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 Ka-Band 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 Ka-Band 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 out-of-phase, 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 two-element 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 Yagi-Uda 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 Yagi-Uda antenna. But higher Rrad is more efficient, so use half wavelength folded dipole antenna (four times Rrad of half-wave 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 (3-30 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 10-3 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