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## [TowerTalk] SWR question

 To: [TowerTalk] SWR question ccc@space.mit.edu (Chuck Counselman) Fri Feb 28 10:49:27 2003
 ```In most discussions about the consequences of high SWR, and perhaps in this one, forward power is confused with _net_ forward power. By _net_ forward power I mean forward power minus reverse power. E-M waves, carrying power, travel in both directions on a transmission line. A directional coupler/bridge whose characteristic impedance Zo matches that of the line distinguishes between and can measure each of these two, forward and reverse, waves separately. From the amplitudes (in volts or amps) of the two waves and Zo, a typical "SWR" meter derives (by some analog or digital process of calculation) the power carried by each of the forward and reverse waves, the percentage of "reflected" (i.e., the ratio of reverse to forward) power, and/or the VSWR. E.g., a VSWR equal to 3 corresponds to a forward-wave amplitude (in volts) equal to two times the reverse-wave amplitude (in volts); maximum voltage on the line occurs where the amplitudes of the two waves are in-phase (2+1=3); minimum voltage on the line occurs where the amplitudes of the two waves have opposite phases (2-1=1); and reflected-power ratio equals 25%. Assuming for simplicity that the line is lossless and that a 100-watt transmitter is matched to the line by a lossless "antenna tuner" or matching network, then: (1) A meter between the transmitter and the tuner will indicate Forward power = 100 watts; Reverse power = 0 watts; % refl. power = 0 %; VSWR = 1:1; and the _net_ forward power is 100 - 0 = 100 watts. (2) A meter between the tuner and the antenna will indicate Forward power = 133.33 watts; Reverse power = 33.33 watts; % refl. power = 25 %; VSWR = 3:1; and the _net_ forward power is 133.33 - 33.33 = 100 watts. Note that the forward power exceeds the power output from the transmitter because the 33.33 watts of reverse power returning to the tuner is reflected from the tuner back toward the antenna. All of the 100 watts put out by the transmitter is delivered to the load, i.e., the antenna, because we have assumed no losses. If the antenna itself has no loss, and if nothing in the near field of the antenna (e.g., the earth) dissipates power, then all of the 100 watts put out by the transmitter is actually radiated. In the real world, both the line and the tuner have losses, and the story is different in several respects: First, some of transmitter's output power is dissipated in the tuner. Second, some of the reverse power returning to the tuner is dissipated in the tuner and not reflected back toward the antenna. Third, some of the forward power, and some of the reverse power, flowing in the line between the tuner and the antenna, are dissipated in the line. The arithmetic is somewhat complicated, but it's easy with a computer. Various computer programs are available, some from the ARRL, and some from the web (e.g., see ). An interesting question for you armchair lawyers out there: Does the FCC's peak-transmitted-power limit of 1500 watts refer to the forward power somewhere, or to the _net_ forward power? IMO, the FCC regulation _must_ refer to _net_ forward power because forward power can be almost anything, depending on where you measure it, for a given transmitter power, for a given radiated power, or for a given power delivered to the antenna. 73 de Chuck, W1HIS ```
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