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Cable talk

Structural Return Loss

What it is and why it matters

Structural Return Loss, SRL is an index of the manufacturing consistency of the coaxial cable.

In every cable, small variations of its physical dimensions lead to impedance variations along its length. Each of these impedance discontinuities causes a small amount of signal to reflect backward. These tiny reflections sum up resulting in a reflected signal.

SRL is the logarithmic ratio of incident signal to reflected signal in a cable, expressed in dB. The higher the absolute value in dB, the weaker the reflections are, the better the cable is.

The following screenshot shows a SRL measurement of a good cable. In each frequency the SRL value is far beyond the limits of EN 50117.

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The following screenshot shows a cable with “bad” SRL. It fails the limit line in many points.

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In the case of terrestrial analogue television broadcast, the use of a cable with bad SRL can result in the appearance of “ghosts” in the picture.

In the case of digital transmitted signals (DVB-T or DVB-S) a cable with bad SRL can result to “freezes”, sound gaps or to a blank screen.

 

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