Proper use of 'patch' antenna

Unanswered Question
Jul 2nd, 2010

I have acquired a number of the flat patch antennas, and I am looking for some guidance in deploying them properly.  I am using cisco 1200 AP's (b/g only), and in some area's I do not need 360' coverage, the patch antennas are better suited, but I cannot find any definitive guidance on their use.  Should they be installed 2 per access point, or is their no benefit to using them in pairs?  If I can use them in pairs, what is the optimum placement (there are 3 foot cables on the patch antennas).  In one large room (appx 76' across, 66' deep), if I could install them in pairs, should they be slightly angled out and down to provide better coverage, or would that make no difference?  I have seen the pages that show the coverage by antenna type, but I was looking for some detailed installation guidance for non engineers.



I have this problem too.
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rob.huffman Fri, 07/02/2010 - 10:52

Hi Gene,

The use of two Antennas is generally referred to as Diversity mode. They are used

in a "pair" type config to mitigate "Multipath"

In a multipath environment, signal null points are located throughout the area. The distance an RF wave travels, how it bounces, and where the multipath null occurs are based on the wavelength of the frequency. As frequency changes, so does the length of the wave. Therefore, as frequency changes, so does the location of the multipath null. The length of the 2.4 GHz wave is approximately 4.92 inches (12.5 cm). The length of the 5 GHz wave is approximately 2.36 inches (6 cm).

In order to increase coverage, conduct a site survey to determine the RF coverage of the antennas. Place access points in the appropriate areas of the installation site. 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.

From this good doc;

Multipath and Diversity

If you provide the patch model #'s we can provide some mounting/installation instructions.

Reference Guide;




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