http://en.wikipedia.org/wiki/Network_ana…
A microwave or radio frequency (RF) network analyzer is used to measure the characteristics of a microwave or RF network. a network consists of a device with 1 or more ports. for example, a simple antenna will will consist of a single port and an amplifier will consist of two ports (an input port and an output port).
The characteristics of interest to a microwave or RF engineer are typically the gain or loss of a signal as it travels through a 2 port network (like an amplifier) or the reflection of a signal that is incident upon a single port network like an antenna. Agilent (www.agilent.com) produces microwave and RF network analyzers and more information may be obtained from their site.
What is a Network Analyzer used for?