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YAGI-UDA ANTENNA
PHYSICAL DESCRIPTION
The Yagi-Uda array has been widely used as a home TV antenna. IT is practically
used in the HF (330 MHz), VHF (30300 MHz), and UHF (3003,000 MHz) ranges.
It is simple to construct low in cost and has high gain, greater than 10 dB.
There are three types of elements within a yagi-uda antenna
DRIVEN ELAMENT
The driven element is the antenna element to which power is applied
.Mostly it is a folded dipole or a half dipole
REFLECTOR
The yagi antenna generally have one reflector .This is behind the main
driven element i.e
sideways from direction of maximum sensitivity.
DIRECTOR
There may be none ,one or more directors in yagi antenna. The
directors are placed in front of driven elements .Each director will add 1 dB of
gain in forward direction. This level reduces with increase in number of directors
ANALYSIS
A full analysis of yagi antenna requires computing the mutual impedances
between the dipole elements .Just considering two elements we can write the
voltage at each feed point as:
1 = 111 + 122
2 = 211 + 222
Z11 and Z22 are ordinary driving points impedances of dipole .Let the driven
element be designated 1 so that V and I supplied to the transmitter
2 = 211 + 222
And so
2 = 2122
1
This is the current impedance induced in parasitic element due to current I1 in the
driven element.