Measuring Transmission Line Length

The SARK-110 supports several methodologies that allow easy measurement of the length of transmission lines. These are explained in this application note for the measurement of a section of approximately 3 meters of RG-58 cable as a practical example.

FDR Method (Cable Test Mode)

The Cable Test Mode uses the Frequency Domain Reflectometry (FDR) measurement technique for performance verification and failure analysis of transmission lines. One of its applications is to determine the length of a cable.

The measurement procedure is as follows:

1) Connect the section of cable to be tested to the analyzer and leave the other end unterminated.

2) Select Cable Test mode.

3) Select the cable type from the dropdown list from menu «Setup» <Cable Type>.

4) Set the more appropriate Zoom option according the expected cable length. For this short cable length, it will be selected «ZOOM» <1st Sixteenth>.

5) For the automatic determination of the cable length, set a marker to track the maximum value of impulse.


6) Wait for the measurement to complete and the marker will automatically position itself at the peak of the impulse response. The distance reading will be the length of the cable; 3.0 meters in this case.


7) The distance value in the marker area has low resolution. Select «Marker 1» <Info> to get the distance with higher resolution; 2.982 meters in this case.

Smith Chart “Unwrap” Method

The Smith Chart “unwrap” method is described in section 4.1. of the User’s Manual as a method to determine the length of a cable and cancel the effect of it using the Transmission Line Subtract feature.

The measurement procedure is as follows:

1) Connect the section of cable to be tested to the analyzer and leave the other end unterminated.

2) Select the cable type from the dropdown list from menu «Setup» <Cable Type>.


3) Select Smith Chart mode.

4) Select «Preset» <Full Span>.

5) Set the estimated cable length in menu «TL Len», but as a negative value as we are subtracting the effect of a transmission line; in this case we will enter a value of 3 meters.


6) We observe in the screenshot below that the trace is in the inductive area of the chart, so we will need to adjust the length value to have the trace around the point of infinite impedance.


7) We set the TL value to -2.9 meters and the trace moved to the capacitive region of the chart. Then the length value must be between -2.9 and 3 meters.

 

8) After checking several intermediate values, we found that the correct length of this coax is -2.958 meters.


9) Besides determining the length of the cable, it can be connected the antenna to the end of the coax cable and the effect of this cable will be discounted.

Half-Wave Method

This method is a variant of the Tuning Stubs method described in section 4.3. of the User’s Manual. This is based on that the Markers Info feature provide information about the electrical length of the transmission line.

The measurement procedure is as follows:

1) Connect the section of cable to be tested to the analyzer and leave the other end unterminated.

2) Set the Stubs-Wavelen parameter to half wave: «Setup» <Stubs> to 000.50


3) In the same Stubs setting, enter the velocity factor of your cable:

 

4) Set the sweep range from the lowest frequency to a frequency where the trace shows at least one rotation on the Smith Chart.

5) Set a marker to detect the Peak Max of |Zs|:

 

6) Move the marker to the second peak (the first peak is at the lowest frequency and the second is at the next half-wave point); in this case it is located at 33,424,390 Hz

 

7) Select «Marker 1» <Info>; the Len reading at the bottom will show the physical length of the cable; in this case 2.960 meters.

Transmission Line Model

If the characteristics of the transmission line are unknown, the analyzer provides a method to obtain them automatically. The procedure is described in section 4.2.4. of the User’s Manual, so it will not be repeated here. The results for the used test cable are shown below as reference.