consisting of four coupling units, with four SWR meters at the top (one for each coupler). To achieve a perfect standing wave ratio of 1:1 would mean that we have succeeded in tuning out all other impedances and that the antenna is matched perfectly  to  the  transmitted  frequency.  With  such  a  low SWR,   the   antenna   would   now   offer   only   its characteristic  impedance.  A  1:1  SWR  is  rarely achieved, of course. There will always be some power loss  between  the  transmitter  and  the  antenna  because  of natural  impedances  that  exist  between  the  two. Nevertheless,  the  objective  is  to  achieve  the  lowest SWR possible. In other words, we want only the characteristic impedance of the antenna remaining. Incident  Waves Various  factors  in  the  antenna  circuit  affect  the radiation of RF energy. When we energize or feed an antenna with an alternating current (ac) signal, waves of energy are created along the length of the antenna. These waves, which travel from a transmitter to the end of the antenna, are the incident waves. Let’s look at figure 2-17. If we feed an ac signal at point A, energy waves will travel along the antenna until they reach the end (point B). Since the B end is free, an open circuit exists and the waves cannot travel farther. This is the point of high impedance.  The energy waves bounce back (reflect) from this point of high impedance and travel toward the feed point, where they are again reflected. Reflected   Waves We call the energy reflected back to the feed point the  reflected   wave.   The  resistance  of  the  wire gradually  decreases  the  energy  of  the  waves  in  this back-and-forth  motion  (oscillation).  However,  each time the waves reach the feed point (point A of figure 2-17), they are reinforced by enough power to replace Figure  2-17.—Incident  and  reflected  waves  on  an  antenna. the lost energy. This results in continuous oscillations of energy along the wire and a high voltage at point A on the end of the wire. These oscillations are applied to the antenna at a rate equal to the frequency of the RF voltage. In a perfect antenna system, all the energy supplied to the antenna would be radiated into space. In an imperfect system, which we use, some portion of the energy is reflected back to the source with a resultant decrease in radiated energy. The more energy reflected back, the more inefficient the antenna. The condition of most  antennas  can  be  determined  by  measuring  the power being supplied to the antenna (forward power) and the power being reflected back to the source (reflected power). These two measurements determine the  voltage  standing  wave  ratio  (VSWR),  which indicates antenna performance. If an antenna is resonant to the frequency supplied by the transmitter, the reflected waves and the incident waves are in phase along the length of the antenna and tend to reinforce each other. It is at this point that radiation is maximum, and the SWR is best. When the antenna is not resonant at the frequency supplied by the transmitter, the incident and reflected waves are out of phase along the length of the antenna and tend to cancel out each other. These cancellations are called power losses and occur when the SWR is poor, such as 6:1 or 5:1. Most transmitters have a long productive life and require   only   periodic   adjustment   and   routine maintenance  to  provide  maximum  operating  efficiency and reliable communications. Experience has shown that many of the problems associated with unreliable radio communication and transmitter failures can be attributed to high antenna VSWR. Dummy  Loads Under  radio  silence  conditions,  placing  a  carrier  on the air during transmitter tuning would give an enemy the opportunity to take direction-finding bearings and determine  the  location  of  the  ship.  Even  during  normal periods of operation, transmitters should be tuned by methods that do not require radiation from the antenna. This precaution minimizes interference with other stations  using  the  circuit. A dummy load (also called dummy antenna) can be used to tune a transmitter without causing unwanted radiation. Dummy loads have resistors that dissipate the RF energy in the form of heat and prevent radiation by  the  transmitter  during  the  tuning  operation.  The 2-17