I would agree with ericgarnel, what type of antenna's are you using, dipole, or patch? Are you using a interconnect cable to expand the range? Also are you utilizing the POE option with the access point? These questions will make a difference when mounting.
Generally speaking, you would want to set these up for full diversity (tx and rx on both antenna ports) to reduce multipathing and achieve the best range possibe. You would also want to set these antennas at the proper distance apart to avoid further multipathing problems. A good way to judge that would be to measure the distance of the antenna ports on the AP and mount the antennas at that distance or in increments of that distance. So if the distance is for example, 4 inches ( I don't recall for sure), mount the antennas 4 inches apart or 8 inches apart, so on and so forth. You do not, however want to mount the antennas too far apart. This would reduce the effectiveness of diversity. The idea is that the AP needs to be able to hear the client on both antennas to be able to determine which on it needs to transmit and/or receive with.
If this is an indoor installation, you would also want to mount the antennas pointing downward (with the cables at the top). For an outdoor situation, you would want to mount them point up. If this is an outdoor situation make sure you are using outdoor rated antennas and lightning arrestor.
Here is a little note to add to Eric,Robert and Ken's great info :) This describes the use of 2 Antennas in "Diversity" Mode;
The purpose of diversity is to overcome multipath reflections. Diversity antennas that share the same physical housing are placed at an optimum distance apart. The maker of the particular antenna determines that distance based on the characteristics of the antenna. When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.Because each antenna is selected by itself, both antennas must have the same radiation characteristics and be positioned to provide similar cell coverage. Two antennas connected to the same access point must not be used to cover two different cells.
"You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly. By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.
A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna and use only that antenna for the remaining portion of that packet.
On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.
One caution with diversity, it is not designed for using two antennas covering two different coverage cells. The problem in using it this way is that, if antenna no. 1 is communicating to device no. 1 while device no. 2 (which is in the antenna no. 2 cell) tries to communicate, antenna no. 2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area from only a slightly different location."
Here is the best description of why to use Diversity Antennas (in laymans terms) from a great NetPro ScottMac. When I read his reference to being at a stadium watching a sporting event, this all finally made sense.Have a look at his great explanation;
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...