*These basic RF measurements often uncover system problems in wired and wireless communications.*

If you observe a signal traveling from a source to a load through a passive system of some sort, you will notice that the signal attenuates by the time it reaches the load, and you will also notice that some of the signal reflects back to the source. **Figure 1** shows an incident signal of power *P _{I}* (blue arrow), reflected signal of power

*P*(red arrow), and attenuated signal of power

_{R}*P*(black arrow) reaching the load. The attenuation and reflection contribute to insertion loss (

_{A}*IL*) and return loss (

*RL*), respectively.

**How do we calculate IL and RL?**

Let’s start with

*RL*, where we will focus on the source and system (

**Figure 2**), which shows an incident signal of voltage

*V*and a reflected signal of voltage

_{I}*V*.

_{R}Looking into the source, we see a source impedance *Z _{S}*, and looking into the load, we see a load impedance

*Z*. If

_{L}*Z*≠

_{S}*Z*, you will have reflections. In recent posts on the Smith chart, I discussed the reflection coefficient:

_{L}*RL*, expressed as a power level in decibels, relates to *G* as follows:

Note there’s a 20 in equation 2 before the log function instead of 10. That’s because *G *is a ratio of voltages. For RL, we need a ratio of power levels, which we can get by squaring the voltages or doubling their logarithms—using the latter approach gives us the 20. **Table 1** shows some sample values for *RL* calculated from equation 2.

**Wait, from your Smith chart posts I know we want to keep ***G* low, which, from equation 2 and Table 1, implies keeping return loss high. How can high losses be good?

*RL* indicates loss from the perspective of the reflected signal. As the highlighted entries in Table 1 show, a 20-dB *RL* indicates that the reflected signal’s power represents just 1% of the incident signal’s power, or in other words, the reflected signal has “lost” 99% of the incident signal’s power. And that’s a good thing, because 99% of the incident signal’s power can continue to the load in Figure 1. Indeed, a 20-dB *RL* is generally considered good performance.

**What about insertion loss?**

Insertion loss refers to the amount of signal power lost in a system such as that in Figure 1 due to return loss as well as dielectric, copper, and other losses. For the Figure 1 system, you can calculate insertion loss in decibels as follows:

**What instruments do I need to measure IL and RL?**

The vector network analyzer (VNA) is the go-to instrument for these measurements. A VNA can measure the scattering parameters

*s*and

_{11}*s*, which relate to

_{21}*RL*and

*IL*, respectively. Teledyne LeCroy elaborates here on S-parameters and their relationship to

*IL*and

*RL*.

You do have alternatives to the VNA. Tektronix offers an application note describing, among other things, how to use an oscilloscope to measure *RL* for 1000BASE-T Gigabit Ethernet. To determine *IL* for the Figure 1 system, you can use a signal generator to generate *P _{I}*, measure

*P*with power meter or spectrum analyzer, and calculate

_{A}*IL*using equation 3. And finally, time-domain reflectometry (TDR) is useful for evaluating

*RL*. TDR enables distance-to-fault (DTF) measurements. If you have several hundred feet of underground cable that is suddenly exhibiting 0-dB

*RL*, DTF can tell you where to start digging. Anritsu has more here.

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