CaTV | What is Signal leakage?

 

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Cable television systems use radio frequency, or RF, signals to provide television, telephone and broadband Internet services to customers. These RF signals normally do not cause interference when cable systems comply with the rules for limiting interference, but the signals can "leak." 

Cable signal leaks occur when the RF signals transmitted within a cable system are not properly contained. Signal leaks can be caused by loose connectors, damaged equipment or cables, or cables that are unterminated (not connected to a device, panel or wall outlet).

What is RF leakage? Signal leakage refers to the loss or egress of radio frequency (RF) signals from a cable system when they are not properly contained. This can result from a multitude of causes but is generally the result of shielding defects within the cable network.

Cable television systems and licensed broadcasters use many of the same frequencies to transmit programming. Cable signal leakage can interfere with any of the over-the-air services using the same frequencies as the cable operator within the vicinity of the cable system.

Cable systems use broadcast TV, radio, aeronautical radio (Federal Aviation Administration), and other channels. Cable operators are considered secondary users of these frequencies and must not interfere with primary use.

Cable networks are closed systems completely isolated from any off-air signals which may be present. This concept allows frequency sharing between off-air and cable signals such as AM/FM radio, remote control cars, wireless phones, CB radio, aeronautical communications, cell phones, and many others.

The reality is that cable networks are not perfectly sealed, and as a result signals from cable networks can escape (egress) and interfere with off-air signals at the same frequencies. Conversely, off-air signals can enter into the cable network (ingress) and interfere with the cable signals.

If the shielding integrity of the cable network is compromised for any reason, then RF signals inside of the coaxial cables and components can leak out and potentially interfere with licensed over-the-air services. Going the other direction, over-the-air signals can leak into the cable network (this is called ingress), and potentially interfere with cable signals.

This cable signal leakage is invisible, and therefore more difficult to pinpoint the source of the leak. A signal leak from one location can potentially mask a leak from a nearby location. Signal leaks can also be intermittent, meaning a slight bend or movement in the cable from one point in time to the next can render the leak undetectable. Metal objects can also interfere with signal leakage detection equipment.

Since this equipment was originally conceived to accommodate the aeronautical VHF frequencies, long standing practices and equipment are not always effective in detecting leakage at higher frequencies or measuring noise-like QAM signals. 

Another challenge facing leakage monitoring systems is determining which detected signals originate from within a specific cable network versus from other sources. To allow this discrimination, leakage monitoring systems will inject a “tag” or unique RF signal into the cable plant that can then be detected by the field monitoring tools. Anytime that the field gear detects this unique tag signal they can be sure that the detected signal is leaking from their specific system and not an overbuilt competitors’ system or other ambient source.

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