Minimum coax length
Is specified in the install manual. According to the upgrade installation manual for the 400 W/500 W series, the GPS antenna coaxial cable must have a minimum of two shields (that is RG 400 or RG 142B). The minimum length is approximately 13 feet of RG 400, producing a loss of 3dB. It's a lot of coax! Adding any connectors or adapters (eg BNC to TNC) increases the loss so you could use less coax. The required loss ranges from 3 to 7 dB according to the manual (see 2.5.4 and 3.10.1).