YAGI-UDA DESIGN OF U.H.F BAND AERIAL TO SUIT LOCAL TV STATIONS PROJECT INDEX: PRJ 079 Presented By: GITAU SIMON WAWERU F17/8261/2004 Supervisor: Mr. S.L OGABA Examiner: Mr. OMBURA
Objective The main objective of the project was to design an antenna for local television stations in the country that suits the local TV channels with varying element lengths. The quality of reception is kept in mind where the length of elements were varied. Problem definition Currently in the country yagi antenna are used but they are made with no criteria or design but pure imitation of foreign country antennas which operate on different TV frequencies as compared to the Kenyans one. Through research it was noted that the antennas in the country are made with over emphasis on cost effectiveness hence compromising on the effectiveness of the antenna
Definition An antenna is basically the structure associated with the transition between a guided wave and free space wave or vice versa. It s a transducer that converts electromagnet energy into RF or vice versa
Reciprocity of antenna interchangeability of the same antenna for transmitting and receiving is antenna characteristics and properties are essentially the same for sending and receiving electromagnetic energy. The more efficient a certain antenna is for transmitting, the more efficient it will be for receiving on the same frequency. the directive properties of a given antenna also will be the same whether it is used for transmitting or receiving.
Figure 1-1. Current and voltage distribution on a dipole antenna.
End-fire array is one in which the principal direction of radiation is along the plane of the array (axis)and perpendicular to the elements. Radiation is from the end of the array, which is the reason this arrangement is referred to as an end-fire array. The currents in the elements of the end-fire array are usually 180 degrees out of phase with each other as indicated by the arrows in the figure.
Directivity. measures the power density an actual antenna radiates in the direction of its strongest emission, relative to the power density radiated by an ideal isotropic radiator antenna radiating the same amount of total power. Where θ and φ are the standard spherical coordinate s angles Radiated power density is the power per unit solid angle such that:
Antenna gain It is defined as the ratio of the radiation intensity of an antenna in a given direction to the intensity that would be produced by a hypothetical ideal antenna that radiates equally in all directions (isotropically) and has no losses. antenna gain is a measure that takes into account the efficiency of the antenna directional capabilities. In contrast, directivity is defined as a measure that takes into account only the directional properties of the antenna and therefore it is only influenced by the antenna pattern
Radiation pattern The radiation pattern of an antenna is the geometric (graphical) pattern depiction of the relative field strength transmitted from or received by the antenna. It is the variation in field intensity as a function of position or angle. The radiation pattern of an anisotropic radiator consists of several lobes. One of the lobes has the strongest radiation intensity compared to other lobes. It is referred to as the Major lobe. All the other lobes with weaker intensity are called Minor Lobes. The width of the main beam is quantified by the Half Power Beamwidth (HPBW), which is the angular separation of the beam between half-power points.
Front-to-back ratio it is the ratio of the energy radiated in the principal direction compared to the energy radiated in the opposite direction for a given antenna. A high front-to-back ratio is desirable because this means that a minimum amount of energy is radiated in the undesired direction. Since completely suppressing all such radiation is impossible, an infinite ratio cannot be achieved. In actual practice, however, rather high values can be attained. Usually the length and spacing of the parasitic elements are adjusted so that a maximum front-to-back ratio is obtained, rather than maximum gain in the desired direction.
YAGI UDA ANTENNA Driven element joined to the transmission line The radiation from the different elements arrives in phase in the forward direction, but out of phase by various amounts in the other directions. The gain is slightly increased by the reflector and further enhanced by the first director element. Additional director elements further increase the gain and improve the front-to-back ratio, up to a point of diminishing returns
Design table for 2% reduction Directors D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 Lengths(m) 0.245 0.240 0.235 0.230 0.225 0.220 0.215 0.210 0.205 0.200 0.195 0.190 Design table for 3% reduction Directors D1 D2 D3 D4 D5 D6 D7 D8 Lengths(m) 0.2425 0.235 0.2275 0.2200 0.2125 0.205 0.1975 0.1900 Design table for 4% reduction Directors D1 D2 D3 D4 D5 D6 Lengths.m 0.24 0.23 0.22 0.21 0.20 0.19 Design table for 6% reduction Directors D1 D2 D3 D4 lengths 0.235 0.22 0.205 0.19
At 2% Maximum gain was achieved at about 490MHz of about 13.5dBi Bandwidth at 13.5 0.7071=9.55dBi =543.75-412.5=131.25MHz has the smallest bandwidth but the largest gain of 13.5dBi
At 3% Bandwidth at 0.7071 12=8.5dBi =562-418=144MHz The bandwidth improved with a reduction in the gain by 5dBi
Bandwidth =0.7071 10.8=7.64 570-412=158MHz The 6% reduction has a higher bandwith but the smallest gain of about 7.64dBi
For 2% radiation pattern for 501MHz For 3% radiation pattern for 501MHz
For 4% radiation pattern for 501MHz For 6% radiation pattern for 501MHz
Advantages of yagi-uda antenna yagi antennas are widely used to achieve high gain in a very simple structure as only one element in the Yagi Uda array is directly driven; this greatly simplifies the construction of the array. Have high directivities as it is an end fire array. Disadvantages of yagi-uda arrays The variation of elements lengths and spacing causes inter related changes in the feed impedance of a yagi array. To obtain maximum possible forward gain experimentally is extremely difficult because for each change of element length it is necessary to readjust the matching either by moving the reflector or by resetting matching device. Shortcomings include effect of poor weather on antennas eg strong winds, obstacles affecting the reception of the signals leading to suspension of antennas on high heights to avoid obstacles. CONCLUSION As the reduction percentage of the lengths is varied from 2% to 6% as seen from the simulation results the gain, directivity, bandwidth and no of elements changes. The no of elements reduces as the percentage is increased because the shortest length of the last director due to the highest frequency remains the same. The reduction in the number of elements lead to a reduction in gain and directivity. The 6% reduction had the largest bandwidth as it had low directivity.2% reduction had the highest gain of 13.5dBi but smallest bandwidth while the 4% reduction had a moderate gain 11.5dBi and a large bandwidth making it suitable for yagi antenna. Reduction greater than 6% leads to poor yagi as the coupling is reduced significantly.