![2. Maximum output design
In maximum output design, the height (h) above
ground is selected such that, the maximum field
strength is obtained from desired elevation angle (β).
The values of tilt angle (θ), leg length (L) and height
above ground (h) is calculated below.
In vertical plane, the relative field intensity of
rhombic antenna is,
E = {2cosθ[sin(2πhsinβ/λ)][sin(πL/λ)(1-cosβ
sinθ)]2}/(1-cosβ sinθ).](https://image.slidesharecdn.com/typicalantennas1-221020062902-03e4865f/85/Typical-Antennas1-ppt-22-320.jpg)
Typical Antennas1.ppt
- 3. Travelling Wave Antennas Travelling wave or non-resonant or aperiodic antennas are those antennas in which there is no reflected wave i.e., standing wave does not travel over such antennas. As against this, there are resonant or tuned or standing waves or periodic antennas in which standing waves exist due to improper termination. Such antenna operates properly on limited band for which they are tuned.
- 4. Since in radio communications which employ ionosphere for reflection, frequently require to operate on a widely spaced frequencies and thus there is a need for an antenna having greater band width. This need of larger bandwidth is met by these travelling wave antenna. In order to avoid reflected waves from the radiator so that only incident travelling wave travel on the antenna, the antennas are terminated at one end other than the feed end.
- 6. The field strength at a distance r from the wire at an angle θ can be shown to be Where E = (60 Irms/ r). (sinθ/1-cosθ). sin (πL/λ{1-cosθ}). L = Length of the wire, Irms = rms value of travelling wave current. If radiation pattern for various lengths are plotted as in Fig., it would be seen that as the length of wire increases, the major lobes get closer and narrower to the wire axis.
- 10. V- Antenna
- 11. V- Antenna
- 12. V- Antenna
- 14. Rhombic Antenna • It consists of two wires arranged in the form of diamond or rhombus. • The basic principle of rhombic antenna depends upon the travelling wave radiator. • Rhombic antenna is used for both transmission and reception. • In case of transmission, the input is applied through a feed line and receiving end is terminated by characteristic impedance.
- 15. Rhombic Antenna
- 16. Rhombic Antenna
- 18. Rhombic Antenna • The rhombic antenna is similar to the two V- antennas connected in series and is suitable for point-to-point communication. • Radiation pattern of rhombic antenna as shown in figure
- 19. Construction depends upon three major factors. They are, 1. Tilt angle(θ) 2. Leg length(L) 3. Height above ground (h).
- 20. Types of Design Basically, there are two types of design. They are, 1. Alignment design 2. Maximum output design. This classification is based upon the elevation angle (β).
- 21. 1. Alignment design In alignment design, the height H above ground is selected such that, angle of main beam is equal to the elevation angle (β). General expression for height is, H = λ/4sinβ. Leg length, L = 0.37λ / sin2β and Tilt angle, θ = 90- β
- 22. 2. Maximum output design In maximum output design, the height (h) above ground is selected such that, the maximum field strength is obtained from desired elevation angle (β). The values of tilt angle (θ), leg length (L) and height above ground (h) is calculated below. In vertical plane, the relative field intensity of rhombic antenna is, E = {2cosθ[sin(2πhsinβ/λ)][sin(πL/λ)(1-cosβ sinθ)]2}/(1-cosβ sinθ).
- 23. Advantages of Rhombic Antenna 1. Rhombic antennas are very much useful for radio communication. 2. It is useful for long distance propagation because of low vertical angle of radiation. 3. The input impedance of single wire antenna is half of the impedance of rhombic antenna. 4. The performance of rhombic antenna is measured in terms of leg length, height and tilt angle. 5. Single wire antennas have less receiving power along main axis whereas, rhombic antennas have more receiving power. So, rhombic antennas are highly directional broadband antennas. 6. The input impedance and radiation pattern does not depend upon frequency so, rhombic antennas are used in broadside arrays. 7. Rhombic antennas are untuned and easily convert from one frequency to another frequency.
- 24. Disadvantages of Rhombic Antenna 1. Transmission efficiency of rhombic antenna is very less. 2. It requires more space.
- 25. Yagi Uda Antenna It consists of a reflector, a driven element and one or more directors. Consider the arrangement of Yagi- Uda antenna shown in figure below. Here resonant half-wave dipole acts as a driven element and parasitic elements are arranged parallel to the driven element.
- 28. • D - Director or Parasitic element • R - Reflector or Parasitic element • DR- Driven element. • The current flowing through the director depends upon the voltage induced in the parasitic elements. • The spacing between the driven element and parasitic element is approximately 0.1 λ (or) 0.15 λ.
- 29. • The driven element is placed between two parasitic elements. • The parasitic element in the back of the driven element is known as reflector and in front of the driven element is known as director. The length of director is approximately 0.45 λ and reflector is 0.55 λ. The length of director, reflector and parasitic element depends upon the frequency.
- 30. The general expressions for 3-element Yagi-Uda antenna is, Reflector length = 500⁄f (MHz) feet Director length = 455⁄f (MHz) feet Driven element length = 475⁄ f (MHz) feet.
- 31. The length of parasitic element determines its reactance. If the length is equal or greater Than λ/2 , it will be inductive and less than λ ⁄2 it will be capacitive. (i) If the less than λ ⁄2 , the current lags the induced voltage. (ii) If the length is greater than λ ⁄2 , the current leads the induced voltage.
- 32. • Hence there is 180° phase difference between the parasitic elements, and therefore which can be analyzed as an end fire-array. • Yagi-Uda antenna is also known as super gain antenna because the gain can be increased by adding a number of directors after the driven element. • The distance between any two elements range from 0.1 λ to 0.3 λ. As the distance between the driven element and parasitic element reduces, the input impedance of driven element reduces.
- 33. Yagi-Uda Array Advantages • ! Lightweight • ! Low cost • ! Simple construction • ! Unidirectional beam (front-to-back ratio) • ! Increased directivity over other simple wire antennas • ! Practical for use at HF (3-30 MHz), VHF (30-300 MHz), and • UHF (300 MHz - 3 GHz)
- 35. Helical antenna The most popular helical antenna (often called a 'helix') is a travelling wave antenna in the shape of a corkscrew that produces radiation along the axis of the helix. These helixes are referred to as axial-mode helical antennas.
- 36. Helical Antenna
- 37. Helical Antenna Directional Circularly Polarized Polarization changes with time Both high gain and wide band
- 39. Geometry D= diameter of helix C= circumference of helix Lo= length of one turn = α= pitch angle = S= spacing between turns N= number of turns Lw= length of helix d= diameter of conductor
- 40. Helical antennas can operate in one of two principal modes: • Normal (broadside) mode • Axial (end-fire) mode. The normal mode, which yields radiation broadside to the helix axis, occurs when the helix diameter is small with respect to a wavelength. The axial mode, the most commonly used mode, provides maximum radiation along the helix axis, which occurs when the helix circumference is of the order of one wavelength. Modes
- 42. Normal Mode Radiation pattern similar to linear dipole The dimensions of the helix are small compared to the wavelength Narrow in bandwidth Radiation efficiency is small Rarely used
- 43. The performance of helical antenna is measured in terms of Axial Ratio (AR). Axial ratio is defined as the ratio of far fields of short dipole to the small loop. Axial Ratio, AR = |Eθ|/|Eφ|.
- 44. Axial Mode General expression for terminal impedance is, R = 140 C⁄λ ohms. Where, R = Terminal impedance C = Circumference. In normal mode, beam width and radiation efficiency is very small. The above factors increased by using axial mode of radiation.
- 45. Axial Mode Half power beam width in axial mode is, Where, λ = Wavelength C = Circumference N = Number of turns S = Spacing. Axial Ratio, AR = 1 + 1/2N
- 46. Advantages Overall length/height of antenna is reduced Can be easily constructed It produce circularly polarized fields.
- 47. Disadvantages Poor reception and transmission properties. Bandwidth too narrow for cellular communication.
- 48. Applications It is used to transmit and receive VHF waves for ionospheric propagation. Wireless LAN Satellite communication Animal tracking