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Guide for antenna selection

I haven't had much experience with external antennas and am curious to know specifics about when/where to use APs with external atennas.  I've read through my CWNA book and generally explains where diferent types of antennas are used.  However, I'd like some scenarios, like something you'd find in a lab book.  Any thoughts?


If not a book, how else are you guys learning to select the correct antenna?  When do you know to deviate from integrated omnis? 

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Re: Guide for antenna selection

The outdoor VRD is a good place to start. However, the 'use case' for external antennas can be broken down into 3 questions:

1. Is it for an AP to AP deployment (ala wireless mesh or bridging)?

2. Is it for a high density deployment?

3. Is it to provide coverage that is farther away than what the integrated antennas will provide for?


There are sub-questions on each of these. But in general, that is where I would start.


For #1, if it's for an AP to AP (mesh or bridging), then the questions come down to three other questions:

A - is there a specific distance you are trying to hit?

B - Is there a specific topology (PtP, PtMP, etc) you have to design to?

C - is there some level of interference you are dealing with?


Obviously distance dictates antenna selection to a great degree. However, if the design also needs to be a PtMP, then the antennas have to be selected such that all the APs within the mesh have to be within each other's beamwidths. Additionally, if it's a dense wifi area for AP to AP, you sometimes have to select the narrowest antennas to provide the maximum amount of interference rejection. If the antennas is wide, it will pick up all interference within the antennas beamwidths.  Doing a PtP with the 275 is really not a good design simple because the APs will pick up ALL the other interference around it, when the design doesn't call for it.


So the TL;DR for #1 is - Select the antennas that provide the most gain to meet your design requirements, in the narrowest beamwidth required to meet the requirement and provide the maximum interference rejection.


For #2, high density deployments are ALL about 'cellularizing' coverage. If you are trying to cover a 2000 seat auditorium, and each AP can support 100 users, you can't just put 20 APs in that auditorium (well, you can, but it will not work well). So in that case, you have to come up with a design that meets the requirements of the environment, and that WILL require the use of small beamwidth, narrow-beam antennas *as much as possible*. That provides the smallest cells of wireless coverage with the maximum amount of interference rejection between APs/cells. The BEST place to read on this condition is here - http://community.arubanetworks.com/t5/Validated-Reference-Design/Very-High-Density-802-11ac-Networks-Validated-Reference-Design/ta-p/230891


For #3, if you have either a large indoor space, or want to provide outdoor coverage for an area larger than what the omnis would cover, using something other than the omni antenna is required. A general guideline outdoors with clear open space is 100-150m radius coverage around the 275 for client coverage. However, if you need to go farther, you can look at the AP-277 (equivalent of an AP-274+ANT-3x3-D100), which can extend that coverage out to 300-400m.


To combine the above, if I need to link up two APs of coverage (use case #1) AND provide outdoor client coverage (#3), then I would use two AP-274s with the narrowest 5Ghz antenna possible (ANT-2x2-5314) and then use the omni 2.4Ghz antennas for client access on both APs (ANT-3x3-2005). 


So hopefully that's a good start, but antenna selection is ultimately decided by the environment, the use case, and the requirements of coverage. The VRDs (outdoor and VHD) are great starting points.

Jerrod Howard
Sr. Technical Marketing Engineer

Re: Guide for antenna selection

Thanks Jerrod.


I've got a real-world use case, now.  I have a basement space that's 75' x 125'.  The ceiling is roughly 15' tall and has HVAC, lighting, and large fans all around, making a ceiling deployment of AP-225s very difficult.  So I'm looking at mounting APs along the walls (basic illustration attached) to provide room coverage.  The way I see it, there are 3 options:


1) Mount several AP-225s on the walls using a horizontal mount.

2) Mount several AP-224s on the walls + ANT-1B antennas.

3) Mount several AP-224s on the walls + Patch antenna.




1) Is it a problem that the wall material is concrete or cinder block?  I've read that mounting APs against the wall can be bad, especially if the material is reflective, like metal.  I believe the reflected signal is considered destructive.  I'm not sure about concrete though, since it should just absorb the signal coming from the back side of the AP, and not reflect it back.


2) I've seen a lot of Cisco deployments done like this.  These were typically wall mounted against drywall, though.  Again, not sure about concrete being an issue.


3) I'm having a hard time finding a panel antenna that has a wide enough beamwidth.  The closest thing I've found is the Terrawave M6060060MP1D33620.    However, I noticed the product documentation states the polarization is vertical only.  I thought a MIMO antenna should be dual polarized.  Is this a concern?  In Ekahau Site Survey I'm getting great side coverage using this antenna, but I'm not sure if real-world performance will be different due to the polarization.


Any thoughts you have on this would be appreciated.

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Re: Guide for antenna selection

What is all along the floor of this basement space (shelving, palattes, open space, tortur devices, etc), and how high are said obstacles along the floor?

To your other questions:
1. Mounting on the wall, esp indoor APs with integrated antennas, *can* alter the effective beamwidths/patterns of the antennas, but in general, since you don't need coverage on the backside of the AP (I wouldn't think), you should be fine. Alot of times customers will wall mount a 225, but then expect coverage behind the AP to the same level as the front-facing, and it's just not the case, so that's why you might have seen recommendations NOT to do that. However, if you don't care about the backplane of the RF (the back of the AP), it's fine.
2. See above. It's worth noting that the type of concrete will really affect any RF behavior. Polished concrete will be more reflective than natural concrete (harsh texture), but in general it's not AS reflective as metal or glass, but is MORE reflective than carpet, wall paper, dirt, etc. 
3. *IF* you wanted to use external antennas, you can easily get away with something like the ANT-35A for the AP-224. The ANT-35A has an effective 100deg H-plane, but remember that doesn't mean the gain drops off completely at 101 deg, there is just a gradual drop in antenna gain. Look at the attached image of the ANT-35A's H-Plane, at position 1, that is maximum gain of 5dBi, and at the beamwidth edge (50deg from Prime) is an effective 2dBi antenna, and while that is where the beamwidth is marked at (the -3dBi demarc), even at 90deg from prime, which would effectively be 'along the wall' when wall mounted, is approx -5dBi gain. So if the radio is at 15dBm, the effective EIRP along the wall from that antenna is still 10dBi (*very* useable). The Terrawave antenna is analagous to our ANT-35A, but their antenna patterns look highly variable and unbalanced between both bands. And besides, you usually want to use the vendors antennas with their APs as we test and support their use (FCC rules and all that). 

So all of that said, I think you are fine with just wall mounting 4x AP-225s. 75ft is NOT an issue to hit wall-mounted facing the other side. So long as there is something to help attenuate the RF from the top to the bottom, you can protect the three 2.4Ghz channels, and if you don't care about 2.4Ghz, then it's not a concern. I think you would the same, or slightly better, performance with AP-224s using the ANT-35A just to help create slightly more narrow wedges of coverage. 

So what is going to be IN the basement and how it will interplay with the RF may help answer which is better. Just always remember that room is only 9400sq ft, and if it's mostly open, that's only ~2300-2400sq ft per AP, which is a pretty safe number with overlap. Chances are both options will work just fine (ignore the 224+1b).

Jerrod Howard
Sr. Technical Marketing Engineer

Re: Guide for antenna selection

Lots of great info.  Thanks for the reply.


The space is going to be a gym.  It'll have a couple of multipurpose rooms along the edges.  Most of the space though is open (no walls) and will have rows of gym equipment.  There are some rows of bikes, ellipticals, treadmills, and then a separate section of weight equipment.  Things like treadmills and ellipticals are pretty low profile, while the weight lifting machines are of course larger and have some height to them.  Given the amount of thick steel in some of the areas, I thought a ceiling mount AP would be best, but in case I'm told I can't use them because of obstructions, I want to have a wall mount option in my back pocket.


If I try out an AP-224, I'll consider the antenna you suggested; it is a pretty good fit.  Ekahau has a poor selection of Aruba antennas so I had to design with an antenna that's in the database.  I'm only given a week after the space is completed to install everything, so I can't really do a survey.  Gotta nail the design the first time!

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Re: Guide for antenna selection

In that case, 225s will most assuredly work fine. Good luck!

Jerrod Howard
Sr. Technical Marketing Engineer
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