There are a ton of variances in the ATC/TCAS system setups. An aircraft can have two directional antennas (top/bottom) or just one directional on the top along with a non-directional on the bottom. ATC transponders have different antenna arrangements also, standalone single bottom, standalone single top/bottom, or shared top/bottom. Wiring diagrams with the proper effectivity for the aircraft must be consulted,
For this failure, we had a shared top/bottom ATC antenna system. Both units showed a bottom antenna failure.
A co-ax switch (or relay) must be used in a shared antenna system. There are two switches. One switches the top antenna to the selected ATC. Same for the bottom.
We found the top connector goes to the #1 (or left) ATC transponder. The bottom connector goes to ATC #2 (or right). The middle connector goes to the antenna. Each plug type is different..... they cannot be mis-connected.
Measurement from the ATC units to the switch were good. The measurement from the center co-ax to the lower antenna was open (3 ohms or less is what should of been measured).
This aircraft had an intermediate plug located about 18 inches from the antenna itself. One slight pull gave us our answer.....
This is not a critical length co-ax (like a radio altimeter or TCAS directional antenna co-ax would be). We were able to replace the jack and correct the fault.