Which 5Ghz antenna can use for a 1242AG.
The documentation do not show any anntenna for 5ghz.
What about the AIR-ANT58G8VOA-N.. the documentation show that it is only for 1400.
That depends what country you're in.
In US, I believe you can only use antennae specifically rated for the AP.
In UK, you can use anything you like, but you are also solely responsible for observing the EIRP limitations.
So from a purely technical standpoint, yes, you can quite happily plug that antenna into a 1242 using a jumper to convert between the two. Note that the performance characteristics Cisco quote will no longer be accurate as the 1400 probably uses a different radio by comparison to the 1242, and the N / RP-TNC jumper will increase RF loss within your setup.