A stack of "fishbone" and Yagi–Uda television antennas
A modern high-gain UHF Yagi television antenna with 17 directors, and one reflector (made of four rods) shaped as a corner reflector.
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
Drawing of Yagi–Uda VHF television antenna from 1954, used for analog channels 2–4, 54–72 MHz (USA channels). It has five elements: three directors (to left) one reflector (to right) and a driven element which is a folded dipole (double rod) to match the 300 Ω twin lead feedline. The beam direction (direction of greatest sensitivity) is to the left.
Electronic symbol for an antenna
Quartet of two-dipole Yagi arrays '(Hirschgeweih) of the German FuG 220 VHF-band radar on the nose of a late-World War II Bf 110 night fighter aircraft.
Antennas of the Atacama Large Millimeter/submillimeter Array.
Yagi–Uda antenna with a reflector (left), half-wave driven element (centre), and director (right). Exact spacings and element lengths vary somewhat according to specific designs.
An automobile's whip antenna, a common example of an omnidirectional antenna.
A portable Yagi–Uda antenna for use at 144 MHz (2 m), with segments of yellow tape-measure ribbon for the arms of the driven and parasitic elements.
Half-wave dipole antenna
Two Yagi–Uda antennas on a single mast. The top one includes a corner reflector and three stacked Yagis fed in phase in order to increase gain in the horizontal direction (by cancelling power radiated toward the ground or sky). The lower antenna is oriented for vertical polarization, with a much lower resonant frequency.
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
A Nakajima J1N1-S night fighter with quadruple Yagi radar transceiver antennas
Cell phone base station antennas
Close-up of Yagi arrays of the ASV Mark II radar fitted beneath a Bristol Beaufort aircraft for anti-submarine warfare.
Standing waves on a half wave dipole driven at its resonant frequency. The waves are shown graphically by bars of color ( red for voltage, V and blue for current, I ) whose width is proportional to the amplitude of the quantity at that point on the antenna.
How the antenna works. The radio waves from each element are emitted with a phase delay, so that the individual waves emitted in the forward direction (up) are in phase, while the waves in the reverse direction are out of phase. Therefore, the forward waves add together, (constructive interference) enhancing the power in that direction, while the backward waves partially cancel each other (destructive interference), thereby reducing the power emitted in that direction.
Typical center-loaded mobile CB antenna with loading coil
Illustration of forward gain of a two element Yagi–Uda array using only a driven element (left) and a director (right). The wave (green) from the driven element excites a current in the passive director which reradiates a wave (blue) having a particular phase shift (see explanation in text, note that the dimensions are not to scale with the numbers in the text). The addition of these waves (bottom) is increased in the forward direction, but leads to partial cancellation in the reverse direction.
Polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects points with 3 dB field power compared to an ISO emitter.
The wave reflected by earth can be considered as emitted by the image antenna.
The currents in an antenna appear as an image in opposite phase when reflected at grazing angles. This causes a phase reversal for waves emitted by a horizontally polarized antenna (center) but not for a vertically polarized antenna (left).
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A Yagi–Uda antenna or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles.

- Yagi–Uda antenna

A number of such dipole elements can be combined into an antenna array such as the Yagi-Uda in order to favor a single horizontal direction, thus termed a beam antenna.

- Antenna (radio)
A stack of "fishbone" and Yagi–Uda television antennas

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UHF half-wave dipole

Dipole antenna

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UHF half-wave dipole
Dipole antenna used by the radar altimeter in an airplane
Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R. The electric field ( E, green arrows ) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive  (+)  and negative  (−) .  Since the length of the antenna is one half the wavelength of the wave, the oscillating field induces standing waves of voltage ( V, represented by red band ) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance R).
Cage dipole antennas in the Ukrainian UTR-2 radio telescope. The 8 m by 1.8 m diameter galvanized steel wire dipoles have a bandwidth of 8–33 MHz.
Real (black) and imaginary (blue) parts of the dipole feedpoint impedance versus total length in wavelengths, assuming a conductor diameter of 0.001 wavelengths
Feedpoint impedance of (near-) half-wave dipoles versus electrical length in wavelengths. Black: radiation resistance; blue: reactance for 4 different values of conductor diameter
Length reduction factor for a half-wave dipole to achieve electrical resonance (purely resistive feedpoint impedance). Calculated using the Induced EMF method, an approximation that breaks down at larger conductor diameters (dashed portion of graph).
"Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna).
Collinear folded dipole array
A reflective array antenna for radar consisting of numerous dipoles fed in-phase (thus realizing a broadside array) in front of a large reflector (horizontal wires) to make it uni-directional.

In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna.

Although they may be used as standalone low-gain antennas, dipoles are also employed as driven elements in more complex antenna designs such as the Yagi antenna and driven arrays.

A Yagi antenna with one driven element (A) called a folded dipole, and 5 parasitic elements: one reflector (B) and 4 directors (C). The antenna radiates radio waves in a beam toward the right.

Passive radiator

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A Yagi antenna with one driven element (A) called a folded dipole, and 5 parasitic elements: one reflector (B) and 4 directors (C). The antenna radiates radio waves in a beam toward the right.
A Yagi antenna for UHF TV reception with 22 parasitic elements; 4 reflectors attached to the vertical bracket at left, and 18 directors attached to the horizontal beam at right. The driven element is attached to the black box next to the reflectors. The antenna is most sensitive to radio waves coming from the right, parallel to the antenna's axis.
Comparison of a Yagi with parasitic elements to a log periodic, with all active elements

In a radio antenna, a passive radiator or parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else.

Multielement antennas such as the Yagi–Uda antenna typically consist of a "driven element" which is connected to the radio receiver or transmitter through a feed line, and parasitic elements, which are not.

Patch antenna gain pattern

Directional antenna

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Patch antenna gain pattern
An early example (1922) of a directional AM radio transmitter, built for WOR, then in New Jersey and targeting both New York City and Philadelphia.
Karl Jansky and his rotating directional radio antenna (1932) in Holmdel, New Jersey, which was the world's first radio telescope, discovering radio emissions from the Milky Way.
Grote Reber's homemade antenna in Wheaton, Illinois (1937), world's second radio telescope and first parabolic radio telescope
Holmdel Horn Antenna in Holmdel, New Jersey (1960s). Built to support the Echo satellite communication program,<ref>{{cite journal |author=Crawford, A.B., D.C. Hogg and L.E. Hunt |title=Project Echo: A Horn-Reflector Antenna for Space Communication |journal=The Bell System Technical Journal |date=July 1961 |pages=1095–1099}}</ref> it was later used in experiments that revealed the cosmic background radiation permeating the universe.<ref>{{cite web |url=http://www.nps.gov/history/history/online_books/butowsky5/astro4k.htm |title=National Park Service: Astronomy and Astrophysics (Horn Antenna) |access-date=2008-05-23 |date=2001-11-05 |url-status=dead |archive-url=https://web.archive.org/web/20080512093810/http://www.nps.gov/history/history/online_books/butowsky5/astro4k.htm |archive-date=2008-05-12 }}</ref>
Parabolic antenna – the 70 m antenna at Goldstone Deep Space Communications Complex in the Mojave Desert, California
Voyager 2 spacecraft. The HGA (a parabolic antenna) is the large bowl-shaped object.
A giant phased-array radar in Alaska
A Yagi-Uda antenna. From left to right, the elements mounted on the boom are called the reflector, driven element, and director. The reflector is easily identified as being a bit (5%) longer than the driven element, and the director a bit (5%) shorter.

A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources.

The most common types are the Yagi antenna, the log-periodic antenna, and the corner reflector antenna, which are frequently combined and commercially sold as residential TV antennas.

HF's position in the electromagnetic spectrum.

High frequency

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ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz).

ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz).

HF's position in the electromagnetic spectrum.
A modern Icom M700Pro two-way radio for marine HF radio communications.
An amateur radio station incorporating two HF transceivers.
A typical Yagi antenna used by a Canadian radio amateur for long distance communication
Boeing 707 used a HF antenna mounted on top of the tail fin

At worst, when a band is "dead", no communication beyond the limited groundwave paths is possible no matter what powers, antennas or other technologies are brought to bear.

The most common antennas in this band are wire antennas such as wire dipoles or rhombic antennas; in the upper frequencies, multielement dipole antennas such as the Yagi, quad, and log-periodic antennas.

A common type of array antenna, a reflective array UHF television antenna. This example consists of eight dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174&ndash;216 MHz) and UHF (470&ndash;700 MHz) TV bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio.

Antenna array

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A common type of array antenna, a reflective array UHF television antenna. This example consists of eight dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174&ndash;216 MHz) and UHF (470&ndash;700 MHz) TV bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio.
Large planar array antenna of a VHF Russian mobile air defense radar, the Nebo-M. It consists of 175 folded dipole antennas. An early phased array, the antenna radiated a vertical fan-shaped beam which could be swept horizontally across the airspace in front of the antenna.
Animation showing how a phased array works.
A rooftop television antenna, an endfire parasitic array consisting of a combination of a Yagi and log periodic antenna
VHF collinear array of folded dipoles
Sector antennas (white bars) on cell phone tower. Collinear dipole arrays, radiating a flat, fan-shaped beam.
108 MHz reflective array antenna of an SCR-270 radar used during World War II consists of 32 half-wave dipole antennas in front of a reflecting screen.
US Air Force PAVE PAWS phased array 420 - 450 MHz radar antenna for ballistic missile detection, Alaska. The two circular arrays are each composed of 2677 crossed dipole antennas.
Some of the crossed-dipole elements in the PAVE PAWS phased array antenna, left
Batwing VHF television broadcasting antenna
Crossed-dipole FM radio broadcast antenna
Curtain array shortwave transmitting antenna, Austria. Wire dipoles suspended between towers
Turnstile antenna array used for satellite communication
Flat microstrip array antenna for satellite TV reception.
The Very Large Array, a radio telescope made of a Y-shaped array of 27 dish antennas in Socorro, New Mexico
HAARP, a phased array of 180 crossed dipoles in Alaska which can transmit a 3.6 MW beam of 3 - 10 MHz radio waves into the ionosphere for research purposes
Array of four helical antennas used as a satellite tracking antenna, Pleumeur-Bodou, France

An antenna array (or array antenna) is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves.

It is usually another name for a Yagi–Uda antenna.

UHF television antenna on a residence. This type of antenna, called a Yagi-Uda antenna, is widely used at UHF frequencies.

Ultra high frequency

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ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter).

ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter).

UHF television antenna on a residence. This type of antenna, called a Yagi-Uda antenna, is widely used at UHF frequencies.
Corner reflector UHF-TV antenna from 1950s

The length of an antenna is related to the length of the radio waves used.

High gain antennas for point-to-point communication links and UHF television reception are usually Yagi, log periodic, corner reflectors, or reflective array antennas.

Circular polarization on rubber thread, converted to linear polarization

Polarization (waves)

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Property applying to transverse waves that specifies the geometrical orientation of the oscillations.

Property applying to transverse waves that specifies the geometrical orientation of the oscillations.

Circular polarization on rubber thread, converted to linear polarization
cross linear polarized
A "vertically polarized" electromagnetic wave of wavelength λ has its electric field vector E (red) oscillating in the vertical direction. The magnetic field B (or H) is always at right angles to it (blue), and both are perpendicular to the direction of propagation (z).
Electric field oscillation
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Animation showing four different polarization states and three orthogonal projections.
A circularly polarized wave as a sum of two linearly polarized components 90° out of phase
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Color pattern of a plastic box showing stress-induced birefringence when placed in between two crossed polarizers.
Paths taken by vectors in the Poincaré sphere under birefringence. The propagation modes (rotation axes) are shown with red, blue, and yellow lines, the initial vectors by thick black lines, and the paths they take by colored ellipses (which represent circles in three dimensions).
A stack of plates at Brewster's angle to a beam reflects off a fraction of the s-polarized light at each surface, leaving (after many such plates) a mainly p-polarized beam.
Stress in plastic glasses
Photomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.
Effect of a polarizer on reflection from mud flats. In the picture on the left, the horizontally oriented polarizer preferentially transmits those reflections; rotating the polarizer by 90° (right) as one would view using polarized sunglasses blocks almost all specularly reflected sunlight.
One can test whether sunglasses are polarized by looking through two pairs, with one perpendicular to the other. If both are polarized, all light will be blocked.
The effects of a polarizing filter (right image) on the sky in a photograph
Colored fringes in the Embassy Gardens Sky Pool when viewed through a polarizer, due to stress-induced birefringence in the skylight
Circular polarization through an airplane plastic window, 1989

All radio (and microwave) antennas used for transmitting or receiving are intrinsically polarized.

So a typical rooftop Yagi or log-periodic antenna with horizontal conductors, as viewed from a second station toward the horizon, is necessarily horizontally polarized.