After successful test phase, I have finally upgraded the gateway located in the Niš center. Upgrade included replacing antenna and putting it outside.
New antenna is omni-directional collinear antenna (VINNANT COL868) ordered from Stanislav. Antenna mounting fixture (AX-L45+) was bought from Antenika and customized by Tehnozavod for this particular use:
Antenna is connected using N-Connector to the Aircell7 cable that goes through the antenna mounting fixture, through the wall, inside the building, and finally into the gateway enclosure. The gateway is build using Raspberry Pi 3 with backplane from Gonzalo and iC880A concatenator from IMST:
After this upgrade, with initial testing, I was able to get 2.5km range in the highly populated urban area - something that can be greatly improved if antenna is placed on the building top roof.
With this gateway, The Things Network coverage in the city of Niš has been increased (beginning of 2018):