It is possible to increase distance between Harmony XB4/XB5R Transmitter and Receiver unit.
There is no such limitation of the antennas that can installed. But you have to actually know, how much maximum distance will be required between transmitter and receiver unit/units for your application. Also, it is important know ,that whether receiver unit will be freely mounted OR it will be mounted inside the cabinet. Because, if the receiver unit is mounted in a free area, the maximum distance between transmitter and receiver unit will be upto 100m ( No need to install Antenna). If the receiver is mounted in a closed cabinet, you can see that distance reduces to 25m. So, in such situation, you can use install a Relay antenna to increase distance upto 40m and if you want more distance than 40m, you can install another Relay Antenna unit.
There is no such limitation of the antennas that can installed. But you have to actually know, how much maximum distance will be required between transmitter and receiver unit/units for your application. Also, it is important know ,that whether receiver unit will be freely mounted OR it will be mounted inside the cabinet. Because, if the receiver unit is mounted in a free area, the maximum distance between transmitter and receiver unit will be upto 100m ( No need to install Antenna). If the receiver is mounted in a closed cabinet, you can see that distance reduces to 25m. So, in such situation, you can use install a Relay antenna to increase distance upto 40m and if you want more distance than 40m, you can install another Relay Antenna unit.