A stack of "fishbone" and Yagi–Uda television antennas
Coherent waves that travel along two different paths will arrive with phase shift, hence interfering with each other.
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.
A diagram of the ideal situation for TV signals moving through space: The signal leaves the transmitter (TX) and travels through one path to the receiver (the TV set, which is labeled RX)
Electronic symbol for an antenna
In this illustration, an object (in this case an aircraft) pollutes the system by adding a second path. The signal arrives at receiver (RX) by means 
of two different paths which have different lengths. The main path is the direct path, while the second is due to a reflection from the plane.
Antennas of the Atacama Large Millimeter/submillimeter Array.
Radar multipath echoes from an actual target cause ghosts to appear.
An automobile's whip antenna, a common example of an omnidirectional antenna.
GPS error due to multipath
Half-wave dipole antenna
Mathematical model of the multipath impulse response.
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
Cell phone base station antennas
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.
Typical center-loaded mobile CB antenna with loading coil
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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In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.

- Multipath propagation

On the other hand, analog television transmissions are usually horizontally polarized, because in urban areas buildings can reflect the electromagnetic waves and create ghost images due to multipath propagation.

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

4 related topics with Alpha

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A portable battery-powered AM/FM broadcast receiver, used to listen to audio broadcast by local radio stations.

Radio receiver

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Electronic device that receives radio waves and converts the information carried by them to a usable form.

Electronic device that receives radio waves and converts the information carried by them to a usable form.

A portable battery-powered AM/FM broadcast receiver, used to listen to audio broadcast by local radio stations.
A modern communications receiver, used in two-way radio communication stations to talk with remote locations by shortwave radio.
Girl listening to vacuum tube radio in the 1940s. During the golden age of radio, 1925–1955, families gathered to listen to the home radio receiver in the evening
A bedside clock radio that combines a radio receiver with an alarm clock
Symbol for an antenna
Symbol for a bandpass filter used in block diagrams of radio receivers
Symbol for an amplifier
Symbol for a demodulator
Envelope detector circuit
How an envelope detector works
Block diagram of a tuned radio frequency receiver. To achieve enough selectivity to reject stations on adjacent frequencies, multiple cascaded bandpass filter stages had to be used. The dotted line indicates that the bandpass filters must be tuned together.
Block diagram of a superheterodyne receiver. The dotted line indicates that the RF filter and local oscillator must be tuned in tandem.
Block diagram of a dual-conversion superheterodyne receiver
Guglielmo Marconi, who built the first radio receivers, with his early spark transmitter (right) and coherer receiver (left) from the 1890s. The receiver records the Morse code on paper tape
Generic block diagram of an unamplified radio receiver from the wireless telegraphy era
Example of transatlantic radiotelegraph message recorded on paper tape by a siphon recorder at RCA's New York receiving center in 1920. The translation of the Morse code is given below the tape.
Coherer from 1904 as developed by Marconi.
Experiment to use human brain as a radio wave detector, 1902
Magnetic detector
Electrolytic detector
A galena cat's whisker detector from a 1920s crystal radio
Marconi's inductively coupled coherer receiver from his controversial April 1900 "four circuit" patent no. 7,777.
Radio receiver with Poulsen "tikker" consisting of a commutator disk turned by a motor to interrupt the carrier.
Fessenden's heterodyne radio receiver circuit
Unlike today, when almost all radios use a variation of the superheterodyne design, during the 1920s vacuum tube radios used a variety of competing circuits.
During the "Golden Age of Radio" (1920 to 1950), families gathered to listen to the home radio in the evening, such as this Zenith console model 12-S-568 from 1938, a 12-tube superheterodyne with pushbutton tuning and 12-inch cone speaker.
De Forest's first commercial Audion receiver, the RJ6 which came out in 1914. The Audion tube was always mounted upside down, with its delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.
Block diagram of regenerative receiver
Circuit of single tube Armstrong regenerative receiver
Armstrong presenting his superregenerative receiver, June 28, 1922, Columbia University
Hazeltine's prototype Neutrodyne receiver, presented at a March 2, 1923 meeting of the Radio Society of America at Columbia University.
Block diagram of simple single tube reflex receiver
The first superheterodyne receiver built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the mixer and local oscillator (left) and three IF amplification stages and a detector stage (right). The intermediate frequency was 75 kHz.
A Zenith transistor based portable radio receiver
A modern smartphone has several RF CMOS digital radio transmitters and receivers to connect to different devices, including a cellular receiver, wireless modem, Bluetooth modem, and GPS receiver.

It is used with an antenna.

The strength of the signal received from a given transmitter varies with time due to changing propagation conditions of the path through which the radio wave passes, such as multipath interference; this is called fading.

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.

Radio wave

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Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below.

Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below.

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.
Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopole radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular, as implied by the phase diagram in the lower right.
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).

Radio waves are generated artificially by an electronic device called a transmitter, which is connected to an antenna which radiates the waves.

Indirect propagation: Radio waves can reach points beyond the line-of-sight by diffraction and reflection. Diffraction causes radio waves to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also partially reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as cell phones, cordless phones, walkie-talkies, and wireless networks. A drawback of this mode is multipath propagation, in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves interfere, often causing fading and other reception problems.

Experimental radar antenna, US Naval Research Laboratory, Anacostia, D. C., from the late 1930s (photo taken in 1945).

Radar

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Detection system that uses radio waves to determine the distance (ranging), angle, and radial velocity of objects relative to the site.

Detection system that uses radio waves to determine the distance (ranging), angle, and radial velocity of objects relative to the site.

Experimental radar antenna, US Naval Research Laboratory, Anacostia, D. C., from the late 1930s (photo taken in 1945).
The first workable unit built by Robert Watson-Watt and his team
A Chain Home tower in Great Baddow, Essex, United Kingdom
Memorial plaque commemorating Robert Watson-Watt and Arnold Wilkins
Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.
3D Doppler Radar Spectrum showing a Barker Code of 13
Brightness can indicate reflectivity as in this 1960 weather radar image (of Hurricane Abby). The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.
Change of wavelength caused by motion of the source.
Radar multipath echoes from a target cause ghosts to appear.
Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is proportional to this time.
Continuous wave (CW) radar. Using frequency modulation allows range to be extracted.
Pulse-Doppler signal processing. The Range Sample axis represents individual samples taken in between each transmit pulse. The Range Interval axis represents each successive transmit pulse interval during which samples are taken. The Fast Fourier Transform process converts time-domain samples into frequency domain spectra. This is sometimes called the bed of nails.
Radar components
AS-3263/SPS-49(V) antenna (US Navy)
Surveillance radar antenna
Slotted waveguide antenna
Phased array: Not all radar antennas must rotate to scan the sky.

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects.

The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment.

Line of sight propagation to an antenna

Line-of-sight propagation

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Characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver.

Characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver.

Line of sight propagation to an antenna
Objects within the Fresnel zone can disturb line of sight propagation even if they don't block the geometric line between antennas.
Two stations not in line-of-sight may be able to communicate through an intermediate radio repeater station.
R is the radius of the Earth, h is the height of the transmitter (exaggerated), d is the line of sight distance

multipath reflection along the street

The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth.