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 Monopole antenna 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. • Both the poles will contribute the field present at point “P”, located at (x, y, z) Ground or Perfect conducting plane Monopole antenna - Theory P • Theory and derivation of Monopole and Dipole are completely same, but the Power output will always half to monopole. V • And Radiation resistance will be half of that for dipole, Rr=36.5Ω Radiation pattern V Monopole antenna • From the ground, at a very small height, a λ/4 antenna is place vertically, called monopole antenna. • AM, FM applications. Examples Bluetooth dongle 1cm Long wire antennas 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 Z0. • 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 Z ~ h 0 Travelling wave structure – E field • The retarded current in the wire shall be described as r Z1 I Im sint C v • Where: v=p.c or p=v/c • “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. P r z1 θ Wave direction z Z0 Z-axis Radiation pattern • We can find the radiation pattern as similar to the dipole case, will result: I p sin wL 0 E .sin 1 pcos 2R 1 pcos 2pc [Krauss] • Here, η is wave impedance. Maxima direction: Also called “Elevation angel” (θmax, θm, α, β) cos 1 0.37 m L θm or β For 3λ and 5λ [Krauss] Minor lob directions, Directivity m=2 m=1 m=0 [Balanis] 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 “Bi- directional radiation”. • This can be possible by “Open circuited long wire” or “Un-terminated”, so called “Stand wave antenna”. P R θ Wave direction Vee antennas β γ=2β By terminating with Z0, we can get Uni-directional radiation Design equation: Φ=2×β where β is θmax of single long wire 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 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 Z • The radiation mechanism is basically 0 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) β 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: www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdf About Dr. Bruce, E: http://ieeexplore.ieee.org/iel5/10933/35478/01685103.pdf Maxima with height Maxima with Length L Design formula • Finally, the design formulae are: (also called BBL formula) Design a rhombic antenna to operate at 20MHz when the angle of elevation angle =10o. 90o 10o 80o sin 10o 0.173 L 12.36;h 1.44 15m L 5.715m;h 3.795m 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 =10o. • Explain resonant and non-resonant modes of Long Wire Antenna. • If we assume the average beamwidth of rhombic antenna as 10o, then design an antenna system such that it will radiate maximum power over the ranges from 10o to 40o for the operating frequency of 10MHz • Explain the working of Open circuited long wire antenna and V antenna with radiation pattern Yagi-Uda Antenna A high frequency and high directive Parasitic array antenna 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. 377-385, May 1927. • 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 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 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 90o (along the axis), but 45 to 60 degree elevated so. 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. 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 Simulation – 9GHz Yagi Uda Atleast 1 or 2 λ Dipole=0.5λ Director=0.45λ Reflector=0.53λ Due to inductive effect at dipole and capacitive effect at parasitic elements fo=8.8GHz @ 8.8GHz θmax @ 8.8GHz θmax Back gain Approx=25dB Front gain Approx=40dB Front to back ratio=40-25 = 15dB Back gain Approx=17dB Front gain Approx=40dB Front to back ratio=42-17 = 25dB Optimized Design of N-element Yagi-Uda. • For the frequency of operation f0, 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. Example (from Balanis) Various Yagi Uda antenna Smallest Yagi Uda antenna!! Ivan S. Maksymov et al, “Optical Yagi-Uda nanoantennas”, http://arxiv.org/pdf/1204.0330.pdf .