Parasitic Element Design Concepts

The most commonly recognized antenna that uses the parasitic element design approach is the Yagi-Uda (or, simply “Yagi” pronounced “Ya-Gee” with a hard “G” as in the word “great”). The Yagi-Uda antenna (Figure 5.12 below) was named after the two Japanese engineers who originated the design. Recall that a metal rod in the reactive near field of a transmitting element will reradiate the electromagnetic field. By cleverly aligning the metal elements of the antenna, the re-radiating elements can serve to either reflect or direct the beam. This is how a Yagi antenna operates.

Figure 5.12 The Yagi-Uda Antenna

The driven element is typically a half-wave (λ/2) dipole radiator, fed with input signal at its center. The parasitic elements are not directly excited with input signal but, rather, reradiate energy received from the driven element. The reflecting element is typically about 4% longer than the driven element and directing elements are typically about 4% shorter. To achieve either the reflecting or directing effect the antenna designer selects the correct length and spacing for the elements.

Antenna Beamwidth and the Law of Reciprocity

Even a simple dipole antenna doesnʼt radiate equally in all directions. The measurement of the directivity of an antenna is called its beamwidth. Since weʼre dealing with three dimensions in the transmission volume thereʼs going to be one beamwidth given for the view from the side (as shown in Figure 5.13 below) and another beamwidth given for the view from the top.

Figure 5.13 The Depiction of an Antenna’s Beamwidth

Copyright 2003 - Joseph Bardwell

An important fact to remember is that the beamwidth angle defines not only how the transmitted signal power will be dispersed but, also, how well the antenna will be able to receive signals. This is because, as provided in the Law of Reciprocity, the same characteristics that make the antenna directional (or provide it with gain) for transmitting, work exactly in reverse with regard to reception.

Copyright 2003 - Joseph Bardwell