39
Experiment
4
Yagi-Uda in VHF Band
Description of The Antenna
The Yagi (or Yagi-Uda) antenna is a linear array of parallel dipoles. One element is
energised directly by a feed transmission line with the others acting as parasitic
radiators. The function of these elements is to enhance the radiation pattern in the
source direction. Generally the reflector will be 5% longer than the driven element (ie
diploe)and the directors will be 5% shorter. Parameter limits are:
• Driven Element: 0.45-0.49 wavelengths.
• Directors: 0.4-0.45 wavelengths.
• Separation between Directors: 0.3-0.4 wavelengths.
• Radii of directors: 0.15-0.25 wavelengths.
• Separation between driven element an parasitics: 0.15-0.25 wavelengths.
Optimization of the Yagi-Uda Antenna can be achieved by simulating the radiation
patterns for various lengths of the elemnets and the spacing between them. Other
factors that effect the radiation pattern are:
• For an antenna with a length of 6 wavelengths or more the overall gain is
independant of the director spacing.
• The reflector size and spacing have negligable effect on the forward gain and
large effects on the backward gain and input impedance.
• The size and spacing of the directors has a large effect on the forward gain,
backward gain and input impedance.
• More than one reflector provides little improvement on the directivity of the
antenna.
• The addition of more directors will increase the gain of the antenna although
after the addition of approximately 5 directors the advantages of adding more
directors decreases significantly.
• The use of a folded dipole will increase the input impedance of the driven
element. This is an advantage as the Yagi design generally has a low input
impedance and the antenna impedance needs to match the transmission line
impedance.
