Optimal Design of Yagi-Udi Antenna for VHF Band Application Using Magus 2.2 Software

Transcription

1 AMERICAN JOURNAL OF SCIENTIFIC AND INDUSTRIAL RESEARCH 01, Science Huβ, ISSN: X, doi:10.551/ajsir Optimal Design of Yagi-Udi Antenna for HF Band Application Using Magus. Software Goshwe, N. Y. and Tijam, I.. Department of Electrical/Electronics Engineering Universit of Agriculture, Makurdi, Nigeria Contact: ABSTRACT This paper presents an optimal design of low cost Yagi Udi antennas. The design targets the field strength in the main beam to be larger than a critical minimum value, the directivit to be large enough over the whole frequenc band, the antenna s input impedance to match the generator reasonabl well over the entire band and that the SWR be small. Nigerian Television Authorit, Makurdi Network centre in Nigeria parameters was used. It transmits at HF, operating on Channel 13 (15-18) MHz. The specification was used to calculate values for the antenna dimensions to be used for the simulation. Antenna Magus. software was used to simulate the optimal designed conditions of the Yagi-Uda antenna. For this design, the director spacing of the designed antenna was varied while maintaining the spacing between the first director and keeping the driven element constant. Optimal design was obtained at a gain of dbi and frequenc of 3.7 MHz. eak real impedance of the antenna was attained at Ω with a corresponding frequenc of 59.7 MHz and the Real impedance at zero-crossing frequenc reased to Ω at Frequenc 00.9 MHz. The SWR of as low as was obtained at frequenc of 1.8 MHz, which falls within the HF operating frequenc. These results show a significant reduction in the value of SWR and rease in both gain and bandwidth. INTRODUCTION Antenna is a transitional structure between free space and a guiding device that is made to efficientl radiate and receive radiated electromagnetic waves. Antennas are commonl used in radio, television broadcasting, cell phones, radar and other sstems involving the use of electromagnetic waves. Antennas demonstrate a propert known as reciprocit, which means that an antenna will maintain the same characteristics whether it is transmitting or receiving (Gar 1996). One of the applications of a one wa wireless communication which antennas pla a ke role is the terrestrial television (T). In terrestrial T sstem, the transmitters (broadcast stations) are transmitting the T signal with high power and ver tall transmitting antenna located on the ground to transmit radio waves to the surrounding area (Bemani and Nikmehr, 009). The choice of a particular antenna depends on factors such as gain, radiation pattern, polarization, bandwidth, resonant frequenc and impedance. The most common antenna used for receiving T signals is the Yagi-Uda antenna (variation of the dipole antenna) which is relativel inepensive to manufacture and design because of the simple - dimensional phsical geometr (Cheng, 1991). The are usuall emploed at HF and higher frequencies (F) because the size of the antenna is directl tied to the wavelength at the resonant frequenc. The Yagi antenna not onl has unidirectional radiation and response pattern, but it concentrates the radiation and response (Barcla, L., 1997, Sun et al 010). The main challenge posed b this tpe of antenna is because it presents difficult design and optimization challenges (Cheng, 1991, Lohn et al 001, Goudos et al 010). Antenna Theor: The main theor of antennas covers directivit, radiation pattern, gain, mutual impedances between linear elements and coupling in receiving modes. Directivit: Directivit describes how much energ is concentrated in one direction in preference to radiation in other directions. It is defined as the ratio of the radiation intensit in a certain direction to the average radiation intensit (erambur and Rajeswari, 003). It is given as U(, ) D(, ) 1.0 U ave

2 Am. J. Sci. Ind. Res., 01, 3(5): 77-8 Where U(θ, ) is the radiation intensit of the antenna while U ave is the average radiation. Directivit can also be defined as a ratio of power densit in a certain range b simpl dividing the numerator and denominator b square of range (r ). B multipling the power densit b the square of the radius r at which it is measured, we obtain the power per unit solid angle (watts per square radian or steradian) or radiation intensit U. It should be noted that the radiation intensit is also independent on the radius (Rhodes, 1999). The radiation intensit can be epressed as: U(, ) U F(, ).0 Where m U m denotes the maimum radiation intensit U m = U(θ ma, ma) F(θ, ) denotes the power pattern normalized to a maimum value of unit in direction (θ ma, ma). The total power radiation is obtained b the radiation intensit over all angles around the antenna: = U(θ, ) dω = U m F(θ, ) dω 3.0 Where dω denotes Sin d d Antenna Gain: It is a measure of how efficient the antenna transforms available power at its input terminals to radiated power, together with its directivit properties (Joe and John, 005). The gain can be epressed as 4U (, ) G(, ) 4.0 in Where G(θ, ) denotes the gain, U(θ, ) denotes radiation intensit of antenna in direction of (θ, ) luding the effects of an losses on the antenna and in denotes the input power absorbed b the antenna. However in practice, input power eperience losses, which ma be due to absorption of nearb structures or loss of power with the obstruction of line of sight (LOS). And radiation efficienc (e r ) of an antenna is given as: e r = in 0 e r Therefore, the maimum gain without the effects of mismatches of impedance or polarization is computed b obtaining the product of the directional characteristic of maimum directivit gain D (, ) multiplied b radiation efficienc, via: 4U (, ) G(, ) er erd(, ) 6.0 Antenna olarization: olarization of an antenna is the polarization of the wave radiated in a given direction b the antenna when transmitting or receiving. Thomas (005) classified the tpes of polarization as: vertical linear, horizontal linear, lefthand circular, right-hand circular, left-hand circular elliptical and right-hand elliptical. olarization of an uniform plane wave describes the shape and locus of the tip of the Electric field E(z, (in the plane orthogonal to the direction of propagation) as a function of time. The electric field phasor consists of and component. E ( z) E ˆ ( z) E ˆ ( z) 7.0 Where E ( z) E o E ( z) E o e e jkz jkz and Ẽ o is comple amplitudes and z denotes the propagation direction Ẽ o is comple amplitudes Besides that, wave polarization depends on the phase of E o relative to that of E o, but not on the absolutes phases of E o and E o. A wave is linearl polarized if E(z, and E(z, are in phase (δ=0) or out of phase(δ =π) (Thomas A, 005). If δ is nonzero, then aial ratio is finite. When δ > 0, E leads E in phase, and the sense of rotation is left and conversel if δ < 0, the sense is right-hand. hasor electric field epression can be epressed as: E( z) ˆ ja ( E Ee ) ˆ e jkz

3 Am. J. Sci. Ind. Res., 01, 3(5): 77-8 Equations 9.0 and 10.0 are the equations for instantaneous electric field and the modulus of the intensit and the direction of the electric field propagated respectivel. E(, E (cos t kz) ˆ E cos( t kz) ˆ And E 9.0 ( z, E Cos ( t kz) E Cos ( t kz) 10.0 The electric field E(z, can be defined in the - plane at a specific z with respect to the zero-phase reference component of E(z, as the lination angle. E tan 1 E ( z, ( z, 11.0 oltage Standing Wave Ratio (SWR): SWR is a measurement of the mismatch between two transmission lines. It provides a measurement of the amount of signal being ected back from the mismatch, which is directl related to the amount of energ that is transmitted (Gar M. 1996). SWR computed as the ratio of the net power to the forward power and can be epressed as the ratio of maimum to minimum voltage on the transmission line. The SWR can be epressed as: Z Z L o 14.0 L Z Z o Where + is the ident wave (magnitude and phase), is the ected wave (magnitude and phase), Z o is the characteristic impedance of the transmission line (magnitude and phase), and Z L is the impedance of the load line (magnitude and phase). If the load impedance is equal to the characteristic impedance of the transmission line, the ection coefficient would be zero because there is no mismatch in this case. In addition, unlike SWR, the ection coefficient has both magnitude and phase. The magnitude of the ection coefficient is then SWR 1 SWR The transmission coefficient τ is defined as the ratio of transmitted to ident waves and is given b L 16.0 or, in terms of impedance, SWR ma min 1.0 Z Z Z o 17.0 L o Where ma is the maimum voltage on the transmission line (feed cable), min is the minimum voltage on the transmission line, is the magnitude of the ident wave, and is the magnitude of the ected wave. The ection coefficient ρ is the ratio of ected to ident waves and is given b 13.0 or, in terms of impedance, Where L is the wave transmitted through the mismatch to the load side (magnitude and phase). The ratio of the ected power to the ident power is given as So that net = 18.0 This can be epressed in a SWR correction factor given in db as 79

4 Am. J. Sci. Ind. Res., 01, 3(5): SWR SWR 1 C SWR log 10 The SWR component covered here is not the onl antenna SWR term related to antenna measurements. If an antenna is not in a free-space environment, energ ected back from other objects will affect the SWR measurement. However, this term is a measure of the antenna's interaction with its environment rather than a measurement of an inherent propert of the antenna. Yagi Uda Antenna Design arameter: According to Labade and Deosarkar (010), one cannot alwas optimize the performance to get the best Directivit, Front-to-back ratio and input impedance at a particular frequenc, as these parameters are complicated functions of element radii, dimensions and relative spacing because of the mutual coupling among the dipoles. Therefore designs are based on the trade-off so that a weighted combination of Table 1.0: Design arameters of a Yagi Uda Antenna Number Elements of Length of Reflector (L R) m Length of Driven Element (L)m Directivit, Front-to back ratio and input impedance needs to be optimized A Yagi antenna develops an end fire radiation pattern and for optimum gains the ector and driven elements are spaced 0.5 to wavelengths, director-to-director spacing is 0.15 to 0.5 wavelengths apart. Reflector length is tpicall 0.5 wavelengths to 1.05 that of the driven element. The driven element is calculated at resonance without the presence of parasitic elements. The directors are 10 to 0% shorter than driven elements at resonance (Adedokun et.al, 010). For this work, boom length is fied at 1.11λ and the directors varied in length in a random fashion from 0.5λ to 0.4λ with an average spacing of less than 0.1λ. In practice, the Yagi-Uda antenna often consists of one ector and two or more directors to provide better gain characteristics. To carr out the design of the antenna, the following parameters were needed. The tpical design parameters of HF are given below. Length of Director (L D) m Spacing of Reflector (S R) m λ 0.5λ 0.456λ 0.5λ 0.15λ In addition, the antenna design targets the field strength in the main beam to be larger than a critical minimum value, the directivit be large enough over the whole frequenc band, the antenna s input impedance match to the generator reasonabl well over the entire band and that the SWR be small. In Table.0: Calculated alues of the Antenna Dimensions Spacing of Director (S D) m this paper, Nigerian Television Authorit, Makurdi Network parameters are used to simulate optimal dimensions of the antenna. It transmits at HF, operating on Channel 13 (15-18) MHz. Using the aforementioned conditions and Table 1.0, the calculated values for the design dimensions for the antenna is tabulated on Table.0. L R (cm) L (cm) L D (cm) S R (cm) S D (cm) Tube diameter (cm) Boom Length (m) Selecting Optimal alues for Antenna Dimension: The gain of Yagi-Uda antenna depends heavil on the number and length of directors. The gain can be reased b reasing the number of director elements while the operating frequenc can be reased b reducing the lengths of all the elements. If reased bandwidth or specified input impedance is required, it is necessar to simulate this antenna repeatedl with variations in the element spacing and length, and to optimize for the specific requirement. In a work b Adedokun et.al, 010, it noted that forward gain can be reased b adding the directors of a Yagi Uda. To optimize the 1 elements Yagi Uda antenna, the spacing between directors was varied while holding the ector eciter spacing and lengths of all elements constant. The best results gave directivit of 13. dbi. For an eperiment that maintained the length parameters and varies the spacing gives directivit of 13.9 dbi. Best results were achieved when there is both spacing and length perturbation of the antenna arra. The initial gain is 13.9 dbi but the computation returned directivit value of 14.9 dbi after varing both the spacing and length of the arra. 80

5 Am. J. Sci. Ind. Res., 01, 3(5): 77-8 Table 3.0 shows the final results with the highest gain achieved. Table 6.0: Directivit optimization for 1 Eelement Yagi-Uda arra for HF band atr= 0.15cm Dipole Length Initial Arra Arra after Spacing L D(cm) L(cm) L D1(cm) L D(cm) L D3(cm) L D4(cm) L D5(cm) L D6(cm) L D7(cm) L D8(cm) L D9(cm) L D10(cm) Directivit 13.9 dbi 13.9 dbi 14.9 dbi Optimum Arra after Spacing and Length erturbation Fig 1.0 Graph of Gain plotted against Frequenc after erturbation of director spacing and all element lengths From the graph in Figure 1.0, it is observed that there was a significant rement in the total gain of the antenna (from dbi at 0.1MHz to dbi at 3.7MHz which is indicated b the red line on the graph) with a corresponding 18.04% rease in the bandwidth of the antenna. Increase in both gain and bandwidth is an advantage for the antenna. RESULTS AND DISCUSSIONS Antenna Magus. software which is a Windowscompatible antenna analsis, modeling and design software package was used to analze the best dimensions selected. This allows simulation of the impedance and patterns of the antenna dimension designed. Antenna Simulation Results: The best performance of the simulated results was obtained when both the director spacing and all the elements length were varied. The result of the simulation of the gain against frequenc after the perturbation is as shown on Figure 1.0. Fig.0: Graph of Impedance plotted against Frequenc after erturbation of director spacing and all element lengths Figure.0 shows that the peak real impedance of the antenna is at Ω which corresponds to frequenc of 59.7 MHz (the thick red line on the graph) and the Real impedance at zero-crossing frequenc reased to 334.0Ω at Frequenc 00.9 MHz compared to simulation of other dimensions. 81

6 Am. J. Sci. Ind. Res., 01, 3(5): 77-8 Fig 3.0: Radiation pattern at the Centre frequenc after erturbation of director spacing and all element lengths From the radiation attern in Figure 3.0, it can be seen that the peak gain at angle (freq) φ = 0 0 is dbi and the peak gain at angle (freq) φ = 90 0 is dbi where θ = 0 0 (16.5 MHz). This is a significant improvement compared to other design dimensions for the antenna. Fig 4.0: Graph of SWR verses Frequenc after erturbation of director spacing and all element lengths Figure 4.0 is the graph of the best SWR against Frequenc from the various simulations. It shows that the frequenc at which SWR is lowest (SWR = 1.463) is now at frequenc of 1.8 as against the net best at SWR = 1.57 at MHz. These results show a significant reduction in the value of SWR which is a good for the antenna. Conclusion : The design of the Yagi Uda antenna for the HF band has been presented. The design and simulation is done using simulation software know as Antenna Magus.. The correlation of the simulated results with the objectives shows that the Bandwidth coverage from 185 MHz to 37 MHz had been covered for the Yagi Uda antenna. This bandwidth is quite satisfactor compared to the terrestrial T frequenc band. The simulation gives a unique set of Yagi antenna dimensions which best satisf optimization goals, but it does not optimize antenna performances over the wider statistical distribution of antenna dimensions. This will make the antenna design ver narrow, sensitive and critical. REFERENCES 1. Adedokun, A. and Adegoe, K. A. (010): Development of 50dB Yagi-Uda antenna for effective communication. acific Journal of Science and Technolog. 11(1): Barcla, L.W. (1997): Radio Sstem arameters. Lancaster Universit: Lancaster, UK. 3. Bemani, M and Nikmehr, S. (009): A Novel Wide-band Microstrip Yagi-Uda arra antenna for WLAN Applications. rogress in Electromagnetics Research. 16: Cheng, D. K. (1991): Gain Optimization for Yagi- Uda Arras. IEEE Antennas and ropagation Magazine. 33(3): Gar M. M. (004): Modern Electronic Communication, 6th Edition, New Jerse: rentice Hall. 6. Goudos, S. K.; Siakavara, K.; afiadis, E. E. and Sahalos, J. N. (010): areto optimal Yagi-Uda Antenna Design Using Multi-objective Differential Evolution. rogress In Electromagnetics Research. 105: Joe, C. and John, D. (000): Newness Radio and RF Engineer s ocket Book (Second Edition), Oford ress. 8. Labade, R.. and Deosarkar, S. B. (010): Design of Yagi-Uda Antenna at 435 MHz for Indian MST Radar International. Journal of Advanced Engineering and Application. 1: Lohn, J. D.; Kraus, W. F.; Linden, D. S. and Colombano, S.. (001): Evolutionar Optimization of Yagi-UdaAntennas. roceedings of Fourth Conference on Evolvable Sstem, Toko. October 3-5, 001: erambur, S. and Rajeswari, C (003): Antennas for Information Super Skwas: An Eposition on Outdoor and Indoor Wireless Antennas. Research Studies ress Ltd. 11. Rhodes, J. E. (1999): Antenna Handbook: US Marine Corps. CN Sun, B. H.; Zhou, S. G.; Wei, Y. F. and Liu, Q. Z. (010): Modified Two-Element Yagi-Uda Antenna with Tunable Beams. rogress In Electromagnetics Research. 100: