Agilent Technologies AN 1287-6 Application Note page 24

A network analyzer consists of either three or
four channels (Figure 12). In network analyzer
terminology, a channel refers to the hardware used
to detect a signal. There are one or two R channels
in a network analyzer. A portion of the stimulus
signal is coupled out and goes to the R channel
for ratioing. The A channel denotes the channel
associated with test port 1. The A channel meas-
ures the reflected signal when making a forward
measurement (stimulus signal is at port 1), and
measures the transmitted signal when making
reverse measurements (stimulus signal is at port
2). The B channel, associated with test port 2,
measures the transmitted signal during forward
measurements and the reflected signal during
reverse measurements.
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Source
A
Port 1
3 Receivers
Source
A
Port 1
Figure 12. Three versus four channels in a test set
Transfer switch
R
B
Port 2
Transfer switch
R1
B
R2
Port 2
4 Receivers