Scalar measurements provide magnitude and frequency information only and are the most commonly used measurement technique. RF and microwave device parameters are usually specified in scalar values such as VSWR, Return Loss, Reflection Coefficient, and power gain in dB. This application note covers one method of measuring load mismatch.

Reflection Coefficient =

Standing Wave Ratio =

Figure 1

Figure 1 shows a mismatch between transmission line and load. In this case, the incident level is 1 volt and the reflected level is .5 volt.

The Reflection Coefficient expressed in dB is Return Loss.

Return Loss = -20 log = 6 dB

Figure 2

Figure 2 shows a Reflectometer Bridge connected between a RF signal generator and a device under test. If an impedance, , equal to the Bridge characteristic impedance, , is connected to port X, all the incident power is absorbed and the reflected power is zero. In other words, the bridge is balanced. If , some incident power is reflected and is indicated on the power meter. Before the device return loss can be calculated, one other piece of information is required. By placing a short circuit termination at port X. All the incident power is reflected and indicated on the power meter. The return loss of the device is the log of the ratio of the two readings expressed in Watts, or the difference of the two readings expressed in dBm (0 dBm = 1 mW).

Return Loss can be converted to reflection coefficient by using the following identity.

Return Loss = -20 log = -10 log


0 1.000 INFINITE
3 0.707 5.828
6 0.501 3.000
10 0.316 1.923
13 0.224 1.580
16 0.158 1.375
20 0.100 1.222
23 0.0707 1.152
26 0.0501 1.105
30 0.0100 1.020