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- Definition : A mathematical function or a graphical representation of the radiation properties of the
antenna as a function of space coordinates.
- Field pattern : E & H fields’
magnitude (Linear scale. Angular
Space)
- Power pattern : Square of
magnitude’s of E&H fields (Linear
scale. Angular Space)
- Power pattern : Magnitude of E &
H fields (dB scale)
RADIATION PATTERN
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- Dividing a field component by its maximum value, we obtain a normalized sphere field pattern which is a
dimensionless number with maximum value of unity. Thus the normalized field pattern for the component
of the electric field is given by
• Patterns may also be expressed in terms of the power per unit area or Poynting vector as S (θ,Φ) at a
certain distance from the antenna. Normalizing this power with respect to its maximum value yields a
normalized power pattern as a function of angle which is a dimensionless number with a maximum
value of unity
RADIATION PATTERN
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• Major lobe (main beam) : The radiation lobe containing the direction of
maximum radiation.
• Minor lobe : any lobe except a major lobe.
• Side lobe : The radiation lobe in any direction other than the intended
lobe.
(Usually a side lobe is adjacent to the main lobe and occupies the hemisphere
in the direction of the main beam.)
• Back lobe : The radiation lobe whose axis makes an angle of
approximately 180◦ with respect to the beam of an antenna. Usually it
refers to a minor lobe that occupies the hemisphere in a direction opposite
to that of the major(main) lobe.
• Minor lobes usually represent radiation in undesired directions, and they
should be minimized.
• Side lobes are normally the largest of the minor lobes.
• The level of minor lobes is usually expressed as a ratio of the power
density in the lobe in question to that of the major lobe. This ratio is often
termed the side lobe ratio or side lobe level. It is an indication of
directivity of antenna.
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- Radiation Pattern Lobes
(a) Radiation lobes and beamwidths of an antenna pattern.
(b) Linear plot of power pattern and its associated lobes and beamwidths
10. Antennas & RF Devices Lab.
Isotropic antenna pattern
Directional antenna pattern
- Having the property of radiating or receiving
electro-magnetic wave more effectively in
some directions than in others.
- This term usually applied to an antenna
whose maximum directivity is significantly
greater than that of a half-wave dipole
Omnidirectional antenna pattern
- Having an essentially non directional pattern in a given plane
- Having a directional pattern in any orthogonal plane. Radiation pattern
may be in XY,YZ, XZ plane
- A Special type of a directional pattern.
2.2.2 Isotropic, Directional, and Omnidirectional Patterns
Isotropic antenna pattern
- A hypothetical lossless antenna pattern having
equal radiation in all directions.
- Ideal, not physically realizable.
- Often taken as a reference for expressing the
directive properties of actual antennas.
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Radian and Steradian
Radian measures plane
angles (2D), while steradian
measures solid angles (3D).
Steradian: solid angle
subtended at the center
of a sphere by an area of
its surface equal to the
square of the radius of
the sphere
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Radiation Power Density
Poynting Vector
• The quantity used to describe the power associated with an electromagnetic wave is the instantaneous
Poynting vector defined as
• The total power crossing a closed surface
Average Power Density
• For applications of time-varying fields, it is desirable to find the average power density.
• The average power density is obtained by integrating the instantaneous Poynting vector over
one period and dividing by the period.
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Radiation Power Density
Average Radiated Power
• The power density associated with the electromagnetic fields of an antenna in its far-field region is
predominately real and will be referred to as radiation density.
• The average power radiated by an antenna (radiated power) can be written as
Power Pattern Versus Average Radiated Power
• The power pattern of the antenna is the average power density radiated by the antenna as a function of
the direction.
• The observations are usually made on a large sphere of constant radius extending into the far field.
14. 14
It is the power radiated from an antenna per unit solid angle(that is it is the power
per steradian)
• We have Sr =
• = (watts/Sr),
• It can be also obtained by simply multiplying the radiation density by thesquare
of the distance
• Total Radiated Power Using Radiation Intensity: The total power is obtained by
integrating the radiation intensity over the entire solid angle of 4π.
Radiation Intensity
15. Beamwidth - The angular separation between two identical points on opposite side the
pattern maximum.
The beamwidth of an antenna is a very important figure of merit and often is used as a
trade-off between it and the side lobe level; that is, as the beamwidth decreases, the side
lobe increases and vice versa.
The beamwidth of the antenna is also used to describe the resolution capabilities of the
antenna to distinguish between two adjacent radiating sources or radar targets.
Half-Power Beam Width (HPBW). In a plane containing the direction of the maximum of a
beam, the angle between the two directions in which the radiation intensity is one-half
value of the beam.
First-Null Beamwidth (FNBW ). Angular separation between the first nulls of the pattern.
Beamwidth
17. Beam solid angle is defined as a solid angle at which the total power is radiated or received
by an antenna.
Unit of Beam solid angle is Steradians (Sr)
BEAM AREA (or BEAM SOLID ANGLE)
ϴ φ
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The ratio of the radiation intensity in a given direction from the antenna to the radiation intensity
averaged over all directions.
Directivity D
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The beam solid angle ΩA is defined as the solid angle through which all the power of the antenna would
flow if its radiation intensity is constant (and equal to the maximum value of U) for all angles within ΩA.
Directivity D
21. The ability of an antenna or antenna system to concentrate the
radiated power in a given direction .
The ratio of the gain to the directivity is the antenna efficiency factor.
G= k D
Where k= efficiency factor
Gain G
22. Antenna Apertures
• The concept of aperture is most simply introduced by considering a receiving antenna, to describe the
power capturing characteristics of the antenna when a wave imposes on it.
• Suppose that the antenna is an electromagnetic horn immersed in the field of a uniform plane as
suggested in Fig.
• Let the Poynting vector, or power density, of the plane wave be S watts per square meter and the area
of the mouth of the horn be A square meters.
• If the horn extracts all the power from the wave over its
entire area A , then the total power P absorbed from the
wave is
• Thus, the electromagnetic horn may be regarded as a aperture, the total power it extracts from a
passing wave being proportional to the aperture or area of its mouth
