Antennas
Yagi Antenna (5)
- Because the Yagi antenna increases gain in one direction
(the forward direction) by reducing it in others,
particularly in the reverse direction it is said to have a front to back ratio.
This ratio represents the property of attenuating signals (both transmit and receive)
off the reverse side (reflector end) of the beam.
A good three element beam would have a front to back ratio of up
to 18dB about three ‘S’ Units
- The additional elements reduce the driven element feed point impedance
from about 70Ω to about 20Ω.
A folded dipole is often used to raise this impedance and
provide a better match for coaxial cable.
More often a matching device called a gamma match is used to give an
almost perfect match to 50Ω coaxial cable