I've been working on an assignment about the IEEE 802.11 standards and I've had some trouble with one aspect of a question. I need to explain the differences between various features of the different 802.11 standards (a, b, g, n, etc). I've been able to find most of the details online, especially through official IEEE documents.
What I've been unable to find any information on is the different "kinds of antennas" used by each standard and how they differ from one another. I know that 802.11n introduced MIMO and thus allowed the use of multiple antennas, and that antennas can be either directional or omnidirectional, but all 802.11 standards seem to be omnidirectional. I don't know what is actually different between the antennas themselves as specified by the IEEE standards.