When you say "In terms of an engineers perspective" do you mean an aeronautical engineer or a maintenance engineer? Please don't consider this request a challenge (as often happens). I just want to understand your perspective.
I'd like to provide some clarification for your comments.
Your statement "the computer checks the antenna impedance for the omnidirectiona upper and lower antenna" implies that both the upper and lower TCAS/ACAS antennas are omni-directional. This is not true. TCAS/ACAS systems utilize either an upper and lower "directional" antenna or an upper directional and a lower omnidirectional (monopole) antenna, but never an upper omnidirectional antenna. It would make the system fairly useless as all of the RAs and TAs would lack relative bearing information.
When an aircraft is fitted with an upper and lower directional antenna, the lower one is placed in an "omnidirectional" mode when the landing gear is down. This reduces the chances of an erroneous traffic bearing due to reflections from the landing gear. Traffic signals received from "omni" antennas produce a "no bearing" traffic alert or display.
I have one more suggestion. I've been involved in curing upper antenna failures on some aircraft (757 especially). Water ingestion starts the problem, usually caused by bad or neglected antenna sealing and/or condensation. The coaxial connectors become worn fairly quickly on the line following antenna troubleshooting because maintainers seem to love to use pliers on them when finger power is all that should be needed and then use oversized probes to do continuity checks. Eventually the connectors on the antenna fail to connect properly and the antenna is garbage.
Installing an antenna disconnect near the antenna can faciliate maintenance without damaging the antenna. Make the connection close enough to the antenna that personell will notice it when accessing the unit.
P.S. Make sure you get the modification approved!