Antenna Theory. Wire Antennas

Transcription

1 Antenna Theory Wire Antennas Monopole Antenna Long Wire or Traveling wave Antennas Yagi Uda Antenna Prof. D. Kannadassan Reference: C. A. Balanis, J.D. Krauss

2 Monopole antenna

3 Image theory, an intro A positive charge +Q (or a charged body) is located a height h from perfect ground, then an image as -Q will form at a depth of h. A dipole will form, and the filed lines will be closed.

4 Both the poles will contribute the field present at point P, located at (x, y, z)

5 Ground or Perfect conducting plane

6 Monopole antenna - Theory Theory and derivation of Monopole and Dipole are completely same, but the Power output will always half to monopole. And Radiation resistance will be half of that for dipole, R r =36.5Ω V P

7 V Radiation pattern

8 Monopole antenna From the ground, at a very small height, a λ/4 antenna is place vertically, called monopole antenna. AM, FM applications.

9 Examples Bluetooth dongle 1cm

10 Long wire antennas

11 Long wire antennas - Theory Consider a thin wire of length L is horizontally placed at a height from ground plane. At one end a RF source is connected and other end is terminated by the characteristics impedance of the wire Z 0. When the wire is excited with a sinusoidal signal, the wave should travel along one direction and will not create any standing wave so called Travelling waves. We are gonna see the expression for electric field at a point P - located at a distance of r from the wire and angle of θ with respect to the length of wire. ~ Wave direction h Z 0

12 Travelling wave structure E field The retarded current in the wire shall be described as Where: v=p.c or p=v/c I I m sin t p is the ratio of the wave velocity at the wire to the free space, called Relative phase velocity. Used to vary with the attenuation constant. r C Z v 1 r P z 1 θ Wave direction z Z 0 Z-axis

13 Radiation pattern We can find the radiation pattern as similar to the dipole case, will result: Here, η is wave impedance. E I 0 p sin wl.sin 1 p 2 R 1 p cos 2 pc cos [Krauss]

14 Maxima direction: Also called Elevation angel (θ max, θ m, α, β) θ m or β cos m L

15 For 3λ and 5λ [Krauss]

16 Minor lob directions, Directivity m=2 m=1 m=0 [Balanis]

17 Bidirectional long wire antenna The maxima direction oriented with the direction of wave in the wire, so by introducing wave on both the direction we can introduce Bidirectional radiation. This can be possible by Open circuited long wire or Un-terminated, so called Stand wave antenna. R P θ Wave direction

18 β Vee antennas γ=2β By terminating with Z 0, we can get Uni-directional radiation Design equation: Φ=2 β where β is θ max of single long wire

19 Rhombic antenna Based on the principle of Traveling wave and Vee antenna, Rhombic antenna is a very high directive antenna has Diamond or Rhombus shape β

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21 Working of Rhombic antenna By properly selecting the tilt angle, the rhombic antenna will give additive effect of radiation pattern of each long wire antenna The radiation mechanism is basically depends on two factors: Tilt angle (φ) Height above the ground (h) These are design parameters of antenna Due to ground effect, the maximum radiation is elevated about an angel (β) Z 0 β

22 Design equations of Rhombic antenna BBL field equation: (Bruce, Beck and Lowry 1935) From this equation, we can deduce the condition to get the maximum power direction with respect to height h and length of line L Original Article: About Dr. Bruce, E:

23 Maxima with height

24 Maxima with Length L

25 Design formula Finally, the design formulae are: (also called BBL formula)

26 Design a rhombic antenna to operate at 20MHz when the angle of elevation angle =10 o. o o o sin o L ; h m L 5.715m; h m

27 Numerical Problems and Review Questions Explain about long wire and Rhombic antenna with its radiation pattern Design a rhombic antenna to operate at 20MHz when the angle of elevation angle =10 o. Explain resonant and non-resonant modes of Long Wire Antenna. If we assume the average beamwidth of rhombic antenna as 10 o, then design an antenna system such that it will radiate maximum power over the ranges from 10 o to 40 o for the operating frequency of 10MHz Explain the working of Open circuited long wire antenna and V antenna with radiation pattern

28 Yagi-Uda Antenna A high frequency and high directive Parasitic array antenna

29 Introduction Prof. S. Uda (japan) was invented this antenna by 1927, and collaborated with H. Yagi S. Uda, "High angle radiation of short electric waves". Proceedings of the IRE, vol. 15, pp , May After the invention, more than 40 researcher were studied on the improvement. Latest article (2011): Application of bacteria foraging algorithm for the design optimization of multi-objective Yagi-Uda array Shintaro Uda Hidetsugu Yagi

30 Principle A folded dipole or ordinary half wave dipole is centered between two types of parasitic elements, called: Directors and Reflectors. The coupling (capacitive) effect between the parasitic elements and active element(dipole), the directional properties are improved a high with endfire pattern Reflector: about 5% greater length than the active element, will reflects the power radiation at backward direction. Directors: 5% lesser length than the active element, will create a converging mechanism and increase the directivity along the forward direction. Spacing between the directors and reflector are depending on the optimality, in most of the case, the spacing should from 0.3λ to 0.4λ (at 1927, the spacing was λ/10) 12 to 20 element yagi-uda antennas are optimum and have improved directivity

31 Radiation properties Basically End-fire radiation pattern, with high directivity (less HPBW) Due to the ground and parasitic element, the pattern maxima at elevation will not be at 90 o (along the axis), but 45 to 60 degree elevated so.

32 Radiation properties We can show that, while increasing the directors, the gain and directivity will improve, however the side lobs will degrade the performance by attracting the noise in unwanted direction.

33 Measurements Forward gain Backward gain (or back gain) Front to Back ratio (diff of F.gain and B.gain) Magnitude of side lobes Input impedance Bandwidth, quality factor

34 Simulation 9GHz Yagi Uda Atleast 1 or 2 λ Dipole=0.5λ Director=0.45λ Reflector=0.53λ

35 f o =8.8GHz Due to inductive effect at dipole and capacitive effect at parasitic elements

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37 @ 8.8GHz θ max

38 @ 8.8GHz θ max Back gain Approx=25dB Front gain Approx=40dB Front to back ratio=40-25 = 15dB

39 Back gain Approx=17dB Front gain Approx=40dB Front to back ratio=42-17 = 25dB

40 Optimized Design of N-element Yagi-Uda. For the frequency of operation f 0, the λ will be estimated. Reflector: (mostly 1 element) length of 0.5λ with spacing of 0.25 to 0.3λ from dipole. Dipole length (active element): 0.475λ Directors (N-2 elements) 0.405λ with spacing of 0.3 to 0.4λ between each element. To match the dipole, usually the QWT section be utilized.

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42 Example (from Balanis)

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44 Various Yagi Uda antenna

45 Smallest Yagi Uda antenna!! Ivan S. Maksymov et al, Optical Yagi-Uda nanoantennas,