The maximum extension that may be added to the standard length of cable supplied with an antenna is 10 meters. Extensions are available in standard lengths of 5 and 10 meters. Longer extensions are possible but such an extension must be installed by a qualified technician. A shorter cable has less losses and therefore is desirable. The advantage of having an outdoor antenna generally diminishes if the cable length becomes excessive.In this video Andre Fourie explains how coax cable affect antenna performance.