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Structural Return Loss Measurement of Coaxial Cable

As an indicator for evaluating a cable’s uniformity return loss is widely used in cable industry. However many domestic enterprises specialized in cable export find that the foreign customers prefer to use structural return loss(SRL) rather than return loss(RL) to measure the quality of the cable products, for example in the United States, and Australia. Then what differences are there between SRL and RL? 

According to American standards, ANSI/SCTE 03 2003 and ASTM D 4566, SRL is defined as:

                                   

Where SRL is structural return loss, dB; Zin is input impedance(complex numbers), Ω; Zavg is average impedance(complex numbers), Ω.

According to the standards: 

                               

Ri=the real part of the input impedance of the cable at various frequency points;

Xi= the imaginary part of the input impedance of the cable at various frequency points;

Ravg=the averaging value of real parts of the cable input impedances at all frequency points;

Xavg= the averaging value of imaginary parts of the cable input impedances at all frequency points;

 According to IEC 61196 or GB/T 17737 standards RL is defined as:

                                        

Where RL is return loss, dB; ZT is the input end impedance when the termination is connected with nominal impedance (complex numbers),Ω; ZL is the calibrated load.

RL can be measured directly with network analyzer, and SRL is obtained through computing the measured data using computer after measuring the input impedance of the measured cable with vector network analyzer, consequently, to implement SRL measurement requires computer program control technique.

Based on the definition of SRL the present InspectionTestCenter sets up a SRL measurement system based on computer program control technique. The following Fig. shows the measurement results of the RL and SRL from a same cable, where the worst value for RL is 21.92 dB, while the worst value of SRL is 24.11 dB, both the worst values occur at the same frequency point.

                                     
                                                                                SRL

                                     
                                                                                  RL 

From the above mentioned definitions it can be seen that the RL reflects the deviation of cable’s
input impedance from the measurement system’s impedance, it not only embodies the structural
nonuniformity of the cable, but also embodies the deviation (or matching degree) of cable
impedance from the measurement system’s impedance; while SRL only reflects the deviation of
cable’s input impedance from the averaging value of the impedances of the cable itself, so what
SRL reflects is the structural nonuniformity of the cable itself only. Although both indicators, RL and
SRL, can reflect the quality of the cable, STL only reflects the structural nonuniformity regardless of
how the cable impedance deviating from the system’s impedance. Unless the averaging value of
the cable’s characteristic impedance is very close to the system’s impedance or SRL always is better
than RL.

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