I'm in Iraq and I was using the "B" side on at 3 miles away with a solid dish "AIR-ANT3338" and was getting 2meg down on by bandwidth and now I'm lucky to get 400K. I have one root hub and 4 non root hubs linking to it with no more than 6 users at each shot. I've replaced the Root hub checked the cable and so on. The Army Camp has grown like you wouldn't believe with everyone using 2.4G so I want to move to "A" side 5G. Do you know if the "AIR-ANT58G28SDA-N will work with the 1242 and if not what is everyone else using? It's hard to find information when you're in Iraq and I never know where my equipment is purchased from.
The 1242 has rp-tnc connectors while the AIR-ANT58G28SDA-N has an N connector. you will need to get a converter to go from N to rp-tnc. Without seeing the antenna, I would guess that you would need a converter with a male "N" and a female "rp-tnc". There will be some minor loss with the adapter. The AIR-ANT58G28SDA-N is designed for the 1400 series, but contact TAC and ask for sure if it will work with the 1242 series.
Your dish antenna is a 21dBi model and like another has suggested its more likely you have some sort of interference. Tried another channel? We run a 2,4GHz link with a 10dBi omni on the root (Aironet 1231G) and gets 36Mbps link with a 18dBi dish 5Km away on the client side (Aironet BR1310).
Are the cable runs to the antennas long? Check this out: