Parasitic Arrays

Parasitic Array defined - an array consist of a basic antenna connected to a transission line plus on or more addition conductors that are not connected to the transmission line.

Parasitic Elements - extra conductors that are added to the array.

Driven Element - the antenna itself.

A common arrangement of a parasitic array.

Types of Parasitic ELements

1. Reflector - is typically about 5% longer than half-wave dipole driven element.- it is spaced from the driven element by a distance of 0.15 to 0.25 wavelenth.

The radiation pattern of a dipole + its reflector

2. Director - It is approximately 4 percent shorter than the half-wave driven element.

- It is mounted in front of the driven element.

- it is spaced by approximately one-tenth to two-tenths of a wavelength from the driven element.

YAGI ANTENNA - made up of a driven element, a reflector and one or more directors. - named after one of its japanese inventors, Yagi and Uda.

- It has a gain of about 8dB when compared to a half-wave dipole.

- the antenna elements are usually made of aluminum tubing and are mounted on an aluminum cross member or boom.

- the resulting antenna is often referred to as a beam antenna.

- gains of 6 to 15 dB are possible with beam angles of 40 degrees to 20 degrees.

Yagi uda Antenna

Yagi Antenna Radiation Pattern

Application of Yagi antenna

- used in communication applications. - at one time, were widely used for TV

reception.

- used in Amateur radio operators.

- mainly used in VHF and UHF.