would you mind explain thoroughly functional test of the transponder and how antenna coupler is used for the test, I believe a test set with a transceiver is installed on the ground which antenna is placed a distance from aircraft and is coupled to the up and bottom ATC antennas, correct me if I'm wrong.
For a functional test we usually use the ATC-601, but there are many other brands available other than Viavi.
The "Power Output or ERP - Effective Radiated Power" test does require the test set to be told the distance to the antenna (upper or lower). This test is usually a failure. I've tried inside hangars and out on the ramp. Finding the perfect distance setting to satisfy the test set is a time consuming and frustrating matter.
When we run tests, we do not run up the altitude with the Air Data Test Set. If the aircraft is placed in the air mode and you crank up the altitude to 30K feet..... you now have an active aircraft reporting to ATC and other aircraft TCAS systems. Major issues could result from such a condition.
If a plane must be run to altitude, couplers (or covers) are placed over the ATC antennas. These are connected by co-ax to the test unit. They prevent radiated transmissions from going out to ATC and flying aircraft.. As a note: most aircraft have both upper and lower antennas. They both must be covered when bringing an aircraft up. You might just be testing one, but the other needs a cover or it needs to be disconnected.
We rarely have issues with altitude reporting from signals from the Air Data Computer. The signal (or output) from the Air Data to the transponders is either there or not. Most control panels will have a "Altitude Reporting" switch which can be selected to either Air Data. That is if the aircraft has been wired to have both Air Data's feeding both ATC's.
The whole "Mode S" test can be accessed (at least with the ATC-601) from instructions provided here.