Specs on distance between antennas

albertt123 · ‎04-09-2008

I work for a company that installs Cisco equipment and was told that the maximum distance for optimal performance is to have the antennas no more than 5.5 inches and up to 20 inches apart. Is this statement true or false? I normally mount the 1242 APs in the center of a 3 foot board and connect 2 2.4GHz antennas, each one tied to opposite ends of the board (pointing down), with about three foot between max. The antenna is then mounted at ceiling height. I've never had anyone complain of having problems with the AP signal and I've never heard of their needing to be a specific distance between antennas. Also, could someone explain to me why two antennas instead of just the one.

Scott Fella · ‎04-09-2008

Take a look at this thread:

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&topicID=.ee6e8c2&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cc024a0

Minimum shoudl be around 5" and maximum should be around 20". Never should you have the antennas exactly one wavelength away from each other. For the frequency of 2400, one wavelength is 4.92"... so any distance that is not a multiple of 4.92 and no more than a multiple of 4 is recommended.

-Scott
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mcorliss · ‎04-15-2008

Two antennas are better than one. This allows you to turn diversity on. Diversity allows the antennas to choose between a cleaner signal from the client. As the signal from a client bounces off surfaces it add a little distortion here and there until it finally reaches the antennas on the AP. Whichever antenna has the least amount of distortion from that client will receive the transmission.

vlad.kabatov · ‎04-15-2008

You don't have to use two antennas, however 1242s can take advantage of antenna diversity which must be explicitly configured on the radio:

(config-if): antenna receive|transmit diversity

Diversity somewhat eliminates multipath distortion. Not sure how the Cisco gear is designed, but some systems utilize a technique where the radio selects which signal from which antenna to use or transmit on.

For best results for antenna diversity, antennas are placed one wavelength apart. 2.4GHz wavelength = 12.5cm, 5GHz = 6cm.

Also, suggested placement of any antennas is about a wavelength away from highly reflective surfaces.

One more point to add to this is clients (especially VoWLAN handsets) are most likely to use the 5GHz diversity, due to size limitations.

Scott Fella · ‎04-18-2008

Here is a site that has the formula for a wavelength:

http://www.pisarsky.com/essays/wavel/

The calculations on Cisco's doc mentions the prefered distance for diversity. From what I have been told by RF engineers is that you should not have antennas one wavelength away or multiples of one wavelength. You should use 1/2 wavelength distances and to be safe no more than multiple of 3. I have been told that multiples of 4 is okay to, but to be safe.... 3. If you take a measurement of the distance apart the antenna ports are on an ap you have and then calculate the wavelength, you can see that it is not exactly one wavelength away. The 1252 ap's have the 2.4ghz and the 5ghz antenna exactly the same distance apart.

If you are installing the ap indoor, you should use diversity. Open areas.... outdoor, you can get away without having two antennas.

-Scott
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albertt123 · ‎04-21-2008

Thanks for the information everyone. I actually mistyped 3 foot. The boards were 2 foot which seems to be one extra multiple too long according to the Cisco (Wow, Cisco is considered a misspelled word in their own forum) documentation. I think I'll just cut it short a couple wavelengths.

sdt · ‎04-18-2008

Cisco's paper on diversity (http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml)says the following: "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"

As an RF engineer, I don't believe you need to be too concerned about "spacing in multiples", since in an indoor environment there is a lot of multipath and the received waveforms are uncorrelated.

The antenna diversity is a "detect and Switch" arrangement. The APs RF chip set periodically monitors both antennas, detects which antenna receives the better signal, and switches to that antenna- for both receive and tranmit.

good luck

Scott

vishalpatil86 · ‎07-13-2011

hi,

i am using two antennas AIR-ANT2460P-R and AIR-ANT1728 with AP 1242.

does that distance rule also apply here.

Pls let me know, bcos i am facing some seriuos signal problems

vishalpatil86 · ‎07-14-2011

one is dipole antenna and other is single patch antenna