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Unformatted text preview: 1/4 wavelength ground plane The 1 / 4 wavelength ground plane antenna is very simple in its construction and is useful for communications when size, cost and ease of construction are important. This antenna is designed to transmit a vertically polarized sig- nal. It consists of a 1 / 4 wave element as half-dipole and three or four 1 / 4 wavelength ground elements bent 30 to 45 degrees down. This set of ele- ments, called radials, is known as a ground plane. Figure 4.8: Quarter wavelength ground plane antenna. This is a simple and effective antenna that can capture a signal equally from all directions. To increase the gain, the signal can be f attened out to take away focus from directly above and below, and providing more focus on the horizon. The vertical beamwidth represents the degree of f atness in the fo- cus. This is useful in a Point-to-Multipoint situation, if all the other antennas are also at the same height. The gain of this antenna is in the order of 2 - 4 dBi. Yagi antenna A basic Yagi consists of a certain number of straight elements, each measur- ing approximately half wavelength. The driven or active element of a Yagi is the equivalent of a center-fed, half-wave dipole antenna. Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called re f ectors and directors, or simply passive elements. A re f ector is placed behind the driven element and is slightly longer than half wavelength; a director is placed in front of the driven element and is slightly shorter than half wavelength. A typical Yagi has one re f ector and one or more directors. The antenna propagates electromagnetic F eld energy in the direction running from the driven element toward the directors, and is most sensitive to incoming electromagnetic F eld energy in this same direction. The more directors a Yagi has, the greater the gain. As more direc- Chapter 4: Antennas & Transmission Lines 111 tors are added to a Yagi, it therefore becomes longer. Following is the photo of a Yagi antenna with 6 directors and one re f ector. Figure 4.9: A Yagi antenna. Yagi antennas are used primarily for Point-to-Point links, have a gain from 10 to 20 dBi and a horizontal beamwidth of 10 to 20 degrees. Horn The horn antenna derives its name from the characteristic flared appear- ance. The flared portion can be square, rectangular, cylindrical or conical. The direction of maximum radiation corresponds with the axis of the horn. It is easily fed with a waveguide, but can be fed with a coaxial cable and a proper transition. Figure 4.10: Feed horn made from a food can. Horn antennas are commonly used as the active element in a dish antenna....
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