Leakage and Signal Quality
Leakage and Signal Quality
This domain tests signal containment and the quality metrics that reveal plant problems. Expect FCC leakage limits, CLI, leakage logging and repair, ingress/egress, shielding integrity, MER/BER, tilt, ripple, and common-path distortion.
Core rules and concepts
- Signal leakage is egress from the plant; ingress is outside signal entering the plant. Both point to shield or connector faults, and ingress often appears first as upstream noise.
- FCC Part 76 sets a routine leakage limit of 20 µV/m at 3 m and a cumulative leakage index (CLI) of 64. Aeronautical bands have stricter limits and testing rules; always follow the current rule and local procedures.
- Leakage is measured in field strength (µV/m), not dBmV. Log location, frequency, level, and repair; re-test after repair.
- Quality is more than level: MER, pre/post-FEC BER, constellation, tilt, ripple, and group delay reveal impairments that a level meter alone misses.
Worked example
A technician measures 25 µV/m at 6 m. In the far field, field strength falls with distance; at half the distance it roughly doubles. The estimated 3 m value is about 50 µV/m, well above the 20 µV/m routine limit. The correct action is to locate and repair the fault, then re-measure at 3 m and document the result.
Traps that cost marks
Confusing µV/m with dBmV; testing leakage with no carrier present; ignoring aeronautical frequencies; assuming low MER always means a bad amplifier when the cause may be a customer drop; failing to re-test after repair.