Tom, I'm assuming you unit is like my 778C, and doesn't have any mapping, just the ability to show position, and mark waypoints. So if you get an antenna for yours, the screen background will be blank. It will show you your track, heading, and the waypoint you are heading to, plus closing distance, but no shoreline features or depth contours. I have a Garmin 498C GPS/Sounder combo which has all the mapping on it. But I do have it linked to my 778C, so I can see speed, position, time, and mark waypoints with depth. I'd go with a unit that has mapping, plus the ability to send NEMA output to your 777C so it receives the GPS signal also. My Garmin had output wires for the GPS signal built into the power cable, I just needed to buy a Humminbird cable with bare wires to connect the 2. The portable DonG has is a nice unit, not sure if you can get a cable for it to link up with your H/B. The nice thing about linking your GPS to your H/B is, even after you pass a brushpile, you can scroll back directly over it, and then mark it.