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Cable talk

Why don't we get all channels?

In a previous post we talked about the Structural Return Loss (SRL) of coaxial cables and why it matters.

Now it will get interesting. There is a special case of bad SRL called periodicity. In this case there is a manufacturing defect repeated at regular length intervals.


As a result the cable has very high loss, selectively at specific frequencies only.

(For those that want to dig into the technical details:
The frequency at which 1/2 of its wavelength coincides with the length between two consecutive imperfections, and its integer multiples, is affected).

The following Vector Analyzer screenshot shows the SRL measurement of a real cable found in the Greek market (let's call it cable X) which has the periodicity problem.


Although SRL is generally "bad" and exceeds the limits in several places, there is a small frequency range, around 500 MHz, where the absolute value of the SRL is extremely low, just 2dB.


The following screenshot shows attenuation measurement of the same cable in dB/100m. At frequencies close to 500 MHz, cable loss is skyrocketed.

The affected frequencies correspond to Channel 24.



Those "lucky" installers who will use this cable in areas where there is a broadcast on channel 24, will certainly have poor or no reception of the corresponding channels 

How long will it take for them to find out the problem is in the cable, and replace it?

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