TCAS II - bottom omni-directional antenna I know that the TCAS II system has a lower antenna which can be either omnidirectional or directional.
How does the functionality of the system change? If the antenna is omni then I think / assume that it cannot know the bearing of an intruding aircraft and therefore can not provide data to position it on a display. Is it only used for transmitting interrogation and/or receiving other data?

Thanks,

Ori
The directionality of a TCAS antenna is ONLY used for positioning the bogey on the TCAS display, it is NOT used for determining time-to-contact. So you're still going to get TAs and RAs from the Omni antenna even if the upper antenna is blanked, but you won't get the display of that traffic.