By Chris Duros, Edwin L. Dickinson
Qualification of cable systems for leakage integrity is being performed by both ground and airborne procedures. A correlation of the airspace and ground measurements was established by the Advisory Committee on Cable Signal Leakage in the late 1970s. This data was taken on a relatively few cable systems and is, therefore, subject to refinement as more data is collected. Some initial observations of recent data provide certain insights into the leakage patterns observed in airborne observations and their sources on the ground. The effects of large single leaks on these patterns, some probable causes, and the implications on ground monitoring procedures are treated in this paper.
You would think that enough had been said about cable signal leakage to last a lifetime. Unfortunately, leakage control probably will last a lifetime and the discussion may never be done. When the FCC rules for qualification of cable systems to the leakage standards become effective in July, 1990 we will only be at the "first hurdle in the race" since qualification must be done yearly and perhaps forever. Leakage is a relatively simple subject on the surface, however, there are many nuances some of which we have yet to learn.
The cable industry has gathered considerable data taken both from ground and airborne measurements. There is a pressing need to investigate correlation between ground and air results. To date, little work has been done toward investigating correlations because of a lack of concurrent ground/airborne data plus the complexity of the situation. Analysis of data taken on a few systems has shown major disparities between ground and airborne results. In these cases the airborne data usually indicates more leakage signal in the airspace than predicted by the groundbased CU. As a matter of fact, flyover measurements of some systems look very bleak indeed, with large sections of the system showing leakage in excess of the limit of ten microvolts per meter (lOuV /m) at 1500 feet above the cable system.