Mohr CT100B Series - Return Loss (S ); Insertion Loss (S ); Cable Loss (S ); Comparison of S

7. TDR Measurement Theory
Figure 7.10. Comparison of S
11
return loss of a 2.4 GHz WiFi patch antenna measured
using a handheld VNA (Agilent® FieldFox N9927A, green) and TDR (CT100HF, blue),
showing good agreement from DC to ~8.3 GHz. Note bandpass in the 2.4 GHz region.
7.11.2. Return Loss (S
11
)
The complex return loss S
11
is expressed as:
Return Loss RL = −20 · log
10
|S
11
|dB
7.11.3. Insertion Loss (S
21
)
The complex insertion loss S
21
is expressed as:
Insertion Loss IL = −20 · log
10
|S
21
|dB
True 2-port insertion loss measurements require a 2-port instrument; however the cable loss
measurement described below uses the reciprocity feature of passive networks to determine
insertion loss from cables and other suitable 2-port networks using a 1-port test instrument.
7.11.4. Cable Loss (S
21
)
Cable insertion loss in dB is additive per unit length. Assuming that port 2 of a 2-port DUT (e.g.,
a coaxial cable) is terminated with a short (or open) adapter such that all (or nearly all) of the
106 CT100B TDR Cable Analyzers Operator’s Manual