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Using an external antenna with a Garmin GPSMAP 276c, 376c, 378 or 478
An external antenna can be used with the GPSMAP 276c, 376c, 378, or 478 series devices. You will need to remove the swivel antenna that is currently attached to the device to uncover the BNC connection used for the external antenna connection.
The external antenna that is compatible with these devices is the GA 38.