S-18 Transmission Line Trainer

S-18 series brochure (PDF) 490K

** Impedance matching designed by Prof Girish Kumar, Dept of Elect Eng, IIT Bombay, India
Laptop shown above is not included
Experiment & Study Areas
Standing Wave Patterns
upto four patterns at a time
Complex Impedance measure R+jX
SMITH Chart Auto and Manual
Study Relationship of VSWR,
shift in minima, reflection co-eff for
  Case 1: Matched Line
  Case 2: Open-Circuited Line
  Case 3: Short-Circuited Line
  Case 4: Inductive (nonresistive) Load
  Case 5: Capacitive (nonresistive) Load
  Case 6: Resistive Load > Zo
  Case 7: Resistive Load < Zo
  Case 8: Resistive-Inductive Load
  Case 9: Resistive-Capacitive Load
VSWR, V(i), V(r) vs FREQ
Impedance Matching
Impedance Variation along
mismatched line
Insertion Loss
TDR (Time Domain Reflectometer)

Brief Description & Key Specifications


STS-3033 Software Features ...

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Standing Wave Pattern

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Four Standing Wave Patterns

Standing Wave Patterns

Standing Wave Patterns are generated depending on the type of load terminating the transmission line. Up to four standing wave patterns can be obtained for concurrent study. Voltage as well as Current SWR patterns can be measured.

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Complex Impedance (R+jX) Measurement

Complex Impedance measurement requires two Standing Wave Patterns, one with the line terminated in a short and the other with the unknown load.
Computations based on the linear difference between the minima of the two patterns (Xvm) and the VSWR of the Unknown Impedance, will yield the values of R+jX of the Unknown Impedance.

The picture on the left shows the measurement of Xvm, calculation based on the entry of Frequency, Xvm, and VSWR to yield the values of R & jX; and plotting the point on the Smith Chart using the manual mode.

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Smith Chart - Auto (Vector Network Analysis)

Automatic measurement and plotting of the Smith Chart is possible using the Auto Smith Mode from 450 to 1300MHz. The plot starts at 450MHz with a Blue Square. Subsequent points at 500MHz to 850MHz are plotted in Red Dots and joined by a line. In the above plot the VSWR is less than 1.3 at 800MHz. (DUT is a Quad Antenna)

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Impedance Matching

Impedance Matching is essential for maximum power transfer to a load. An ideal load would be 50+j0 ohm. However real life loads (R+jX) require matching networks to transform them as close to the ideal as possible, where VSWR 1.0 (generally VSWR <2 is acceptable)
A user can fully understand what impedance matching implies and get a theoretical and experimental feel of the matching process.

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VSWR vs Frequency Mode

This mode is useful for the study of Incident Voltage (Vi) & Reflected Voltage (Vr) of a device under test (DUT) and computation of VSWR vs Frequency
DUT shown is a 800MHz QUAD Antenna connected via the Directional Coupler. The Plots are taken over the 750 - 850 MHz freq range. The user can locate the minimum VSWR to identify the resonant freq of the antenna. The VSWR is measured as 1.015 at the resonant frequency of 810.5MHz.

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Insertion Loss (Scalar Network Analysis)

The user can measure the Insertion Loss or thru loss of a passive device or measure gain of an active device of, say an amplifier.
The start/stop frequency and the no. of steps are to be selected. Initially the RF output is measured and stored by the software. The DUT is then connected and the measurement is normalized against the stored values and the Insertion Loss (or gain) vs Frequency is plotted.
Two cursors give readout of Frequency and dB Loss. A text file of Freq & Loss can be saved.


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Impedance Variation along mismatched Transmission Line

When there is a mismatch, the impedance will be alternatively higher and lower than the Zo across the line. The V and I standing waves above have been obtained with a short termination.
The Impedance Plot is obtained by computing the V/I at each point and plotting it. Cursor readout of the min & max values is 0.707 ohm & 4566 ohm which agrees with the theoretical values of 0 & infinity .

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Spectrum Analysis

In this mode the Frequency Spectrum of an input signal is measured & plotted. The start, stop frequency, no. of steps and step size are user selectable via software

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RF Recorder Mode (Level & VSWR Monitoring)

In this mode the RF Signal is connected to the Receiver. Measurement freq is set via the software. The dB level is measured and displayed Digitally and also plotted as a Scroll Graph.
The RF Recorder Mode can be used for exploration and behaviour of RF signals in real time.

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TDR (Time Domain Reflectometer)

The S-18R TDR is a simple but powerful tool used to evaluate transmission lines. Any oscilloscope having bandwidth >20MHz will display the incident and the reflected pulses.
Users can analyze the pulse response and measure time intervals to understand effect of cable terminated in a short, open, characteristic impedance. They can also study velocity factor, attenuation and cable fault location.