A Yagi-Uda antenna is a widely used antenna design due to its high forward gain capability, low cost and ease of construction, it is commonly used as a roof top television receiver. It consists of a number of metal rods called elements arranged on a central support beam. The elements are a dipole - which is the only driven element - arranged with a number of parasitic elements, of which there are two types;
A reflector
One or more directors
The function of the parasitic elements is to improve the radiation pattern in the forward direction. The reflector is placed behind the dipole and is slightly longer, it provides 3dB of additional forward gain, but having more than one reflector has
little benefit. The directors are placed in front and slightly shorter in length than the dipole provide an additional 3dB of forward gain each.
The parasitic elements provide forward gain by redistributing the energy of the EM signal generated by the dipole, since they are not driven they can only
redistribute energy in one direction at the expense of other directions.
For a particular operating frequency a typical design would be;
Element Length (l) Separation (l)
Reflector > 0.50 0.15 – 0.25
Driver 0.45 1 -
Directors 0.40 – 0.45 0.30 – 0.40
A polar plot of the gain verses orientation (radiation pattern) is useful when characterizing antennas. Some important features that appear on
plot are;
Forward gain – expressed in dB relative to an isotropic source or a standard dipole (in direction of maximum gain) represents the improvement in signal level to a reference antenna.
Front to back ratio – ratio of signal level in the forward direction to the signal level in the back direction (rotated 180o) also expressed in dB.
Beamwidth – angle between directions where the power is half the value at direction of maximum gain, hence -3dB. It gives a measure of the directivity of the antenna.
Side lobes – these are unwanted peaks in the gain at angles other than in the forward 1 Since an EM wave has a reduced velocity in the conductor a factor of 0.95 is usually applied, so in practice the driver is generally 0.475l long.
Richard Laugesen Page 1 3/11/2001 direction, they reduce the amount of useful
energy contained in the forward direction. Other characteristics that do not appear on the polar plot but which are equally important are;
Bandwidth – the range of frequency over which the antenna exhibits acceptable characteristics.
Radiative Resistance – want the impedance ofthe antenna to match the impedance of the transmission cable used to drive it otherwise
signal loss and high voltages in the cable may occur. It is difficult to optimize all of these characteristics simultaneously so the aim when designing an antenna depends on the requirements of the situation in which the antenna will be used. Optimization is achieved by simulating the radiation pattern of the antenna while varying the
lengths and separations of the elements.