b377,
You are totally missing the point.
At the moment the main wheels touch (which is the moment you would like the radalt to read zero), the real signal path length between transmitter output and receiver input (in the case of the 737) is still 57 ft, made up out of the cables between the R/A unit and the antennas and the path from the antenna to the ground and back.
So there is no question of additional "transmission delay lines", analog or digital.
You simply offset the indicator, so it reads 0 ft, when the R/A measures 57 ft.
Since the exact AID varies between aircraft (which use the same R/A) that offset is made pin-selectable.
CJ