Questions about antennas

Unanswered Question

Two general questions about antennas (on Cisco Aironet Access Points): 

1) Does “diversity” in antennas always mean there must be 2 (or more) antennas?  In other words, if there is only 1 antenna, is there any reason the AP should be set for diversity?

2) If you can’t actually put your hands on an Access Point, is there any way to tell what kind of antenna is connected to it, or whether it’s plugged into the left side or the right side?

Thanks …

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Rob Huffman Mon, 02/15/2010 - 08:59
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Hi Susan,


Good questions


You have to use two antennas to be in "Diversity" mode but not to cover two different cells/areas. For most installations, if you just used one antenna connected to the Right-hand/Primary, connector you should be good, if you have some obstacles in the cell area you can try using Diversity mode(2xAntennas);




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




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






show run is the appropriate command to view Antenna settings. Here are the commands to reset the Antenna settings to default,diversity (left-right) etc;




antenna




Use the antenna configuration interface command to configure the radio receive or transmit antenna settings. Use the no form of this command to reset the receive antenna to defaults.




[no] antenna {receive | transmit} {diversity | left | right}


Syntax Description




receive


Specifies the antenna that the access uses to receive radio signals




transmit


Specifies the antenna that the access uses to transmit radio signals




diversity


Specifies the antenna with the best signal




left


Specifies the left antenna




right


Specifies the right antenna








Defaults




The default antenna configuration is diversity




Examples




This example shows how to specify the right receive antenna option:




AP(config-if)# antenna receive right






This example shows how to set the receive antenna option to defaults:




AP(config-if)# no antenna receive




Cheers!

Rob

Leo Laohoo Mon, 02/15/2010 - 13:58
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