An antenna would depend on what brand/model router you have. For example a new AirPort Extreme Base Station does not have a connection for an external antenna but some D-Links and others have antenna connections so it would help to know what you have.
If you get a range extender make sure it's the same brand as your current router since Wireless Distribution System technology is proprietary meaning only Apple wireless products can extend an Apple network wirelessly and Linksys can only extend Linksys so on and so forth.
There are settings that can be changed on the router itself such as multicast rate, channel, IEEE Protocol (i.e. 802.11b/g/n and which one is best depends on the wireless capability of your laptops and any other devices you have such as iPhones, game consoles and printes etc.) but those are quite difficult (and a last resort for me) but can help if nothing seems to work.
Hope this helps.🙂
