I just bought a new HDS system for my boat and I already have a 520c with GPS and was wondering if I could use the same External antenna on it and if so how and what do I need?

If they are both NMEA 2000 supported systems you will be able to use just the one antenna. I have the setup information and will send you a copy via fax or email if you like.

Lowrance(LEI) sells an instalation kit that contains all the t-connectors and extension cables you need.

Your manuel should have a section that will explain the setup for you

The new HDS systems are internal antenna units so you dont have to run to an external antenna as you did with your 520

I'm right in the middle of networking my lowrance units (522 and 28) so have srent an enormous amount of time researching this topic.  Start here.

http://www.lowrance.com/en/Products/Marine/LowranceNET-Marine-Networking-System/

Mike, I got all mine up and running yesterday. Let me know if I can help.

Sorry for the hijack

The new HDS systems are internal antenna units so you dont have to run to an external antenna as you did with your 520

The internal likes to lose it self all the time

I can't speak for the HDS units, but if you connect your 520 and it's external antenna to the NMEA network, any other unit's internal antenna will be turned off by the network.

I noticed the new hds box they show a model 4000 antenna  anyone know if the 3000 works with the new hds