The Early Days of CLI Leakage

The new regulations required the operator to monitor, measure with a specific technique, and log all system leakage.  The operators soon realized that this was a difficult task with their current equipment, as signal level meters had insufficient sensitivity to measure 20 uV/m at even 10ft, much less be able to see it at easement or street level. For the close in measurements, filters and amplifiers were externally added to signal level meters, which just would have sufficient sensitivity to capture a 20uV/m leak, but the system was awkward to handle and had no use in monitoring.  A second problem for the operator came about when the marketing department did not want a channel space to be taken up by a test carrier. They wanted a sellable service here.  Bandwidth was in demand.

Just a few months before the CLI regulations became effective; a product known as the Searcher Plus hit the market. This seemed to solve all the testing problems stated earlier and became a valuable asset in leakage evaluation.  The product was a crystal tuned receiver with a calibrated display capable of resolving 2uV/m and with a variable audio tone which increased in frequency as the receive level became stronger.  The unit was sold with a vehicle mount which automatically connected it to an external antenna used for drive around monitoring.  To implement leakage monitoring, the operator would drive out the system and listen for leakage audio. When the receiver went off, the operator would drive until the peak of the signal was heard, stop the vehicle and prepare to find the source. The technician would remove the unit from the vehicle mount and attach it to a dipole antenna and a three-meter collapsing pole and turn the antenna until a null in the audio was heard. The antenna elements were generally pointing at the leak source.  Heading in that direction, the technician would position the dipole directly under the leak, if possible, approximately 3 meters away, spin the dipole slowly and secure the peak reading from the meter.

This technique worked fine until in the presence of micro-gap radiation from power lines. To solve this problem, a circuit was added which detected the presence of the Vertical Blanking Interval (VBI) in a NTSC signal. When detected, the circuit would allow the audio tone to announce the leakage and not sound off on other noise radiation. This post detection circuitry also, through its narrow filter, allowed significantly more receiver sensitivity yielding measurements at much greater distances.

All these advancements happened in relatively rapid succession but were just the beginning of technological innovation trajectory in cable signal leakage technologies.


Part 1 of a 3-part series, see the  other posts in this series:


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