An antenna is an electrical device, usually a specially sized and shaped metal body, that converts electrical power to radio waves for a transmitter and radio waves to electrical power for a receiver. Beyond this simple definition there is a large amount of technology that has to be implemented if it is actually to work as intended.
The two great early researchers in antenna design and construction were the German physicist Heinrich Hertz and the Italian builder of radios Guglielmo Marconi. Hertz set out to validate James Clerk Maxwell’s earlier prediction, on purely theoretical grounds, that electromagnetic energy in the form of waves would propagate through space. Hertz accomplished this by placing two spark coils, not electrically connected, in close proximity. An electrical spark jumping an air gap in the receiving coil would coincide with a similar arc made in the transmitting coil connected to a power source. Hertz always believed that his apparatus had no useful application, serving only to demonstrate Maxwell’s theoretical work on electromagnetic wave propagation.
Marconi saw the potential for long-distance radio transmission and built units that worked at successively greater distances. Earlier investigators had seen the possibilities but Marconi was the first to build working equipment. Intuitively, it was known that conductive electrodes would be needed for antennas, but their optimal form wash ascertained only after lots of theoretical and experimental work.
Some antennas are omnidirectional, transmitting and receiving in all horizontal directions. This works well for radios in moving vehicles and when the signal is relatively low frequency as in the AM band. Directional antennas are appropriate for higher frequencies. The extreme case is a precisely aimed parabolic dish for satellite Internet or TV reception or for point-to-point microwave communication.
The most common omnidirectional antenna is a vertical metal rod. This whip antenna is generally a quarter of a wavelength long, the exact measurement being a compromise for the band of frequencies that is intended to be received.
A dipole antenna is twice as long, and it consists of two rods arranged end to end with the transmission line connected at the center. Optimum reception is from a point along a line perpendicular to the dipole. There are blind spots at either end. This type of antenna can mount on a swivel base and be rotated for optimum signal strength.
The dipole results in equal voltages of opposite polarities applied to a two-conductor transmission line, so that there is balanced transmission into the receiver.
More complex antenna designs are based upon the dipole concept. The object is to increase the directionality and therefore gain of the antenna, and the way this is done is by introducing additional elements. A phased array is constructed by connecting two or more uniformly spaced dipoles to an electrical network.
Log periodic dipole arrays are used in VHF reception. This type of antenna is constructed by mounting dipole elements on a support beam. These dipole elements are placed at intervals that are logarithmic functions of the frequencies they are intended to receive. Besides the spacing, the lengths of the elements also vary, so they are in a state of resonance with each frequency.
All elements are connected to the transmission line, but those that are not in resonance with the channel that is currently tuned in convey very little current and are essentially invisible to the TV receiver. The feeder that connects the dipoles to the transmission line continuously crosses over so that adjacent pairs have opposite polarity and are out of phase. In conjunction with balanced transmission, this twisted-wire approach minimizes reactive losses.
The Yagi antenna, invented in Japan in 1926, superficially resembles the log periodic dipole array, but it operates quite differently. Also used for VHF as well as UHF reception, it consists of one driven element connected to the transmission line with additional parasitic elements that have no electrical connection to the receiver. They function as resonators, picking up the signal and reradiating it for the benefit of the driven element. The Yagi antenna is characterized by very narrow bandwidth resulting in extraordinarily high gain.
A conventional antenna has elements that are usually a quarter-wavelength long at the frequency of interest. In recent years, researchers have devised so-called meta materials that can function as antennas but with physical dimensions that are much smaller than a quarter wavelength. Meta materials aren’t really materials in the usual sense of the word. They basically consist of special conductive patterns that act to produce novel properties, such as a negative permeability, unavailable in ordinary antenna elements. Meta materials and electrically small antennas comprised of them are at present a hot research topic.
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