Rogues Gallery

One common design of distribution system has a long cable running the length of the premises, with tap-off units at intervals. The signal available on this cable should be far stronger than is needed by an individual TV set because the tap-off units are designed to extract a very small percentage of it and feed it to one or more downleads. The majority of the signal passes along the main cable to the next tap-off unit.

The tap-off unit in the first photograph has a tap value of 26dB, meaning that only 5% of the signal should pass to the downleads, or to put it another way the signal on the main cable should be more than twenty times stronger than is needed by a TV set.

In this case someone has simply bypassed the tap-off unit by connecting three of the downleads to the main cable, presumably to increase signal strength at these three outlets. This is an absolute botch for many reasons. A severe impedance mismatch is caused, so signals will bounce up and down the cables causing all sorts of problems. Signal levels at every outlet further down the line will be very low indeed. If the main cable is carrying power to a repeater amplifier a voltage will be present on the downleads.

The second photograph shows a 15dB tap. Since this tap is at the end of the line the installer has correctly terminated it with a 75 ohm resistor. But then the botcher-in-chief has come along and added his bits of wire.
The perpetrator should have found out why the signal levels on the main cable were low and corrected the fault.

Thanks to Bob Whelan for these photographs.

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