Wireless Access Point(1200) Antenna extentions?

Unanswered Question
Feb 22nd, 2007

Dear All,

I am presently rolling out WLAN for my department and I was wondering if anyone could advise me about dealing with a wireless signal coverage issue? ;

The particular building is a classic quadrant (Square) but the building is constructed of very thick re-enforced concerte as the main buisness of our facility is partical/nuclear physics research, thus at present on AP per floor is not providing every office/lab with good signal strength.

If you install one access point is it possible to apply the antenna cable extenders to give a better/purposly direct signal range? - I am thinking of putting the AP in the middle of the square and then extending the antenna in opposite directions so that the quadrant is completely covered in lovely WLAN connectivity!!

Any pointers/advice on such an install would Fanatastic.

Thank you.

Nathan - Oxford UK.

I have this problem too.
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rob.huffman Thu, 02/22/2007 - 05:47

Hi Nathan,

I think a second (and maybe third) AP per floor is likely what you are going to need :( You cannot use two antennas to cover two different Cells/Areas. The purpose of connecting two antennas is to provide Diversity type coverage. Have a look;

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.***

" ** Multipath ** - 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."

From this Antenna reference guide;

http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.html

Here is the best description of why to use Diversity Antennas (in laymans terms) from Scott Mac last year. 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;

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.1dd7905b/6#selected_message

Another good doc;

Multipath and Diversity

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml

Hope this helps! Another few AP's will make a huge difference.

Rob

rob.huffman Mon, 02/26/2007 - 05:44

Hi Nathan,

You are most welcome :) Best of luck with your Wireless rollout! Let us know if you have anymore questions.

Rob

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