DESIGN OF A 10- ELEMENT YAGI-UDA U.H.F AERIAL WITH EQUAL LENGTHS OF DIRECTORS TO SUIT LOCAL TV STATIONS PROJECT INDEX: PRJ 078 By ODUOR THOMAS KIZITO

Transcription

1 DESIGN OF A 10- ELEMENT YAGI-UDA U.H.F AERIAL WITH EQUAL LENGTHS OF DIRECTORS TO SUIT LOCAL TV STATIONS PROJECT INDEX: PRJ 078 By ODUOR THOMAS KIZITO REG NO: F17/8235/2004 Supervisor: Mr. S.L OGABA Examiner: Dr. GAKURU MUCEMI 1

2 Objectives To carry out a survey of the TV channel (UHF) frequency allocations in Kenya. To construct a 10-element Yagi Uda antenna with equal lengths of directors and varying spacings between the directors. To test the antenna on an antenna testing kit and a television set. 2

3 What is a Yagi-Uda? A Yagi Uda is a narrowband antenna that operates between 300MHz and 3GHz.This is a highly directional antenna that is mainly used in television reception. Its mainly used to Capture reception the V.H.F and U.H.F band 3

4 Status of TV frequencies in Kenya STATUS OF TV FREQUENCIES Location: Identity TV channel Station ID Frequency allocated Status (MHz) 1 K.B.C 23 K.B.C channel On air Nairobi 2 K.B.C 29 K On air 3 K.B.C 31 Channel On air 4 Royal Media Services 5 Royal Media Services 6 Nation Media Group 7 Kitambo communications 34 Citizen 575 On air 39 Citizen 622 On air 42 N.T.V 639 On air 45 Aljazeera - Not detected 8 Capital Group 47 C.N.B.C 679 On air GOD TV 695 On air 10 Stellavision 56 Aljazeera 751 On air 11 K.T.N Baraza Limited 59 K.T.N 775 On air 12 Radio One IPP 62 E.A.T.V 799 On air 4

5 Antenna parameters Some important antenna parameters to consider are: Radiation pattern Directivity Input impedance VSWR Return loss Gain 5

6 How the Yagi works Yagi-Uda Antenna is a parasitic linear array of parallel dipoles. which is energized directly by a feed transmission line while the others act as parasitic radiators whose currents are induced by mutual coupling. As leading elements absorb power the, diffraction bends the adjacent rays toward the antenna. As more directors are added, the more power is absorbed hence greater gain. An antenna has an Aperture area. which is proportional to its gain. 6

7 Construction considerations Factors to be considered in the construction include: 1. Upper frequency, centre frequency, lower frequency of the U.H.F band. 2. Material type Resistivity Skin depth Electrical corrosion 7

8 Antenna Element Calculations Reflector lengths Was done using lowest frequency of the UHF band F=487MHz Wavelength=0.625m Length=315mm Dipole lengths dipole length was done using centre frequency F=643MHz Wave length= m length=234.3mm Director lengths Director lengths were calculated using the upper frequency F=799MHz Wavelength=0.3754m Length=178.3mm 8

9 Spacing Calculations Since I decided to work with 10 channels in the UHF band, I chose a 10 element Yagi. Each element spacing was represented by a channel frequency to come up with 1o different spacings.(for simulation). For the practical part, optical ray geometry was used. 9

10 Spacings cont d Element Frequency(MHz) Spacing formula(m) Wavelength (m) Spacing(mm) Reflector-Dipole λ Dipole-Director λ Director1-Director λ Director2-Director λ Director3-Director λ Director4-Director λ Director5-Director λ Director6-Director λ Director7-Director λ Director8-Director λ

11 Constructions: Insulator and Perforated reflector 11

12 Ray Geometry of the paraboloidal reflector The center of the dipole was located using ray geometry as shown alongside. Incident rays on the dish come from the transmitter and congregate at the focal point. The diameter of the dish was 315mm and the distance was found to be 11.35cm. 12

13 Simulations: Radiation patterns. Azimuth Elevated 13

14 Comparison of Data Azimuth Elevation Elevation angle Outer ring 0.0 deg 9.97dBi Azimuth angle Outer ring 3D max gain 0.0 deg 9.97dB 9.97dB 3D max gain Slice max gain Front /back 9.97dBi angle= 0.0deg 18.36dB Slice max gain Font/back Beamwidth 9.97 angle=0.0 deg 18.36dB deg; 24.8 deg Beamwidth Side lobe gain Front/Sidelobe db angle=215.0 deg db Side lobe gain Front/sidelobe 2.6 angle=65.0 deg 7.37 db 14

15 SWR Demonstration 15

16 Practical testing Element Frequency Wavelength,λ Simulation Practical spacing variations (λ mm) (MHz) (m) spacing(mm) 0.18λ 0.19λ 0.2λ 0.21λ 0.22λ R-DR DR-D =0.6V =0.65V D1-D =0.65V =0.8V =0.85V =0.8V =0.5V D2-D =0.55V =0.6V =0.7V =0.7V =0.8V D3-D =1.0V =1.0V =1.0V =1.0V =1.0V 16

17 Practical testing cont d D4-D =1.15V =1.1V =1.2V =1.2V =1.2V D5-D =1.35V =1.4V =1.4V =1.35V =1.30V D6-D =2.0V =2.1V =2.25V =1.6V =1.5V D7-D =2.25V =2.35V =2.3V =2.65V =2.65V 17

18 Optimization through graphing Optimized spacing for Dipole-director 1 Optimized spacing for Director 1-Director Graph of Field voltage Vs Spacing data 2 spline Graph of feild voltage Vs spacing data 3 spline Field voltage V Field voltage V spacing (lambda mm) Spacing(lambda mm) 18

19 Conclusions and Recommendations As directors were increased, the field voltage also increased. The centre frequency of the antenna was 643MHz.As frequency increased above the centre frequency, the gain declines abruptly and below centre frequency. With digital TV coming up, Yagi s will be rendered obsolete in urban areas but still useful in the rural areas. When tested on a SAMSUNG TV the reception was clear with particular bias towards CITIZEN 2. A computerized spectrum analyzer with graphical plotter could be used to obtain accurate results. Further design could be carried out at high frequencies that could be employed in Wi-Fi communications. 19

20 Thank you 20