The questions related to antenna gain were substantially changed in this question pool, and, as a result, the question numbers of other questions were changed.
To make their signals more effective, some amateurs use directional antennas. Directional antennas, such as Yagis and quads, direct most of the power output in a particular direction, making the signal seem more powerful. They are also more sensitive to receiving signals from a particular direction.
The “gain” of a directional antenna is the relative increase in power radiated in the direction in which the antenna is pointing. The gain is usually specified in decibels, or dB. Look at this specification very carefully, because the gain may be specified in relation to either an isotropic antenna (dBi) or in relation to a dipole (dBd). When referring to antenna gain, dBi refers to an isotropic antenna, dBd refers to a dipole antenna. (G9C20)
dBi is strictly a theoretical figure, as the isotropic antenna is strictly a theoretical construction. dBd is a more realistic specification. When stated in dBi, the gain of an antenna will always be higher than if it is stated in dBd. dBi gain figures are 2.15 dB higher than dBd gain figures. (G9C19)
A characteristic related to the antenna gain is the “front to back ratio.” The “front-to-back ratio” of a Yagi antenna is the power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction. (G9C07) The “major lobe” or “main lobe” of a directive antenna is the direction of maximum radiated field strength from the antenna. (G9C08)
Yagis are perhaps the most common type of directional antenna. A Yagi antenna consists of a driven element, a reflector, and one or more directors. The reflector and directors are called parasitic elements. The approximate length of the driven element of a Yagi antenna is 1/2 wavelength. (G9C02) The reflector is normally the longest parasitic element of a three-element, single-band Yagi antenna. (G9C04) In a three-element, single-band Yagi antenna, the director is normally the shortest parasitic element. (G9C03)
By changing the physical characteristics of the elements and the spacing between the elements, you can change the characteristics of the antenna. For example, larger diameter elements increase the bandwidth of a Yagi antenna. (G9C01) The gain increases when you increase boom length and add directors to a Yagi antenna. (G9C05)
All of these choices are correct when talking about Yagi antenna design variables that could be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth (G9C10):
- The physical length of the boom
- The number of elements on the boom
- The spacing of each element along the boom
While a Yagi antenna is a great antenna, you can improve the performance of this antenna by stacking one on top of another. The gain of two 3-element horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically is approximately 3 dB higher than the gain of a single 3-element Yagi. (G9C09) The advantage of vertical stacking of horizontally polarized Yagi antennas is that it narrows the main lobe in elevation. (G9D05)
Although the driven element of a Yagi antenna is similar to a dipole, the other elements cause the feedpoint impedance to be significantly lower than 72 ohms. The purpose of a gamma match used with Yagi antennas is to match the relatively low feed-point impedance to 50 ohms. (G9C11) An advantage of using a gamma match for impedance matching of a Yagi antenna to 50-ohm coax feed line is that it does not require that the elements be insulated from the boom. (G9C12)
You can also make directional antennas using loop antenna elements. The elements of a quad antenna are square loops. Each side of a quad antenna driven element is approximately 1/4 wavelength. (G9C13) Each side of a quad antenna reflector element is slightly more than 1/4 wavelength. (G9C15) Assuming one of the elements is used as a reflector, the reflector element must be approximately 5 percent longer than the driven element, for the antenna to operate as a beam antenna. (G9C06)
The forward gain of a two-element quad antenna is about the same as the forward gain of a three-element Yagi antenna. (G9C14) The polarization of the radiated signal changes from horizontal to vertical when the feed point of a quad antenna is changed from the center of either horizontal wire to the center of either vertical wire. (G9C18)
The elements of a delta loop beam are triangular. Each leg of a symmetrical delta-loop antenna is approximately 1/3 wavelength. (G9C17) The gain of a two-element delta-loop beam is about the same as the gain of a two-element quad antenna. (G9C16)