Methods for measuring cable path loss in RF Testing
When you're using a network analyser to measure S21, it generates a signal on port 1, and receives a signal on port 2. S21 is a measurement of the ratio of those signals.

While a network analyser does much more than a signal source and power meter can (S21 is the complex ratio), for this cable loss application, if you display simply the magnitude of S21, you can regard it as a signal source and power meter in a single box.