A stack of "fishbone" and Yagi–Uda television antennas
Animated diagram of waves from an isotropic radiator (red dot). As they travel away from the source, the waves decrease in amplitude by the inverse of distance, shown by the declining contrast of the wavefronts. This diagram only shows the waves in one plane through the source; an isotropic source actually radiates in all three dimensions.
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 depiction of an isotropic radiator of sound, published in Popular Science Monthly in 1878. Note how the rings are even and of the same width all the way around each circle, though they fade as they move away from the source.
Electronic symbol for an antenna
Diagram of antenna and resistor in cavity
Antennas of the Atacama Large Millimeter/submillimeter Array.
An automobile's whip antenna, a common example of an omnidirectional antenna.
Half-wave dipole antenna
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).
frame

Isotropic radiators are used as reference radiators with which other sources are compared, for example in determining the gain of antennas.

- Isotropic radiator

A hypothetical antenna that radiates equally in all vertical as well as all horizontal angles is called an isotropic radiator however these cannot exist in practice nor would they be particularly desired.

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

5 related topics with Alpha

Overall

UHF half-wave dipole

Dipole antenna

3 links

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.

A turnstile antenna comprises two dipoles crossed at a right angle and feed system which introduces a quarter-wave phase difference between the currents along the two. With that geometry, the two dipoles do not interact electrically but their fields add in the far-field producing a net radiation pattern which is rather close to isotropic, with horizontal polarization in the plane of the elements and circular or elliptical polarization at other angles. Turnstile antennas can be stacked and fed in phase to realize an omnidirectional broadside array or phased for an end-fire array with circular polarization.

Example of omnidirectional antenna; a whip antenna on a walkie-talkie

Omnidirectional antenna

2 links

Example of omnidirectional antenna; a whip antenna on a walkie-talkie
Radiation pattern of a 3λ/2 monopole antenna. Although the radiation of an omnidirectional antenna is symmetrical in azimuthal directions, it may vary in a complicated way with elevation angle, having lobes and nulls at different angles.
Vertical polarized VHF-UHF biconical antenna 170–1100 MHz with omnidirectional H-plane pattern

In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis (azimuthal directions), with power varying with angle to the axis (elevation angle), declining to zero on the axis.

Note that this is different from an isotropic antenna, which radiates equal power in all directions, having a spherical radiation pattern.

Three-dimensional antenna radiation patterns. The radial distance from the origin in any direction represents the strength of radiation emitted in that direction. The top shows the directive pattern of a horn antenna, the bottom shows the omnidirectional pattern of a simple vertical antenna.

Radiation pattern

2 links

Three-dimensional antenna radiation patterns. The radial distance from the origin in any direction represents the strength of radiation emitted in that direction. The top shows the directive pattern of a horn antenna, the bottom shows the omnidirectional pattern of a simple vertical antenna.
Typical polar radiation plot. Most antennas show a pattern of "lobes" or maxima of radiation. In a directive antenna, shown here, the largest lobe, in the desired direction of propagation, is called the "main lobe".  The other lobes are called "sidelobes" and usually represent radiation in unwanted directions.
A rectangular radiation plot, an alternative presentation method to a polar plot.

In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.

Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.

Examples of sound levels in decibels from various sound sources and activities, taken from the "How loud is too loud" screen of the NIOSH Sound Level Meter app

Decibel

1 links

Relative unit of measurement equal to one tenth of a bel (B).

Relative unit of measurement equal to one tenth of a bel (B).

Examples of sound levels in decibels from various sound sources and activities, taken from the "How loud is too loud" screen of the NIOSH Sound Level Meter app
A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)

dBi: dB(isotropic) – the gain of an antenna compared with the gain of a theoretical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

dBd: dB(dipole) – the gain of an antenna compared with the gain a half-wave dipole antenna. 0 dBd = 2.15 dBi

Diagram showing directivity: the highest power density of this antenna is in the direction of the red lobe

Directivity

0 links

Diagram showing directivity: the highest power density of this antenna is in the direction of the red lobe

In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction.

Therefore, the directivity of a hypothetical isotropic radiator is 1, or 0 dBi.