Travelling Wave, Broadband Antennas, Frequency-independent Antennas

Frequency-independent (FI) antennas are radiating structures capable of maintaining consistent impedance and pattern characteristics over multiple-decade bandwidths. Their finite size limits the lowest frequency of operation, and the finite precision of the center region bounds the highest frequency of operation.

1. Travelling Wave and Broadband Antennas / Frequency – Independent Antenna

2. Travelling wave is transfer energy from one place to another, without transferring the matter.

3. Folded Dipole consists of a two parallel half-wave dipoles connected at the ends forming a narrow wire loop Feed point is at the center

4. Modes of current Transmission line mode - fields from the current cancel in the far field Antenna line mode - fields from the current reinforce in the far field  

5. Half wave folded dipole - it provides a four field increase in impedance over the dipole version At resonance, the impedance of a half-wave dipole antenna is approximately 70 Ohms, so that the input impedance for a half-wave folded dipole antenna is roughly 280 Ohms. Often used for twin-lead transmission lines  

6. There are a number of advantages or reasons for using a folded dipole antenna: Increase in impedance   Wide bandwidth

7. Applications FM radio antennas As driven elements for rooftop Yagi television antennas

8. Yagi Uda simple to construct and has a high gain, typically greater than 10 dB. operate in the HF to UHF bands (about 3 MHz to 3 GHz) invented in Japan, with results first published in 1926 work was originally done by Shintaro Uda , but published in Japanese. presented for the first time in English by Yagi

9. Elements of Yagi Driven element - power is applied; a half wave dipole or often a folded dipole Reflector - behind the main driven element Director - placed in front of the driven element

10. Advantages Antenna has gain allowing lower strength signals to be received. Yagi antenna has directivity enabling interference levels to be minimized. Straightforward construction. The construction enables the antenna to be mounted easily on vertical and other poles with standard mechanical fixings

11. Disadvantages For high gain levels the antenna becomes very long Gain limited to around 20dB or so for a single antenna

12. Long Wire random wire antennas require a length greater than a quarter-wavelength (λ/4) of the radio waves The radiation pattern of a straight random wire antenna is unpredictable and depends on its electrical length Above the 0.6λ results in four azimuth angles at which the gain is maximum. As the length of wire in wavelengths increases, the number of lobes increases and the maxima become increasingly sharp.

13. Biconical Antenna a broad-bandwidth antenna made of two roughly conical conductive objects, nearly touching at their points. For an infinite antenna, the characteristic impedance at the point of connection is a function of the cone angle only and is independent of the frequency. Practical antennas have finite length and a definite resonant frequency.

14. Triangular sheet, Bow Tie and Wire Simulation Geometrical approximation that make the structure can be lighter and smaller retaining desired electrical features triangular sheet bow tie wire simulation

15. The triangular sheet and bow tie antennas are geometrical approximations to the solid or shell conical or biconical antenna. Wires are simulated along the periphery of its surface reduces significantly the weight and wind resistance of the structure computed input impedances and radiation patterns of wire bow-tie antennas computed using Moment Method

16. Helical Antenna is a travelling wave antenna in the shape of a corkscrew that produces radiation along the axis of the helix antenna.

17. The parameters of the helix antenna are defined below. D - Diameter of a turn on the helix antenna. C - Circumference of a turn (C=pi*D). S - Vertical separation between turns , pitch angle, which controls how far the helix antenna grows in the z-direction per turn N - Number of turns on the helix antenna. H - Total height of helix antenna, H=NS

18. Operating principle normal mode axial mode

19. In the normal mode or broadside helix, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. the antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis These are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas.

20. In the axial mode or end-fire helix, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarized radio waves. These are used for satellite communication.

21. input impedance half-power beamwidth is maximum directive gain beamwidth between nulls is is approximately use C=λ S=0.23*C, G=0.8*N

22. References http://pediaa.com/difference-between-stationary-and-progressive-waves/ http://www.radio-electronics.com/info/antennas/dipole/folded_dipole.php http://www.antenna-theory.com/antennas/foldeddipole.php http://www.radio-electronics.com/info/antennas/yagi/yagi.php Straw, R. Dean (2003). The ARRL Antenna Book, 20th Edition. Newington, Connecticut, USA: The ARRL, Inc. p. 944. ISBN 0-87259-904-3. Practical Antenna Handbook ( Carr , Mc Graw Hill) - Chapter 9 Silver, Ward (2007). The ARRL General Class License Manual, 6th Ed. USA: American Radio Relay League. p. 6.6. ISBN 978-0872599963. Zhuohui Zhang,Analysis and design of a broadband antenna for software defined radio, ProQuest, 2007 http://www.antenna-theory.com/antennas/wideband/bowtie.php http://www.antenna-theory.com/antennas/travelling/helix.php Tomasi , Wayne (2004). Electronic Communication Systems - Fundamentals Through Advanced. Jurong , Singapore: Pearson Education SE Asia Ltd. ISBN 981-247-093-X. Balanis , Constantine (2005). Antenna Theory Analysis and Design, 3 rd Edition.John Wiley & Sons. Inc. Hoboken, New Jersey

23. Travelling Wave Antennas Composed of Transmission line structures that radiate. By designing the length, the bandwidth and the gain can be varied By designing the size and shape of the structure, beamwidth and polarization can be controlled.

24. Discone and Conical Skirt Monopole

25. Discone Antenna Modification of the Dipole A coaxial line is fed at the Center of the cone. Formed by Disk and Cone

26. Discone Antenna Characteristics : Omnidirectional Polarization: Vertical Similar Radiation pattern to a Dipole. Performance as a function of frequency is similar to a high pass filter. Can operate over frequency ranges of up to 10:1 Low angle of radiation

27. Physical Aspects Cone Element -should be a quarter wavelength at the minimum operating Frequency.  

28. Physical Aspects Disc - Must have an overall length of 0.7 times a quarter wavelength. - The Smaller the Diameter the higher the frequency.  

29. Physical Aspects Space Between the cone and disc -For VHF/UHF region it is around 15 millimeters. Insulator - Separates the disc and the cone

30. Discone Antenna

31. Radiation Pattern at 7MHz

32. Radiation Pattern at 14MHz

33. Radiation Pattern at 21MHz

34. Radiation Pattern at 28MHz

35. Application Military Amateur Radio Radio Scanner UHF and VHF communication

36. Conical Skirt Monopole Similar to Discone except the disk is replaced by a monopole which usually has a measure of quarter wavelength . Its behavior is similar to a discone .

37. Sleeve Antenna Radio Wave Antenna Used for both transmitter and receiver. Composed of a 2 elements: -Straight Metal element(rod or wire) -Straight Metal Tube(sleeve)

38. Sleeve Antenna Notable Characteristics: Can Receive and transmit Wide range of Radio Frequencies Flexible. Can be classified as: -dipole antennas -half wave dipole -omnidirectional antennas -coaxial antennas 3. Sturdier.

39. Application Used in High Frequency band applications: CB radio Base station Maritime ship to shop and ship to shore radio antennas

40. Spiral Antenna Characteristics: Has very large bandwidth. Fractional bandwidth as high as 30:1 Circularly polarized HPBW is approx. 70-90 degrees

41. Log-Periodic Spiral Antenna Also known as equiangular spiral antenna. 1954, Edwin Turnur played with dipole antenna. Has wide beamwidth with angle less than 70 degrees Log-Periodic spiral

42. An Arm of the antenna can be defined in polar form as: Where : = constant which controls the initial radius of the spiral antenna a = rate of the spiral antenna as it turns  

43. Parameters that Affects the Radiation of the Antenna Total Length of the spiral ( Flare Rate (a) Feed structure Number of Turns  

44. Total Length of the Spiral(   -This determines the lowest frequency operation. -Lowest frequency occurs when the wavelength is equal to Circumference of the spiral  

45. Flare Rate (a) Rate of which the spiral grows with angle. If flare rate is too large the spiral will be tightly wrapped. If flare rate is too small it will act more as a dipole. Common value a=0.22

46. Feeding Structure Use a balun to have a balanced current on both arms. This also determines the high end of the operating band.  

47. Number of Turns(N) This is also part of the design parameter. ½ to 3 turns works well according to experiments 1.5 is a Good number of turns in application

48. Archimedian Spiral Antenna Arms can be defined as: where: a = flare rate r = radius of the antenna which linearly increases with angle phi.

49. Archimedian Spiral Antenna The has two arms identical that is 180 degrees rotated which each other.

50. Applications Defense industry for sensing application. GPS Military aircraft

51. Log-Periodic Antenna Is a broadband, Multi-element , Directional, narrow-beam antenna which has impedance and radiation characteristics that are regularly Repetitive. Most of the time the components are dipoles.

52. Log-Periodic Dipole Antenna The Log-Periodic Dipole Antenna can vary its Radiation Characteristics by varying the scaling constant( τ ) and the spacing constant( σ ).

53. Using Scaling constant

54. Spacing Constant Half Apex Angle

55. Average Gain

56. Average E-Plane Beamwidth

57. Average E-Plane Beamwidth

58. Radiation Pattern at 12MHz

59. Far Field Pattern

60. Notable Characteristics Wide Bandwidth, even higher than Yagi- Uda Moderate Directivity Good gain over a wide range of frequencies

61. Application Television Antennas (UHF/VHF)

62. Broadband Antenna This are designed to operate effectively over a wide range of frequencies A rule of thumb in designing antenna is that you can increase the frequency band of operation of your antenna by increasing its volume.

63. Broadband Dipole A dipole can be made more broadband by increasing the radius A of the dipole. Also known as wideband dipoles. By varying increasing the radii: -the resonant frequency lowers as the radius increases -the bandwidth increases

64. Cage Dipole The Bandwidth of a single wire dipole can be increased by using a thicker radiator. It does not have to be a single wire, multiple wire “caged” to increase thickness can be applied. Spacers/spreaders should be 0.02 wavelength or less.

65. Disadvantages Construction Maintenance

66. Sources: http://www.antenna-theory.com/antennas/travelling/spiral.php http://www.qrz.ru/schemes/contribute/arrl/chap9.pdf Antenna Theory Analysis and Design by Constantine Balanes Modern Antenna and Design 2 nd ed. By Thomas Milligan