Your question involves many subtle effects. UWB design for distance measurement is very different than making antennas for good reception range.
Size of obstacle. If it is relatively small, say under 2 cm, it will have minimal effect. As it gets larger, it will begin to affect the first path, or line of sight path, which is what determines system distance accuracy.
Type of obstacle. If it is metal, it will block the signal. If it is wet (like human body, or water), it will also stop the signal. If it is other material, like wood or concrete, then it will attenuate the signal depending on the amount you pass through and the characteristics of the material (like how much moisture is in it).
Proximity of obstacle. If it is near either end, say within 20-30 cm, it can affect the antenna pattern and delay, especially if metal.
Size of antenna. Making an antenna larger often creates problems with good timing characteristics. Basically, the antenna delays varies depending on which way the signal comes in. For timing, a point antenna is ideal. For signal, you want a huge dish. A compromise has to be reached.
Amplified antenna. Can be of some advantage for receive as an LNA can have lower noise figure than the Decawave chip itself. About the best you can do is a 3 dB improvement. For transmit, there is no advantage as you can’t exceed FCC limits and the Decawave can already reach that. Adding a receive amplifier is fairly easy for a receive only node, but our nodes both transmit and receive so that adds complexity such as RF muxes and reduces the advantage to about 2 dB.
Antenna delay. You can set antenna delay to compensate for cable or antenna changes so TWR works properly. For best results, this requires a per unit calibration.
RF parameters. Future release of the demo code will allow changing the RF parameters (frequency, preamble length, preamble code, PRF, bit rate, etc). This will allow longer transmit distances but at the expense of air time.