study of antenna array - kuet

Study of Antenna Array

Dr. Md. Mostafizur Rahman

Professor

Department of Electronics and Communication Engineering (ECE)

Khulna University of Engineering & Technology (KUET)

Arrays of Antenna

Usually the radiation pattern of a single element is relatively wide, and each element

provides low values of directivity (gain). In many applications it is necessary to design

antennas with very directive characteristics (high gains) to meet the demands of long

distance communication. This can only be accomplished by increasing the electrical

size of the antenna. For increasing the electrical size of the antenna following systems

can be used ;

Enlarging the dimensions of single elements often leads to more directive

characteristics.

Is to form an assembly of radiating elements in electrical and geometrical

configuration.

This new antenna , formed by multi elements, is referred to as an array.

In an array of identical elements, there are at least five controls that can be used to

shape the overall pattern of the antenna. These are :

The geometrical configuration of the overall array (linear, circular, rectangular,

spherical etc.)

The relative displacement between the elements

The excitation amplitude of the individual elements

The excitation phase of the individual elements

The relative pattern of the individual elements

Individual antennas of an antenna array system is also termed as Elements.

Also a uniform linear array is one, in which the elements are fed with a current of equal

magnitude with uniform progressive phase shift along the line.

“Phase” in an antenna arrays and ordinary circuit has same meaning i.e. two currents in

two elements are said to be in phase if they reach their maximum values, flowing in the

same direction at the same instant.

Antenna Array is one of the common method of combining the radiations from a

group or array of similar antennas in which the phenomena of wave-interference is

involved. The total field (not power) produced by an antenna array system at a great

distance from it, is the vector sum of the fields produced by the individual antennas

of the array system. The antenna arrays are used for the improvement of gain and

directivity.

An antenna array is said to be linear, if the individual antennas of the array are equally

spaced along a straight line.

Arrays of Antenna

Various Forms of Antenna Arrays

There are different types of Antenna Array Such as ;

Broadside Array• Number of identical parallel elements are set up along a line drawn perpendicular to their respective

axes.

• Individual elements are equally spaced along a line and each element is fed with current of equal

magnitude.

• Dipole elements are fed in the same phase and separated by ½ wavelength.

• This antenna produces a highly directional radiation pattern that is broadside or perpendicular to the

array axis or to the plane of the array.

• This broadside antenna is bidirectional in radiation, but the radiation pattern has a very narrow

beam width and high gain.

• Bidirectional pattern of a broadside array can be converted into unidirectional by installing

an identical array behind this array at a distance λ/4 and exciting it by current leading in

phase by 90 degree or π/2 radian.


End Fire Array

• Similar to broadside array except dipoles are fed 180 degrees out of phase

• Radiation pattern is maximum off the ends that is parallel to array axis.

• Elements (half wave length dipoles) are spaced one half wavelength apart.

• Radiation pattern is bidirectional, with narrower beam widths and high gain.

• A highly unidirectional antenna can be created by careful selection of the

optimal number of elements with the appropriately related spacing.

Collinear Array

Parasitic Array


The figure illustrates the radiation pattern of end-fire array and broad side array.

•Both, the end fire array and broad side array, are linear and are resonant, as they consist of resonant elements.

•Due to resonance, both the arrays display narrower beam and high directivity.

•Both of these arrays are used in transmission purposes.

•Neither of them is used for reception, because the necessity of covering a range of frequencies is needed for

any kind of reception.

End-fire Array VS Broad Side Array


A parasitic element is an element, which depends on other’s feed. It does not have its

own feed. Hence, in this type of arrays we employ such elements, which help in increasing the

radiation indirectly.

The elements, which are added do not possess an electrical connection between them to

the driven element or the feed. They are positioned so that they lie in the induction field of the driven

element. Hence, they are known as parasitic elements. These parasitic elements are not directly

connected to the feed.

Construction & Working of Parasitic Array

Let us look at the important parts of a Parasitic array and how they work. The main parts are;

• Driven element

• Parasitic elements

• Reflector

• Director

• Boom

Parasitic Array


Driven elementThe antennas radiate individually and while in array, the radiation of all the elements sum up to form the

radiation beam. All the elements of the array need not be connected to the feed. The dipole that is

connected to the feed is known as a driven element.

ReflectorIf one of the parasitic element, which is 5% longer than driven element, is placed close to the driven

element is longer, then it acts as a concave mirror, which reflects the energy in the direction of the

radiation pattern rather than its own direction and hence is known as a reflector.

DirectorA parasitic element, which is 5% shorter than the driven element, from which it receives energy, tends to

increase radiation in its own direction and therefore, behaves like convergent convex lens. This element

is called as a director. A number of directors are placed to increase the directivity.

BoomThe element on which all these are placed is called a boom. It is a non-metallic structure which provides

insulation, so that there will not be any short circuit between the other elements of the array.

These are all the main elements, which contribute the radiation. This can be better understood with the

help of a diagram


Collinear Array

A Collinear array consists of two or more half-wave dipoles, which are placed end to end. These

antennas are placed on a common line or axis, being parallel or collinear.

The maximum radiation in these arrays is broad side and perpendicular to the line of array. These arrays

are also called as broad cast or Omni-directional arrays.

Frequency range

The frequency range in which the collinear array antennas operate is around 30 MHz to 3GHz which

belong to the VHF and UHF bands

Construction of Array

These collinear arrays are uni-directional antennas having high gain. The main purpose of this array is

to increase the power radiated and to provide high directional beam, by avoiding power loss in other

directions.

The above images show the pictures of collinear arrays. In figure 1, it is seen that collinear array is

formed using folded dipoles, while in figure 2, the collinear array is formed by normal dipoles. Both

types are half-wave dipoles used commonly.


The radiation pattern of these collinear arrays is similar to that of a single dipole, but the array pattern

of increasing number of dipoles, makes the difference.

The radiation pattern of collinear array when made using two elements, three elements and four

elements respectively are shown in the figure given below.

The broad side array also has the same pattern, in which the direction of maximum radiation is

perpendicular to the line of antenna.

Advantages

• Use of array reduces the broad ends and increases the directivity

• Minor lobes are minimized

• Wastage of power is reduced

Disadvantages

• Displacement of these antennas is a difficult task

• Used only in outdoor areas

Applications

• Used for VHF and UHF bands

• Used in two-way communications

• Used also for broadcasting

purposes

Radiation Pattern

References Antenna Theory by Constantine A Balani

Antenna & Wave Propagation by K D Prasad

Antenna Theory (TutorialsPoint)

https://www.tutorialspoint.com/antenna_theory/antenna_theory_broad_si

de_array.htm

study of antenna array - kuet

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