Parasitic arrays are used at frequencies ranging from approximately 5 MHz into the microwave range for obtaining directivity and forward gain. Examples include the Yagi antenna and the quad antenna.
A parasitic element is a conductor that forms an important part of an antenna system, but is not directly connected to the feed line. Parasitic elements operate by means of EM coupling to the driven element. When gain is produced in the direction of the parasitic element, the element is a director. When gain is produced in the direction opposite the parasitic element, the element is a reflector. Directors are a few percent shorter than the driven element; reflectors are a few percent longer.
A three-element Yagi antenna. Dimensions are discussed in the text.
The Yagi antenna, sometimes called a “beam,” is an array of parallel, straight elements. A twoelement Yagi is formed by placing a director or a reflector parallel to, and a specific distance away from, a single λ/2 driven element. The optimum spacing for a driven-element/director Yagi is 0.1λ to 0.2λ, with the director tuned 5 percent to 10 percent higher than the resonant frequency of the driven element. The optimum spacing for a driven-element/reflector Yagi is 0.15λ to 0.2λ, with the reflector tuned 5 percent to 10 percent lower than the resonant frequency of the driven element. The gain of a well-designed two-element Yagi is approximately 5 dBd. A Yagi with one director and one reflector, along with the driven element, increases the gain and f/b ratio compared with a two-element Yagi. An optimally designed three-element Yagi has approximately 7 dBd gain. An example is shown in above figure. (This drawing should not be used as an engineering blueprint.)
The gain, f/b ratio, and f/s ratio of a properly designed Yagi antenna all increase as elements are added. This is usually done by placing extra directors in front of a three-element Yagi. Each director is slightly shorter than its predecessor.
A quad antenna operates according to the same principles as the Yagi, except full-wavelength loops are used instead of straight half-wavelength elements. A two-element quad can consist of a driven element and a reflector, or it can have a driven element and a director. A three-element quad has one driven element, one director, and one reflector. The director has a perimeter of 0.95λ to 0.97λ, the driven element has a perimeter of exactly λ, and the reflector has a perimeter of 1.03λ to 1.05λ. These dimensions, as all antenna element dimensions, are electrical dimensions (taking the velocity factor of wire or tubing into account), and not free-space dimensions.
Additional directors can be added to the basic three-element quad design to form quads having any desired numbers of elements. The gain increases as the number of elements increases. Each succeeding director is slightly shorter than its predecessor. Long quad antennas are practical at frequencies above 100 MHz. At lower frequencies, their construction tends to be unwieldy.