802.11 Antenna's

I was just putting together a proposal for a WLAN for a class project.. and a question came to mind. When changing or adding antenna's to a device, you need to have all of the same antenna's for it to be beneficial, and in most cases the signal is only going to be as good as the least "powerful" antenna (for lack of a better word).

My question is: What about when using directional antenna's? Let's pretend I have an AP with 2 external antenna's, and I want to put (non-omni) directional antennas on it. How does that work? Do I replace both antenna's with directional ones, and point them both in the same direction?

Also, antenna's are obviously for different bands.. so you need to make sure that's compatible... but what about MIMO, and dual band? Some antenna's are sold as "802.11n" (etc, etc, etc) and claim to be compatible with all 802.11n devices... but it seems like most antenna's *don't* specify a lot of this.. and it makes me question if you need to match all those things up, or not. Some people/stores/ads act like if the antenna connector fits, the antenna will work... and sometimes it doesn't specify MIMO, frequency, or anything, a lot of the time,

When getting an antenna, what are ALL of the things that need to match up? Connector type, obviously. Frequency, also.. but do you need to make sure of *everything else* (like MIMO, dual-band)?

And (sorry for so many questions), but how do scientists measure the radiation pattern of antenna's? Is there anyway for a normal person to measure the pattern of the waves with some type of equipment?

Thanks. Just trying to fill in some of the "holes" in what I know about this. In books, this makes sense.. until I see all the products being sold online for wireless devices... and antenna's especially.

Thanks.

Edit: Also.. how come different frequencies don't interfere with others? I've never heard of an explanation as to why.. just that they don't (as much as frequencies on the same band, anyway). I didn't wanna start a new thread just for this question.

I run two 5db and one 8db on my router. The strength does drop if I replace the 8 with the 5.

I have used a 9db directional to fire down a hallway in a housing estate that was smothered in WAPs. Worked for the customer as they were then able to get signal strong enough.

You can also use extension cables to go between the WAP and the antenna to spread them out.

Often though I have found its preferable to plug in Power plugs and another WAP.