Chapter 3 Antenna Types Part 1.

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Chapter 3 Antenna Types Part 1 Table of Contents Antenna Arrays
Helical Antenna
Microstrip Patch Antenna / Slot Antenna Horn Antennas Reflector
Antennas Emerging Antenna Technologies 4 weeks 3.1 Antenna Arrays A
KaBand Array Antenna
The antennas we have studied so far have all been omnidirectional
no variation in . A properly spaced collection of antennas, can
have significant variation in leading to dramatic improvements in
directivity. A KaBand Array Antenna Antenna Arrays (Contd..)
An antenna array can be designed to give a particular shape of
radiating pattern. Control of the phase and current driving each
array element along with spacing of array elements can provide beam
steering capability. For simplification: All antenna elements are
identical The current amplitude is the same feeding each element.
The radiation pattern lies only in xy plane, =/2 The radiation
pattern then can be controlled by: controlling the spacing between
elements or controlling the phase of current driving for each
element Antenna Arrays (Contd..)
For simple example, consider a pair of dipole antennas driven in
phase current source and separated by /2 on the z axis. z z Assume
each antenna radiates independently, at far field point P, the
fields from 2 antennas will be 180 outofphase, owing to extra /2
distance travel by the wave from the farthest antenna fields cancel
in this direction. At point Q, the fields in phase and adds. The E
field is then twice from single dipole, fourfold increase in power
broadside array max radiation is directed broadside to axis of
elements. Antenna Arrays (Contd..)
z Modify with driving the pair of dipoles with current sources 180
out of phase. Then along z axis will be in phase and along y axis
will be out of phase, as shown by the resulting beam pattern
endfire array max radiation is directed at the ends of axis
containing array elements. Antenna Arrays (Contd..)
Pair of Hertzian Dipoles Recall that the far field value of E field
from Hertzian dipole at origin, Slide 52 of Chap2 P3 But confining
our discussion to the yz plane where phi = /2, Antenna Arrays
(Contd..)
Consider a pair of z oriented Hertzian dipole, with distance d,
where the total field is the vector sum of the fields for both
dipoles The magnitude of currents are the same with a phase shift
between them. Where, Antenna Arrays (Contd..)
For amplitude variation, For phase variation, Antenna Arrays
(Contd..)
Thus, the total E field becomes: With Eulers identity, the total E
field at far field observation point from two element Hertzian
dipole array becomes : array factor Antenna Arrays (Contd..)
Antenna Arrays (Contd..)
To find radiated power, It can be written as: Antenna Arrays
(Contd..)
Unit factor, Funit is the max time averaged power density for an
individual antenna element at =/2 An array factor, Farray is Where,
This is the pattern function resulting from an array of two
isotropic radiators. Excitation phase Phase due to path difference
Example 3.1 Three isotropic sources, with spacing d between them,
are placed along the z axis. The excitation coefficient of each
outside element is unity while that of the center element is 2. For
a spacing of d= /4 between the elements, find the Array factor
Normalized array factor Angles where the nulls occur Angles where
the maxima occur Antenna Arrays (Contd..)
N  Element Linear Arrays The procedure of twoelement array can be
extended for an arbitrary number of array elements, by simplifying
assumptions : The array is linear antenna elements are evenly
spaced, d along a line. The array is uniform each antenna element
driven by same magnitude current source, constant phase difference,
between adjacent elements. Antenna Arrays (Contd..) Antenna Arrays
(Contd..)
The far field electric field intensity : Where, Manipulate this
series to get: For smaller value of, With the max value as :
Antenna Arrays (Contd..)
Nulls of Array Factor The nulls of array factor when AFn is set to
0, Antenna Arrays (Contd..)
Maxima of Array Factor The maximum values of the AF is occur when,
Antenna Arrays (Contd..)
Half Power Beam Width (HPBW) of main lobe The HPBW can be
calculated when AFn is set to, Antenna Arrays (Contd..) Antenna
Arrays (Contd..)
Parasitic Arrays Not all the elements in array need be directly
driven by a current source. A parasitic array typically has a
driven element and several parasitic elements The best known
parasitic array is YagiUda antenna. Antenna Arrays (Contd..)
Parasitic Arrays (Cont) On one side of the driven element is
reflector length & spacing chosen to cancel most of the
radiation in that direction, as well as to enhance the direction to
forward or main beam direction. Several directors (four to six)
focus the main beam in the forward direction high gain and easy to
construct. Antenna Arrays (Contd..)
Parasitic Arrays (Cont) Parasitic elements tend to pull down the
Rrad of the driven element. E.g. Rrad of dipole would drop from 73
to 20 when used as the driven element in YagiUda antenna. But
higher Rrad is more efficient, so use half wavelength folded dipole
antenna (four times Rrad of halfwave dipole!) Yagi Uda Array
Antennas Yagi Uda Array Antennas (Contd..) Yagi Uda Array Antennas
(Contd..)
Applications Amateur radio TV antenna (usually single or a few
channels) Frequency Range HF (330 MHz) VHF ( MHz) UHF ( MHz) Yagi
Uda Array Antennas (Contd..)
Typical Sizes Director lengths: (0.4 0.45) Feeder length: (0.47
0.49) Reflector length: (0.5 0.525) Reflector feeder spacing: (0.2
0.25) Director spacing: (0.3 0.4) Yagi Uda Array Antennas (Contd..)
Yagi Uda Array Antennas (Contd..)
Design sizes and Directivities Length (in ) 0.4 0.8 1.2 2.2 3.2 4.2
Directivity in dB 7.1 9.2 10.2 12.25 13.4 14.2 Total no of elements
3 5 6 12 17 15 D0 (/2) = 1.67 = 2.15 dB 0.001 d/0 0.04 0.002 D/0
0.04 Yagi Uda Array Antennas (Contd..)
Design Procedure Specify: Center frequency (fo) Directivity do/
(diameter of parasitic elements) D/ (diameter of boom) Find:
Lengths of directors and reflector(s) Spacing of directors and
reflector(s) Example 3.2 Given: Center frequency (fo) = 50.1 MHz
Directivity (relative to /2) = 9.2 dB d = 2.54 cm D = 5.1 cm Using
these parameters, design a Yagi Uda Array antenna by finding the
element spacing, lengths and total array length. Solution to
Example 3.2 Solution:
= 5.988m = cm; d/ = ; D/ = 8.52 x 103 Step 1: Find L and N from
Table 10.6 L = 0.8 (from the given directivity of 9.2 dB) N = 5 (3
directors, 1 reflector, 1 feeder) For d/ = : Marked in Fig by dot
() Solution to Example 3.2(Contd..)
Step 2: From Fig , draw vertical line through d/ = Step 3: Measure
l between l3, l5 and l4. Transpose to l3, l5 to find l4. Read l4.
Step 4: From Fig , for D/ = l = Marked in Fig by (x) Solution to
Example 3.2(Contd..)
Therefore, Total array length: 0.8 The spacing between
directors:0.2 The reflector spacing: 0.2 The actual elements
length: L3 = L5 : 0.447 L4 : 0.443 L1 : 0.490 Solution to Example
3.3(Contd..) Design Curves to Determine Element Lengths
Solution to Example 3.2(Contd..) Design Curves to Determine Element
Lengths . Fig 10.25 X Solution to Example 3.2(Contd..)
Fig 10.26