Yagi-Uda

Experiment No. - 1(e) Objective :- To measure the antenna characteristics for Yagi-Uda antenna: a) Radiation pattern on polar plot. b) Beam width c) Gain of antenna. Apparatus Required :• RF Transmitter and Receiver • Half wave dipole antenna and Yagi-Uda antenna. • Antenna tripods with connecting cables. Theory: An example of a multi-element parasitic array is the Yagi Uda Antenna (figure 1, views A and B). The spacings between the elements are not uniform. 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 directors and the reflector in the Yagi antenna are usually welded to a conducting rod or tube at their centers. This support does not interfere with the operation of the antenna. Since the driven element is centerfed, it is not welded to the supporting rod. The center impedance can be increased by using a folded dipole as the driven element. The Yagi antenna shown in figure 1, view A, has three directors. In general, the greater number of parasitic elements used, the greater the gain. However, a greater number of such elements causes the array to have a narrower frequency response as well as a narrower beamwidth. Therefore, proper adjustment of the antenna is critical. The gain does not increase directly with the number of elements used. For example, a three-element Yagi array has a relative power gain of 5 dB. Adding another director results in a 2 dB increase. Additional directors have less and less effect.

Procedure : 1. Set the distance between the centers of the top of tripods to around 1m.Remove any stray object from around the antennas, especially in the line of sight. Avoid any unnecessary movement while taking the reading. 2. Connect the Yagi-Uda antenna, with its axis horizontal, to one of the tripods. Connect the tripod with the receiver. This antenna work as the receiving antenna. 3. Now connect a half wave dipole antenna to the other tripod, such that the elements of the half wave dipole antenna remain horizontal. Connect the tripod with the transmitter. This will work as transmitter. 4. Now rotate the helical antenna in the horizontal plane through 360ο in steps of 5 degrees. Note down the readings at the receiver display at each step. 5. Note the maximum reading out of whole set of readings. This will form the 0db Reference. Subtract each reading from 0 to obtain a normalized data for power density. Now use this new set of readings for drawing normalized power pattern. 6. Plot the readings on a polar graph. 7. From the Polar plot locate the 0db point and note down the corresponding angle. This gives the direction of main lobe. 8. Locate -3db points on its either side of the 0 db point. 9. Connect the –3db points to the origin by straight lines. The angle between these two straight lines is the half power beam width or -3db beam width of the radiation pattern in the E-plane. 10. Note down the maximum level in db on the back lobe which is approximately in the diametrically opposite direction to that of the 0 db level. Numerical value of that level is the Front to back ratio in db. Observation: Table1: Record of data to draw normalized power pattern of Yagi-Uda antenna Sl. No. 1

Angle in degree 10

Power density in db

2 3 4 …… …… …… … 35

20 30 40 …………………

……………..

350

Result and discussion: (a) (b) (c) (d)

Shape of the radiation pattern is ……………….. Half power beam width is ………………………………….. Front to back ratio is …………………… Gain of antenna is…………………..