To simplify the description of the types of measurements a MS2085A/89A Site Master VNA can make, the reflection and transmission measurements are defined in terms of scattering parameters, or S-parameters. For a 2-port and 1-path network, two fundamental S-parameters can be measured, and they are defined as SXY. For a 2-port VNA, measurements of signals leaving Port 1 are called forward measurements, and those leaving Port 2 are called reverse measurements. Signals that leave and return to the same port are designated reflection measurements, and those that leave one port and return to another port are designated transmission measurements.
S-parameters are an abbreviated designation for these measurements, and are used as shown in the following list:
• S11: Forward Reflection
• S21: Forward Transmission
The first number (X) in SXY is the port number into which the signal is being injected, and the second number (Y) is the port number from which the signal is leaving. The S-parameter is a ratio of these two signals.
Additional Examples:
S11
Forward Reflection represents the measurement in which the signal leaves port 1 and is reflected back to port 1.
S21
Forward Transmission represents the measurement in which the incident signal is transmitted from port 1 and is received at port 2.
User
Custom user-defined S-parameters can be selected from the list of options included in the numerator and denominator submenus, in addition to user-defined S-parameter power source port.
User-Defined Numerator/Denominator Combinations
Denominator
Numerator
A1
B1
B2
1
A1
S11 Forward Reflection
S21 Forward Transmission
B1
B2
1
S21 (Ext. Sens)
Forward Transmission represents the measurement in which the signal leaves port 1 and is transmitted to port 2. In this case an external USB power sensor is used as port 2.
VNA Architecture
A VNA can have only 1-port, in which case it measures only reflection signals. A VNA can have 2-ports, in which case it measures both reflection and transmission. A 2-port VNA can also have two different capabilities: 1-path 2-port, or full-reversing. The 1-path 2-port design allows reflection measurements only at one of the two ports and allows transmission measurements only in one direction. With Options 904/906 the MS2085A/89A Site Master is a 1‑path 2‑port, VNA that allows S11 and S21 measurements with a single connection.
