Cisco 1250 dipole antenna spacing

Question

When using dipole antennas such as AIR-ANT 4941 with the 2.4 Ghz 1250 AP, must the antennas be spaced EXACTLY as far apart as the antenna connectors on the AP, or can the dipoles be spread further out, say to 4" spacing
thank you
Scott

Answer 1

Let's talk a bit about Cisco diversity..
When a conventional Access Point such as an AP-1240 is in operation, it starts off favoring the right (primary) antenna, this is the antenna the Access Point typically uses to communicate to the 802.11a or b/g client. When the client starts to get out of range and retries result, the Access Point then samples the secondary antenna and if the signal is better it will use same. A client out in the very fringe area could benefit by as much as a 3 dB gain in certain null or problematic fringe areas.
Lets talk about how the 1250 Diversity works...
With an 802.11n Access Point such as the AP-1250 (unlike the older conventional Access Point) each antenna has a dedicated radio so there is no delay when it comes to switching antennas (to determine which one is the best). An added benefit when the signals are OFDM (as is with 802.11g and 802.11a clients) the Access Point can hear the signal on all antennas at the same time and then correlate or compare them all at the same time to enhance readability (that's essentially what MRC does). This has the benefit of hearing the signal much better in noisy or bad multi-path environments where a single antenna (receiver) would be problematic.
To put it into more simple terms... Think of a conventional (non-802.11n) AP much like an old fashioned television with "rabbit ear" antennas... when you are watching and the picture is not so good you can see "ghosts" images slightly off from the image that you are viewing. To fix that "multi-path" problem that is caused by the TV signal reflecting off of objects, you physically move the rabbit ear type antenna so as to minimize reflections. The Access Point basically does the same thing but rather then physically moving the antenna (like in the case of the TV) it electrically moves the antenna by switching to the other antenna on the Access Point.
Now with 802.11n (instead of this electrical switching of the antenna) think of several antennas with a method that dynamically "puts" all of these antennas together (without having to choose just one) for best performance. This is why all clients including older 802.11a/g clients benefit from this method.
So when it comes to antenna placement, you can look at multiples of a given wavelength but it's not nearly as critical as you might think. Basically you don't want the antennas so far apart that they cover different cell sizes - and they could (but shouldn't be) any closer then they are on the AP-1250 AP which is approximately 2 & 3/4 inches.
If you put them much closer then that, you run the risk that the Dipole elements will play off each other and perhaps take on additional directional properties.
Fred Niehaus N8CPI
TME Cisco WNBU.