The antenna both increases the receive gain and the transmitted power. One of those rare situations where you benefit coming and going.
Generally the higher the gain the more directional the antenna. Antennas don’t create more signal, they focus it. A theoretically perfect omni-directional antenna would have a gain of 0. Just about all real world omni-directional antennas have a donut or toroidal shaped gain pattern, you get very little signal directly above or below the antenna and 1-2 dB of gain on all sides of the antenna. The other extreme would be a parabolic dish antenna which can have a gain of 40 dB but only in one very narrow direction.
You are correct, the maximum transmit power limits are normally in the direction of peak power so you don’t gain anything from the transmitted gain, if your antenna has 2 dB of gain you need to dial the power down 2 dB. My mistake there, sorry. But you do benefit from the receive antenna gain, this effectively increases your sensitivity, if you are pointing in the correct direction.
So ideally you’d have an omni-directional transmit antenna and send out as much energy as possible equally in all directions. And then have a high gain directional antenna for receiving pointed in exactly the correct direction. In reality you’re normally going to have omni-directional antennas on everything and just get a 1-2 dB benefit on the receive side.
Ground bounce is a special case of multipath. For most multipath the reflected signal path is multiple wavelengths longer, often hundreds or more, UWB is very immune to this.
However if you have two antennas mounted above the ground then at a certain separation you will hit a point where the multipath signal off the ground has traveled exactly 1/2 of a wavelength further than the direct signal. At that point the direct and reflected signals cancel out at the receive antenna meaning that there is virtually no signal power to detect. If there’s no signal power there isn’t a lot you can do about it, you can’t detect the signal because there is nothing to detect.
The wide bandwidth of UWB both helps and hinders this, because it’s wide you aren’t going to fade out all of the signal at the same time so there is always going to be something to detect but it will be weaker. The downside is that because it’s a wide bandwidth you’re going to see the effects over a larger range of distances than you would with a narrow signal.
For two antennas at the same height the fade distance is (4height^2 - wavelength^2)/(2wavelength)