are generally less than vertical ones. Sea water provides highly conducting surfaces that reflect almost all incident wave power.
Common questions
Who invented the first practical radio antenna and when?
German physicist Heinrich Hertz established the first practical radio antennas for scientific demonstration in 1886. He used dipole antennas at the focal point of parabolic reflectors to both transmit and receive signals.
What is the origin of the word antenna and who popularized it?
The word antenna comes from Italian where l'antenna means tent pole. Guglielmo Marconi called his elevated horizontal wire antenna simply l'antenna because it resembled a tent pole on his father's estate near Bologna starting in 1895.
How does an antenna convert electric current into radio waves?
An antenna converts an alternating electric current into radio waves during transmission. It intercepts power from a radio wave to produce an electric current during reception while radio waves travel through space at the speed of light with almost no transmission loss.
Why do omnidirectional antennas radiate energy equally in all directions?
Omnidirectional antennas radiate energy approximately equally in all horizontal directions as demonstrated by a vertical whip antenna found on portable radios. Directional beam antennas concentrate radio waves in specific angles instead using passive elements like those in a Yagi-Uda array.
How long should a half-wave dipole antenna be for a 1 meter wavelength signal?
An antenna designed for 1 meter wavelength would be approximately 50 cm from tip to tip. Each element is approximately one quarter wavelength long and standing waves form within the conductor when the signal frequency matches the design length.