GPS antenna With the C80 installed, do I need to replace my old Raymarine GPS antenna with the new one they list with the C80. Mine was used with an older Raymarine chartplotter.

If your old antena is a Seatalk or NMEA antena you can use it for your C80
If you GPS antenna is older than five year change it.

You can use any GPS unit with either NMEA0183 or SeaTalk output (probably will be som you can't use seeing I said all)

As long as the old antenna still works, it will work with the C80. I would however, recommend that you replace it simply because the newer antenna (the Ray 125) is much better and faster than the older 120.

If it is an RS 120 or an RS112 antenna you do not need to change it.These are both active antennas in other words they do all the gps computing in the unit itself. You need to look at the wire from the antenna. If it is a coax cable(2 wire) it will not work.If it is 3 or 4 wires it will work. If 3 wire (red ,silver yellow) just hook up color for color.If 4 wire you are going to have to figure out the wiring for it from the units wiring diagram.That could be a big enough pain that you might be inclined to change to the new antenna. If you wire the new RS125 antenna you must wire it to the seatalk buss. The seatalk wires are Red Silver and Yellow.The wire from the RS125 are red yellow silver green and brown.Wire as follow to your sea talk cable.Yellow to Yellow, red to red , silver to silver, green to red, brown to silver.
Hope this helps.