How can I link portals from own faction without portal key in ingress?

Solution 1:

You have to go there and hack the (destination) portal to obtain the key (yes, you need to hack "friendly" portals anyway to get supplies), or trade keys with team members by dropping/acquiring them.

Also, resonators "drop" keys which are used for links when one end is blown up that you can pick up.

Solution 2:

The only way to link two portals is to have a portal key form the destination portal.

There are only two ways to get a portal key right now:

• Hack a portal (actually hacking in #ingress is just a search for goodies)
• Pick up a key lying on the ground (a key can be dropped by another player or fell out of the portal as a result of a powerful xmp blast)

Official info is here: http://support.google.com/ingress/answer/2808251?hl=en&ref_topic=2799270