Engineering information VSWR/return loss conversion Reflection can be estimated by reflection coefficient, which is the ratio of reflected wave voltage (current) to incident wave voltage (current). Reflection coefficient has a magnitude and phase and can be represented by complex numbers. Another parameter for reflection is voltage standing wave ratio (VSWR). VSWR is de- fined from the magnitude of reflection coefficient and, therefore, does not have a phase. Return loss com- pares the power in the reflected wave with that in the forward wave. The unit for return loss is decibel. Return loss can be calculated from VSWR and vice-versa. VSWR to return loss VSWR Ref- lection coeffi- cient Return loss dB 1.01 0.0050 46.06 1.02 0.0099 40.09 1.03 0.0148 36.61 1.04 0.0196 34.15 1.05 0.0244 32.26 1.06 0.0291 30.71 1.07 0.0338 29.42 1.08 0.0385 28.30 1.09 0.0431 27.72 1.10 0.0476 26.44 1.11 0.0521 25.66 1.12 0.0566 24.94 1.13 0.0610 24.29 1.14 0.0654 23.69 1.15 0.0698 23.13 1.16 0.0741 22.61 1.17 0.0783 22.12 1.18 0.0826 21.66 1.19 0.0868 21.23 1.20 0.0909 20.83 1.21 0.0950 20.44 1.22 0.0991 20.08 1.23 0.1031 19.73 1.24 0.1071 19.40 1.25 0.1111 19.08 1.26 0.1150 18.78 1.27 0.1189 18.49 1.28 0.1228 18.22 1.29 0.1266 17.95 1.30 0.1304 17.69 Return loss dB 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 Ref- lection coeffi- cient 0.0100 0.0112 0.0126 0.0141 0.0158 0.0178 0.0200 0.0244 0.0251 0.0282 0.0316 0.0355 0.0398 0.0447 0.0501 0.0562 0.0631 0.0708 0.0794 0.0891 0.1000 0.1122 0.1259 0.1413 0.1585 0.1778 0.1995 0.2239 0.2512 0.2818 VSWR 1.020 1.023 1.026 1.029 1.032 1.036 1.041 1.046 1.052 1.058 1.065 1.074 1.083 1.094 1.106 1.119 1.135 1.152 1.173 1.196 1.222 1.253 1.288 1.329 1.377 1.433 1.499 1.577 1.671 1.785 Power handling of RF coaxial assemblies Two potential failure modes must be considered when determining the power handling capability of an RF coaxial cable: • Peak power (voltage breakdown) • Average or CW power The peak power (voltage breakdown) occurs when the voltage gradient between the cable center conduc- tor exceeds a limiting value causing the signal to arc across the path of the least resistance. Generally, the path of the least resistance is located at the cable/ connection junction. Catastrophic breakdown is not the only problem: the existence of corona, usually around the center conductor, produces other dele- terious effects. Corona cutting is a concern with PTFE insulators whereupon the PTFE is eroded causing the formation of cavities (usually without carbonisation). Coaxial cable assemblies are typically rated with the peak power handling much lower than what the inter- face can handle. To maximise peak power of the cable assembly, a high voltage (HV) connector should be used. A higher voltage potential is achieved by over- lapping the dielectric thereby increasing the airline arc path. A drawback to this design is that connectors which are modified in this way generally have greater VSWR at higher frequencies. If the transmission line has reflections, the voltage and the current along the line will have maximums and minimums. The cause of this nonuniform distribution is superposition of the incident and reflected waves. Breakdown is a function of the maximum voltage. Higher reflection results in lower voltage handling. Even high-performance assemblies with low VSWR can have poor peak power handling if they are connected to an unmatched load. Peak power handling is de- pendent on frequency since the typical value of VSWR is proportional to the increase in frequency. The most common breakdown at high altitudes (usually greater than 70 000 ft) is ionisation breakdown in the air path. For vacuum and space applications, the main type of breakdown is multipaction breakdown. For average power rating of a cable with a pulsed sig- nal, multiply the peak power rating by the duty cycle. Frequency range, ambient temperature, altitude, physical size, and the thermal properties of each layer of construction are the primary factors which deter- mine the average power handling capability of an RF coaxial cable. The average power failure occurs when the level of power transmitted results in resistive 193
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