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.

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

dummy load, instead of the antenna, is conected to the output of the transmitter, and the normal transmitter tuning procedure is followed.

Most Navy transmitters have a built-in dummy load. This permits you to switch between the dummy load or the actual antenna, using a switch. For transmitters that do not have such a switch, the transmission line at the transmitter is disconnected and connected to the dummy load (figure 2-18). When transmitter tuning is complete, the dummy load is disconnected and the antenna transmission line is again connected to the transmitter.