Standing Wave Ratio

used to extract energy from a passing radio wave, maximum signal pickup results when the antenna is placed physically in the same direction as the electric field component. For this reason, a vertical antenna is used to receive vertically polarized waves, and a horizontal antenna is used to receive horizontally polarized waves. At lower frequencies, wave polarization remains fairly constant as it travels through space. At higher frequencies, the polarization usually varies, sometimes quite rapidly. This is because the wave front splits into several components, and these components follow different propagation paths. When antennas are close to the ground, vertically polarized radio waves yield a stronger signal close to the Earth than do those that are horizontally polarized. When the transmitting and receiving antennas are at least one wavelength above the surface, the two types of polarization are approximately the same in field intensity near the surface of the Earth. When the transmitting antenna is several wavelengths above the surface, horizontally polarized waves result in a stronger signal close to the Earth than is possible with vertical polarization. Most shipboard communication antennas are vertically polarized. This type of polarization allows the antenna configuration to be more easily accommodated in the limited space allocated to shipboard communications installations. Vertical antenna installations often make use of the topside structure to support the antenna elements. In some cases, to obtain the required impedance match between the antenna base terminal and transmission line, the structure acts as part of the antenna. VHF and UHF antennas used for ship-to-aircraft communications use both vertical and circular polarization. Because aircraft maneuvers cause cross- polarization effects, circularly polarized shipboard antennas frequently offer considerable signal improvements over vertically polarized antennas. Circularly polarized antennas are also used for ship- to-satellite communications because these antenntas offer the same improvement as VHF/UHF ship-to- aircraft communications operations. Except for the higher altitudes, satellite antenna problems are similar to those experienced with aircraft antenna operations. Standing Wave Ratio Another term used in antenna tuning is standing wave ratio (SWR), also called voltage standing wave ratio (VSWR). A simple definition could be the “relative degree of resonance” achieved with antenna tuning. When tuning an antenna, you must understand the SWR when expressed numerically. You will hear SWR expressed numerically in nearly every tuning procedure. For example, you will hear such terms as “three-to-one,” or “two-to-one.” You will see them written 3:1 SWR, 2:1 SWR, or 1:1 SWR. The lower the number ratio is, the better the match between the antenna and the transmitter for transmitting RF signals. For example, a 2:1 SWR is better than a 3:1 SWR. As you approach resonance, you will notice that your SWR figure on the front panel meters will begin to drop to a lower numerical value. A good SWR is considered to be 3 or below, such as 3:1 or 2:1. Anything over 3, such as 4:1, 5:1, or 6:1 is unsatisfactory. The SWR becomes increasingly critical as transmitter output is increased. Where a 3:1 SWR is satisfactory with a 500-watt transmitter, a 2:1 SWR may be considered satisfactory with a 10-kilowatt transmitter. Most antenna couplers have front panel meters that show a readout of the relative SWR achieved via antenna tuning. Figure 2-16 shows a multicoupler, Figure 2-16.—AN/SRA-33 antenna multicoupler. 2-16