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I am looking for the correct antenna for a aironet air-ap1242ag-a-k9
The 1242 AP will require at least 2 External Antennas. Have a look (there are Dual RP-TNC connectors for 2.4GHz b/g radio and 5 GHz a radio);
Antenna Connectors - AP 1242
Dual RP-TNC connectors
Cisco Aironet 2.4 GHz and 5 GHz Antennas and Accessories
The choice you make will depend on the Coverage pattern you desire. There are a number of Models for the 1242 listed here;
Hope this helps!