5. TDR Measurement Theory
Figure 5.10.
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.
5.11.2. Return Loss (S
11
)
The complex return loss
S
11
is expressed as:
Return Loss
RL
=
−
20
·
log
10
|
S
11
|
dB
5.11.3. 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.
5.11.4. 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 incident voltage trace is reflected back to the TDR instrument, insertion loss (
S
21
) is half of
the corresponding
S
11
return loss measurement in dB:
Cable Loss
CL
=
RL
2
dB
CT100B TDR Cable Analyzers Operator’s Manual
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