Aircraft will be picked up using ADS-B transmissions. The maximum range of the system is based on line-of-sight, this typically less than 200 nautical miles (370 km) due to the curvature of the earth.
Its important to place your antenna in a position where it has a clear view of the sky and is not blocked by any buildings/trees/hills in any direction to get the best possible reception. To increase reception place the antenna in high position so it has a unobstructed view. Furthermore place the antenna on a metallic ground plane, A biscuit tin-lid or car roof is sufficient.
Transmissions are also affected by atmospheric conditions such as ducting. Have a look at the site below, the more ducting more likely you are to receive further transmissions.
http://www.dxinfocentre.com/tropo_eur.html
