RESONANT AND NON-RESONANT ANTENNAS. Difference between Resonant and Non- Resonant Antennas 2/20/

Presentation on theme: "RESONANT AND NON-RESONANT ANTENNAS. Difference between Resonant and Non- Resonant Antennas 2/20/"— Presentation transcript:

1 RESONANT AND NON-RESONANT ANTENNAS

2 Difference between Resonant and Non- Resonant Antennas 2/20/2021 2

3 Con….. 2/20/2021 3

4 Con….. 2/20/2021 4

5 The physical length 2/20/2021 5

6 6 The voltage current distribution along the resonant wire working a fundamental frequency (i.e. λ /2) and the first harmonic (i.e. λ /2), second harmonic (i.e. 2 λ /2) and third harmonic (i.e. 3 λ /2) are as shown below.

7 The field strength for the Resonant type long wire antenna: 2/20/2021 7

8 The field strength for the Non-Resonant type long wire antenna: 2/20/2021 8

9 For the resonant long wire antenna 2/20/2021 9 1) Radiation Resistance is given by 2) The angle of maximum Radiation is given by:

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11 Travelling Wave Antenna: 2/20/2021 11

12 The strength of the electric field at a distance r away from the radiator is given by: 2/20/2021 12

13 Different values of angle of major lobe and amplitudes of the lobes for the increasing length. 2/20/2021 13

14 Radiation pattern for different length of travelling wave antenna 2/20/2021 14

15 Helical Antenna: 2/20/2021 15

16 Helical Antenna: 2/20/2021 16 Helical antenna is basic, simple broadband VHF and UHF antenna which provides circular polarization. It consists of a thick copper wire Wound in the form of screw thread. In general, the helix is used with ground planes such as  Flat ground planes  Cylindrical cavity or frustrum cavity. In general the helical antenna is fed with co-axial transmission line in which the central conductor is connected to the helix at the feed point, while the outer conductor is attached to the ground plane.

17 Modes of Helical Antenna 2/20/2021 17 In general, a helical antenna can radiate in many modes. But the most important modes of radiation are as follows. 1. Normal or Perpendicular mode of Radiation. The field radiated by the antenna is Maximum in the plane normal to the axis and Minimum along the axis 2. Axil or Endfire or Beam mode of Radiation. The field radiated by the antenna is Maximum in the plane along the axis In axial mode there is only one major lobe with maximum intensity along the axis of the helix.

18 2/20/2021 18 The dimensions of the helix in the axial mode are not critical. Hence the end fire or axial helical antenna can be used to achieve circular polarization over a wider band width. For space communication applications, the helical antennas are suitable as they have wide bandwidth., higher directivity and circular polarization. To transmit or receive VHF signals through ionosphere generally an array of helical antennas is widely used for space and satellite communication.

19 Helical Antenna: 2/20/2021 19

20 Helical Antenna: 2/20/2021 20

21 2/20/2021 21 The mode of radiation depends on the diameter of the helix i.e D, The spacing between the turns S which is measured between two centers of the turns. The circumference of the helix is denoted by C = π D The number of turns are denoted by N. The axial length of one complete turn is denoted by L.

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23 Pitch Angle “ α ”: 2/20/2021 23 The pitch angle is defined as the angle between a line tangential to the helix wire and the plane normal to the axis of helix. The pitch angle then can be expressed as

24 Normal or Perpendicular mode of Radiation: 2/20/2021 24

25 Limiting Conditions on Helix: 2/20/2021 25

26 Helical Antenna with Spherical Co-ordinates: 2/20/2021 26

27 Far Field Helix at different Limiting Conditions The Far Field of a Small Loop is given by The Far Field of a Small Dipole is given by 2/20/2021 27

28 Axil ratio 2/20/2021 28 From above equations, it is clear that the two fields are in phase quadrant (phase=90). To get axil ratio (AR) of the elliptical polarization, we should take the ratio of the magnitudes of the field due to short dipole to that to loop. Hence the axil ration is given by

29 2/20/2021 29 CONDITION 1: AR =0, The elliptical polarization becomes linear horizontal polarization. CONDITION 2: AR =∞, The elliptical polarization becomes linear vertical polarization

30 2/20/2021 30 CONDITION 3: When AR =1, The elliptical polarization becomes circular polarization Thus the condition for circular polarization is given by

31 2/20/2021 31 Normal mode radiation pattern with circular polarization for helical antenna.

32 Axil or Endfire Mode of Radiation: 2/20/2021 32

33 2/20/2021 33 Terminal impedance The beam width between half power points is given by Beam width between first nulls The maximum directive gain in the axial mode is given by: The axial ratio (AR) is given by:

34 Radiation pattern of helical antenna in axial mode: 2/20/2021 34

35 Applications of Helical antenna: 2/20/2021 35 A single helical antenna or an array of helical antennas are useful in transmitting or receiving VHF signals through the ionosphere. The helical antennas are most extensively used in the Satellite Communication and Space probe Communication. In axial mode the dimensions are not critical hence bandwidth and radiation efficiency both are greater, hence axial mode helical antennas are used to achieve circularly polarized waves over wide band width.

36 Micro Strip Antenna: 2/20/2021 36 Strip lines are essentially modifications of the two wire lines and co-axial lines. These are basically planar transmission lines that are widely used at frequencies from 100 MHz to 100 GHz.

37 V Antenna: 2/20/2021 37

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