All Wifi devices including client devices and broadband routers communicate over specific wireless channels. Each Wifi channel is designated by a number that represents a specific radio communication frequency. The use of these channels is regulated by national laws that may vary from country to country.
Kerio Control NG100W and NG300W includes a wireless package which contains the wireless regulatory database used by the Central Regulatory Database Agent (CRDA) to configure wireless devices to operate within the radio spectrum allowed in the local jurisdiction. This package is common among Linux distributions.
In Germany, Netherlands and Bulgaria, it is illegal to use Wifi channels above 140 as they are limited to devices known as Short Range Devices (SRD). These SRD devices have low capability of causing interference to other wireless and radio equipments with a maximum transmit power of 25mW and Kerio Control definitely doesn’t fall under this category.
Therefore, in Kerio Control when you set channels above 140 in these countries, the transmit power reduces to 20mW to abide by the laws and hence you face issues with the strength of the signal and the Wifi performance. Thus, it is recommended to Kerio Control users from Germany, Netherlands and Bulgaria to use channels less than 140 for optimal power performance.
To configure Wifi channels in Kerio Control, refer to http://go.gfi.com/?pageid=control_help#cshid=1976