The antennas for the Paralinx Arrow and Arrow+ systems are both omni directional. However, the best signal is accomplished when orienting the units towards each other. On the transmitter and receiver, the antennas are located on the end opposite of the HDMI connector.
Check this video out below for more details on best orientation to get the best range. (Skip to about 4 Minutes 10 seconds, but the whole video is good!)