By Jonathan Imberi
Directional antennas, or beam antennas, have two big advantages over dipole and vertical antennas. The first advantage is that a beam antenna concentrates most of its transmitted signal in one compass direction. Directivity or gain is provided in the direction the antenna is pointed. This makes your signal sound stronger to other operators and vice versa, when compared with non-directional antennas.
The second important advantage of beam antennas is the reduction in the strength of signals coming from directions other than where you point it. By reducing the interference from stations in other directions you can increase your operating enjoyment in the desired direction. Beam antennas find their most use on 15 and 10 meters and are very popular on the VHF and UHF bands.
A beam antenna's radiation pattern can be found on a graph of the antenna's gain and directivity. The diagram below shows the radiation pattern of a typical Yagi beam antenna:
As shown in the diagram below, the Yagi beam has several elements attached to a central boom. These elements are placed in a straight line along the boom and are parallel to each other. The boom length has the largest effect on gain in a Yagi antenna: the longer the boom the higher the gain.
Feed line connects to the driven element. On a three element Yagi like the one showed above, the driven element is located in the middle. The element located at the front of the antenna, nearest the favored direction, is called the director. The element located directly behind the driven element is called the reflector. The driven element is one-half wavelength long at the antenna's intended frequency. The director is just a little bit shorter than one-half wavelength, with the reflector being slightly longer than one-half wavelength.
Although Yagi antennas can have more than three elements, rarely is there ever more than one reflector. The extra elements are used as directors. For example, a four element Yagi has a reflector, a driven element, and two directors. The directors and reflectors are also known as parasitic elements, because they are not fed directly. You may even hear beams referred to as "parasitic beam antennas".
The direction of maximum radiation is from the reflector on through the driven element to the director in a beam antenna. The region of maximum radiation from a directional antenna is referred to as the "major lobe" or "main lobe". You may communicate in different directions with a beam antenna simply by rotating the array in the azimuthal, or horizontal, plane to point it in the desired direction.
Because you may want to rotate your beam antenna often, you will need something to turn them. For a single-band beam on six or two meters, you can use a TV mast, hardware and rotator. Most large HF beam antennas are mounted on towers, for which you will need a heavy duty mount and rotator.
The cubical quad antenna (aka "quad") also uses parasitic elements. These elements are usually just simple wire loops. The total length of the wire in the driven element is approximately one electrical wavelength. In a typical quad, as shown below, there are two elements: a driven element and a reflector. A two element quad could also be constructed with a driven element and a director, rather than a reflector. More elements can always be added to the design, such as a reflector and/or one or more directors. The typical radiation pattern of a quad antenna is very similar to that of the Yagi shown in the diagram above. Each element of the quad is usually square in shape, with each of these element loops being one electrical wavelength. Each side of the square would then be one quarter wavelength long.
Polarization on a quad antenna is determined by where the feed point is located on the driven element, making it possible to change the polarization of the signal. If the feed point is located in the center of one of the vertical sides, the transmitted wave would be vertically polarized, just as if the feed point was located in the center of a horizontal side, the waves would be horizontally polarized. This type of antenna can also be turned 45 degrees so that it resembles the shape of a diamond, at which point if it was fed at the side corner, the transmitted wave would be vertically polarized. In this same manner, if it was fed at the bottom corner, the transmitted wave would be horizontally polarized.
A delta loop antenna, as shown in the picture below, is very similar to the quad antenna. The delta loop antenna has triangular elements rather than square elements, although the total loop length is still one wavelength. You must divide the total length by three to find the length of each side element. As with the quad, the radiation pattern of the delta loop is very similar to the pattern of the Yagi as shown above.