TWR (Traveling Wave Ratio) is the reciprocal of the SWR. The traveling wave ratio characterizes the degree of matching between the antenna and the feeder (it is also said to characterize the matching between the transmitter output and the feeder). TWR is defined as the ratio of the minimum voltage amplitude in the transmission line to the maximum voltage amplitude in the same line. TWR is usually used to describe the performance of open-wire lines, but sometimes it is also used to describe the performance of coaxial cable transmission lines. SWR is almost always used for coaxial lines.
To measure the traveling wave ratio, measurement lines, complete impedance meters, as well as panoramic VSWR meters are used (these measure only the magnitude, without phase).
The traveling wave ratio is calculated as follows:
TWR = (Winc ? Wref) / (Winc + Wref)
where Winc is the incident power and Wref is the reflected power. Ideally, SWR = 1; values up to 1.5 are considered acceptable in the VHF range.
TWR is expressed in relative units: the greater the TWR value, the more efficiently the signal is transferred from the antenna to the transceiver. Full matching occurs when the antenna resistance Ra and the characteristic impedance of the feeder Rf are equal (Ra = Rf). In this case, a traveling wave mode is achieved, in which current and voltage along the feeder length have neither maxima nor minima, and TWR equals one. In this situation, all the power from the transmitter is delivered to the antenna. The higher the TWR value (in various antenna designs, it ranges from 0.25 to 0.9), the more efficient the signal transfer from the antenna to the transceiver.
SWR, Cable Shortening Factor, Characteristic Impedance, Reactive Power.