Directivity


Directivity
Diagram showing directivity

In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in all directions) radiating the same total power.

An antenna's directivity is a component of its gain; the other component is its (electrical) efficiency. Directivity is an important measure because most emissions are intended to go in a particular direction or at least in a particular plane (horizontal or vertical); emissions in other directions or planes are wasteful (or worse).

The directivity of an actual antenna can vary from 1.76 dBi for a short dipole, to as much as 50 dBi for a large dish antenna[1].

Contents

Definition

The directivity of an antenna is the maximum value of its directive gain. Directive gain is represented as D(θ,ϕ), and compares the radiation intensity (power per unit solid angle) U that an antenna creates in a particular direction against the average value over all directions:

D(\theta,\phi) = \frac{U}{\mbox{Total radiated power}/\left(4\pi\right)}

...where:

  • θ and ϕ are the standard spherical coordinates angles
  • U is the radiation intensity, which is the power density per unit solid angle such that:
\mbox{Total radiated power}=\int_{\phi=0}^{\phi=2\pi}\left(\int_{\theta=0}^{\theta=\pi}U\sin\theta\,d\theta\right)d\phi
  • is the total solid angle for a sphere (also the surface area of a unit sphere, similar to being the total angle for a circle and the perimeter of a unit circle).
  • The denominator, \mbox{Total radiated power}/\left(4\pi\right), represents the average radiated power density

Directivity is represented as simply D, and is the maximum directive gain value found among all possible solid angles:

D = \max\left(\frac{U}{\mbox{Total radiated power}/\left(4\pi\right)}\right)

The equation can also be written as:

D = \frac{4\pi}{\frac{\int_0^{2\pi}\int_0^{\pi}F(\theta,\phi)sin(\theta)d(\theta)d(\phi)}{F(\theta,\phi)|_{max}}}

The word directivity is also sometimes used as a synonym for directive gain. This usage is readily understood, as the direction will be specified, or directional dependence implied. Later editions of the IEEE Dictionary[2] specifically endorse this usage; nevertheless it has yet to be universally adopted.

Relation to beam width

The beam solid angle, represented as ΩA, is defined as the solid angle which all power would flow through if the antenna radiation intensity were constant and maximum value. If the beam solid angle is known, then directivity can be calculated as:

D = \frac{4\pi}{\Omega_A}

...which simply calculates the ratio of the beam solid angle to the total surface area of the sphere it intersects.

The beam solid angle can be approximated for antennas with one narrow major lobe and very negligible minor lobes, by simply multiplying the half-power beamwidths (in radians) in two perpendicular planes. The half-power beamwidth is simply the angle in which the radiation intensity is at least half of the peak radiation intensity.

The same calculations can be performed in degrees rather than in radians:

D \approx 4\pi\frac{(\frac{180}{\pi})^2}{\Theta_{1d}\Theta_{2d}} = \frac{41253}{\Theta_{1d}\Theta_{2d}}

...where Θ1d is the half-power beamwidth in one plane (degrees) and Θ2d is the half-power beamwidth in a plane at a right angle to the other (degrees).

In planar arrays, a better approximation is:

D \approx \frac{32400}{\Theta_{1d}\Theta_{2d}}

Expression in decibels

The directivity is rarely expressed as the unitless number D. Rather, the directivity is usually expressed as a decibel comparison to a reference antenna:

D_{dB} = 10 \cdot \log_{10}\left[\frac{D}{D_{reference}}\right]

The reference antenna is usually the theoretical perfect isotropic radiator which radiates uniformly in all directions and hence has a directivity of 1. The calculation is therefore simplified to:

D_{dBi} = 10 \cdot \log_{10}\left[D\right]

Another common reference antenna is the theoretical perfect half-wave dipole which radiates perpendicular to itself with a directivity of 1.64:

D_{dBd} = 10 \cdot \log_{10}\left[\frac{D}{1.64}\right]

Accounting for polarization

When polarization is taken under consideration, three additional measures can be calculated:

Partial directive gain

Partial directive gain is the power density in a particular direction and for a particular component of the polarization, divided by the average power density for all directions and all polarizations. For any pair of orthogonal polarizations (such as left-hand-circular and right-hand-circular), the individual power densities simply add to give the total power density. Thus, if expressed as dimensionless ratios rather than in dB, the total directive gain is equal to the sum of the two partial directive gains.[2]

Partial directivity

Partial directivity is calculated in the same manner as the partial directive gain, but without consideration of antenna efficiency (i.e. assuming a lossless antenna). It is similarly additive for orthogonal polarizations.

Partial gain

Partial gain is calculated in the same manner as gain, but considering only a certain polarization. It is similarly additive for orthogonal polarizations.

In other fields

The term directivity is also used in acoustics, as is a measure of the radiation pattern from a source indicating how much of the total energy from the source is radiating in a particular direction. In electro-acoustics, these patterns commonly include omni-directional, cardioid and hyper-cardioid microphone polar patterns. A loudspeaker with a high degree of directivity (narrow dispersion pattern) can be said to have a high Q.[3]

References

  1. ^ Antenna Tutorial
  2. ^ a b Institute of Electrical and Electronics Engineers, “The IEEE standard dictionary of electrical and electronics terms”; 6th ed. New York, N.Y., Institute of Electrical and Electronics Engineers, c1997. IEEE Std 100-1996. ISBN 1-55937-833-6 [ed. Standards Coordinating Committee 10, Terms and Definitions; Jane Radatz, (chair)]
  3. ^ Rane Professional Audio Reference Home
  • Coleman, Christopher (2004). "Basic Concepts". An Introduction to Radio Frequency Engineering. Cambridge University Press. ISBN 0-521-83481-3. 

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Look at other dictionaries:

  • directivity — n. the property of a microphone or antenna of being more sensitive to sounds coming from one direction than from another. Syn: directionality. [WordNet 1.5] …   The Collaborative International Dictionary of English

  • directivity — kryptingumas statusas T sritis fizika atitikmenys: angl. directivity vok. Richtfähigkeit, f; Richtwirkung, f rus. направленность, f pranc. directivité, f …   Fizikos terminų žodynas

  • directivity — noun 1. the property of a microphone or antenna of being more sensitive in one direction than in another (Freq. 1) the antenna received signals with no directivity • Syn: ↑directionality • Derivationally related forms: ↑directional (for: ↑ …   Useful english dictionary

  • directivity — noun Date: 1928 the property of being directional …   New Collegiate Dictionary

  • directivity — noun The effect of earthquake motion propagation being greater in the direction of the rupture …   Wiktionary

  • directivity — n. property of being directional (of a microphone or antenna) …   English contemporary dictionary

  • directivity — di·rec·tiv·i·ty …   English syllables

  • directivity pattern — kryptinė diagrama statusas T sritis automatika atitikmenys: angl. directional pattern; directivity pattern vok. Richtdiagramm, n rus. диаграмма направленности, f pranc. diagramme de rayonnement, m; diagramme directionnel, m …   Automatikos terminų žodynas

  • directivity diagram of laser radiation in free space — kryptinė lazerio spinduliuotės diagrama laisvojoje erdvėje statusas T sritis radioelektronika atitikmenys: angl. directivity diagram of laser radiation in free space vok. Freiraumlaserrichtdiagramm, n; Laserrichtdiagramm im freien Raum, n rus.… …   Radioelektronikos terminų žodynas

  • antenna directivity diagram — kryptinė antenos diagrama statusas T sritis automatika atitikmenys: angl. antenna directivity diagram vok. Antennenrichtcharakteristik, f; Antennenrichtdiagramm, n rus. диаграмма направленности антенны, f pranc. diagramme de directivité d antenne …   Automatikos terminų žodynas


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