Antenna (radio)

• 1. Introduction

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  In 1886, a German physicist named Heinrich Hertz built the first antennas, placing dipole elements at the focal point of parabolic reflectors. He was trying to prove that something invisible was real. James Clerk Maxwell had predicted electromagnetic waves in his 1867 theory, but no one had yet caught them in the act. Hertz used his crude structures to transmit and receive those waves, and the proof held. An antenna, in the language of radio-frequency engineering, is the structure that converts alternating electric currents into radio waves, and then converts those waves back into currents. It is the interface between two worlds: energy moving as waves through space, and energy moving as electrons in metal. Every radio receiver and transmitter needs one. But how does a piece of metal persuade an electric current to leap off into empty space? Why must some antennas be aimed with care while others can sit on a table pointing nowhere in particular? And why does the ground beneath an antenna matter as much as the antenna itself? The answers run from a tent pole in Italy to the physics of standing waves trapped inside a wire.

• 2. A Tent Pole In Bologna

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  In the summer of 1895, Guglielmo Marconi began testing his wireless system outdoors on his father's estate near Bologna. He soon started experimenting with long wire aerials suspended from a pole. In Italian, a tent pole is called l'antenna centrale, and the pole carrying the wire was simply called l'antenna. The name stuck to the device rather than the support. Until that moment, the radiating and receiving elements of wireless apparatus had no special name. Engineers called them terminals. Because of Marconi's prominence, his casual word spread first among wireless researchers and enthusiasts, then to the general public. The word travels with a regional split. American English keeps antenna, while British English often says aerial, and the two are used interchangeably. Occasionally aerial means something narrower: an elevated horizontal wire. The same year his vocabulary took hold, Marconi began developing antennas practical for long-distance wireless telegraphy. By 1898 he had opened a factory in Chelmsford, England, to manufacture his invention.

• 3. The Trapped Wave

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  The majority of antenna designs rest on a single idea: resonance. Inside a metal element, moving electrons reflect off the surfaces where the dielectric constant changes, much as light reflects when optical properties shift. The end of a conductor is one such surface. When a radio signal's electric field induces a voltage in the wire, a current starts flowing, and when that current reaches the open end, it reflects, which amounts to a 180 degree change in phase. If the conductor is a quarter wavelength long, the current shifts 90 degrees on the way out, flips 180 degrees at the end, then shifts another 90 degrees coming back. The total is a full 360 degrees, returning it in step with the source. The current adds to itself instant by instant, building a standing wave with maximum current at the feed point. This is why a resonant antenna's element is about a quarter of the wavelength, or an odd multiple of that. The same element will also resonate when fed at three times its design frequency, so antennas can be operated harmonically. The half-wave dipole, two such elements arranged end-to-end along one axis, is probably the most widely used antenna design of all. Its physical arrangement places the two halves 180 degrees out of phase, so one element drives current into the line while the other pulls it out.

• 4. The Mismatch Problem

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  Consider a half-wave dipole built for signals with a wavelength of one meter, roughly 50 centimeters from tip to tip. If its length-to-diameter ratio is 1000, the element has an inherent impedance of about 63 ohms, purely resistive. Feed it one ampere and it demands 63 volts, radiating 63 watts of radio frequency power once losses are set aside. Now feed the same antenna a signal at 1.25 meters. The induced current arrives at the feed point out of phase, the net current drops while the voltage holds, and electrically the antenna now looks like a very high impedance. The antenna and its transmission line no longer match, so the signal reflects backward into the antenna and the output falls. The mismatch on a feedline is measured by a parameter called the standing wave ratio, or SWR. Because a current reflects wherever the electrical properties change, the junction between feedline and antenna must present the same impedance on both sides, or power bounces back toward the transmitter. The cure is impedance matching, often handled by a circuit called an antenna tuner or transmatch placed between transmitter and antenna. This narrow tolerance is why resonant designs are inherently narrow-band, useful only over a small range of frequencies clustered around resonance. A true antenna tuner does more than cancel reactance; it transforms the leftover resistance to match both the feedline and the radio, using at least two adjustable elements.

• 5. Shrinking The Antenna

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  At 30 megahertz, a true quarter-wave monopole stands almost 2.5 meters tall. Many installations cannot spare that height, so engineers shorten the element to 1.5 meters and add a loading coil. As a monopole or dipole is cut shorter for a given frequency, its impedance becomes dominated by a series capacitive reactance, a negative quantity. A loading coil supplies an equal and opposite inductive reactance, cancelling the capacitance and leaving a pure resistance. The coil is then said to have lengthened the antenna to an electrical length of 2.5 meters, even though it is physically shorter. The trick has a cost. The resistive impedance that remains is much lower than a true quarter-wave monopole would offer, often requiring a further transformer to reach the desired transmission line. As antennas grow shorter still, the radiation resistance plummets roughly as the square of the length, and the system behaves like a resonant circuit with a higher Q factor and a narrower bandwidth. That bandwidth can shrink until it no longer covers the transmitted signal's spectrum. Bandwidth, more than anything else, sets the size of antennas at one megahertz and below. The most familiar shrunken antenna sits inside portable AM receivers: a loopstick, a loop of wire wound around a ferrite rod, resonated by a tuning capacitor that tracks the receiver's dial across the broadcast band.

• 6. Borrowing From Light

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  When a radio wave hits a large conducting sheet, it reflects with the phase of its electric field reversed, exactly as a mirror reflects light. Placing such a reflector behind an otherwise non-directional antenna redirects the power that would have escaped backward, raising the gain by a factor of at least two. A corner reflector can squeeze all of an antenna's power into a single quadrant of space or less. The reflector need not be solid metal; a curtain of rods aligned with the antenna's polarization works just as well while cutting weight and wind load. The same optical thinking produces the parabolic reflector, whose curved surface focuses an incoming wave onto a feed antenna at its focal point, yielding an effective area close to the size of the dish itself. Other antennas exploit passive elements rather than mirrors. A Yagi-Uda array places several parallel half-wave elements of very specific lengths along a boom that serves only for support. Just one element connects to the transmitter; the rest are passive radiators that reshape the field, producing large gain in one direction at the expense of a limited bandwidth. The log-periodic dipole array looks similar but operates on an opposite principle. Every element connects electrically to its neighbor with a phase reversal, and the self-similar geometry holds gain and impedance steady across a very wide bandwidth, which is why these serve as rooftop television antennas.

• 7. Two Ways To Measure A Beam

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  Antenna gain measures how sharply an antenna concentrates its radiated power, and unlike amplifier gain it adds no power at all. An amplifier increases total signal power; an antenna only redistributes a fixed total, sending more energy in some directions and less in others. Antennas are passive devices that conserve power. Gain is expressed two ways. One compares the antenna to a hypothetical isotropic radiator that emits equally in every direction, giving units of decibels-isotropic, or dBi. The other compares it to a half-wave dipole, giving decibels-dipole, or dBd. Because a half-wave dipole already has a gain of 2.15 dBi, any figure in dBi runs exactly 2.15 decibels above the same figure in dBd. A high-gain antenna, such as a parabolic satellite television dish, reaches farther and delivers cleaner signal, but it must be aimed carefully at its target. A low-gain antenna, such as the whip on a portable radio or cordless phone, covers a wide angle, so its orientation hardly matters. Gain is bound to effective area, the patch of a passing wave's power an antenna can capture. A half-wave dipole has an effective area of about 0.13 wavelengths squared when viewed broadside. By reciprocity, an antenna's transmitting gain in any direction is proportional to its receiving effective area in that same direction, so the two figures describe one quantity from two sides.

• 8. The Ground Below

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  Ground reflection is one of the common forms of multipath, and for a terrestrial antenna the earth is often the most important object nearby. The antenna's height above ground, along with the ground's permittivity and conductivity, shapes both the radiation pattern and the driving point impedance. When a wave strikes the ground, part passes into the soil and part reflects according to the Fresnel coefficients. A very good conductor reflects almost all of it, flipped 180 degrees in phase, while a lossy dielectric ground absorbs a large share of the power. The behavior changes with frequency. From very low frequencies up to about 30 megahertz the ground acts as a lossy dielectric, though at lower mediumwave frequencies it behaves as a good conductor, which AM broadcast antennas between 0.5 and 1.7 megahertz depend on. Between 3 and 30 megahertz, much of a horizontally polarized antenna's energy reflects off the ground, with nearly total reflection at the grazing angles that matter for ground wave propagation. That phase-reversed reflection can cancel or reinforce the direct wave depending on antenna height and elevation angle. Vertical polarization behaves differently. The ground reflects it poorly except at grazing incidence or over highly conducting surfaces like sea water, yet at the grazing angle the reflection arrives in phase with the direct wave, providing a boost of up to 6 dB. Above 30 megahertz the ground turns into a poorer reflector overall. When its surface irregularities stay much smaller than the wavelength, it reflects specularly, and a receiver effectively sees both the real antenna and a mirror image of it buried beneath the soil.