The general guideline for using an external antenna is to obtain radio coverage in a even globe around the external antenna. Note that the external antenna does not extend the maximum range, which is still 15-25 m totally.
An example of a situation where external antenna could be used is when the placement options are limited for the communication hub.
The following figure illustrates the difference in radio pattern for external and internal antenna. (Note that this is only an simplified picture of the radio reception for each antenna, showing the theoretical characteristics. The real radio coverage also vary for each installation site.)
