Yougi Uda Anteena

Yagi-Uda Antenna

Let the driven element be kept at the origin and one parasitic element at z=-.125 . Let both dipoles are oriented along x axis. Due to the driven element (1), the current will be induced in (2) and this element is called parasitic element. The current induced in the second element depends on its length, radius of the element and spacing.
Let the current in driven element and parasitic element be 1 A and 1 2
0.7Ð1400 A, respectively.

AF = I1e

jb d

1

cos q + I 2 e
9 9

jb d

2

cos q

AF = 1 + 0.7e

If you solve then |AF|=1.172 for theta 7p p j( - ) cos q = 0 deg.|AF|=0.309 for theta = 180 deg.

It indicates that the incident field is being reflected by the parasitic element. If the length of the parasitic element is greater then the driven element, the current on the parasitic element leads the reflector element it acts as reflector. 1 A and

0.7Ð - 1400 A j( -

AF = 1 + 0.7e

7p p - ) cos q 9 9

If you solve then |AF|=1.172 for theta = 180 deg.|AF|=0.309 for theta = 0 deg. In this case parasitic element is director. Optimum distance between reflector to driven element varies between 0.15-0.25 Lambda. Length of reflector to director decreases in order of about 0.5 Lambda.

The input impedance of folded dipole increases in order of n^2 (impedance of dipole). Balanced feed is given to the antenna. If you use a co-axial then BALUN (balanced to unbalanced transformer is used . Application (a)TV reception in VHF Band (30-300 MHz). (b)It is also used in HF, VHF, UHF and Microwave Link.