Since you are planning to cover an outdoor area, it would be ideal to have a bridge connecting the wireless LAN networks. There are a wide variety of Cisco bridges available in the market. You can go for 1400 bridge.
On the antennas here is a complete reference guide which gives you an idea of the different range of antennas that cisco provides:
thanks for your replies. The command for blocking the respective indoor channels is excactly what I was looking for. Also the link to the various antennas was helpful. I had an older link where not all antennas were described.
I'm now planning to install 3 AP1242 with sector antennas (AIR-ANT5117S-N) on the 5 GHz band to function as backbone for about 12 AP1242 in the field.
I would be interested in recommendations, how far in distance these antennas can go and what kind of "receiver antennas" I should install on the field APs.
On the 802.11b/g side where the clients reside, I plan to go for AIR-ANT2506. (or what about a diversity antenna?).
What about the 1400? Is there any advantage against the AP1242 for using them?
Diversity is generally only needed in situations where you encounted multi-path interference, so if you have lots of metal in the area (for example) then it might be worth a go. If you go with diversity, I THINK you're supposed to keep the antennae about a wavelength apart, so ~12.5cm.
For the 5GHz links, try to keep the TX and RX antennae the same gain, although it's likely you'll need an omni-directional for your Root Bridge, and directional antennae for your None-Root Bridges. If you have different gains at the root / none-root, you can encounter problems with transmit power mis-matches - one AP can hear the other, but not vice versa.
The 1400 comes with a built-in directional antennae, or with connectors for adding external antennae. Most importantly though, it is rated for outdoor use, where as the 1240 is an indoor AP, which will therefore need cables running between the AP & Antenna.
2.4GHz antennae is entierly dependant upon AP positioning, coverage requirements, etc. 2506 is probably a good place to start if you're not very experienced with spec'ing antennae.