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sdt
New Member

Cisco 1250 dipole antenna spacing

When using dipole antennas such as AIR-ANT 4941 with the 2.4 Ghz 1250 AP, must the antennas be spaced EXACTLY as far apart as the antenna connectors on the AP, or can the dipoles be spread further out, say to 4" spacing

thank you

Scott

20 REPLIES
Green

Re: Cisco 1250 dipole antenna spacing

They should be at least ~5 inches apart (approximately one wavelength) or more, out to ~one meter or so (it's not critical).

Less than one wavelength and the pattern is distorted and there may be some coupling of the elements (which causes the distortion).

The antennas should both be looking a the same area, for diversity, not coverage of a wider area / multiple rooms, East/WEst, etc.

Good Luck

Scott

Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

For the 2.4GHz, the spacing should be 4.92 and use multiplier up to 4. So max should be 19.62 inches apart. Hope this helps.

-Scott
*** Please rate helpful posts ***
Green

Re: Cisco 1250 dipole antenna spacing

I'm curious; where'd you get the 4X (19.62") figure?

Too close is usually an issue (and doesn't do much for "diversity"), but aside from "they should be looking at the same area" .. I never really heard of a max spec for radiator separation.

I'm not saying it's wrong, I just can't figure why it'd be an issue.

Thanks

Scott

Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

Scott,

This info was provided by Cisco. Also it was in one of their older docs regarding diveristy.

-Scott
*** Please rate helpful posts ***
Hall of Fame Super Red

Re: Cisco 1250 dipole antenna spacing

Hi Guys,

Here is the clip in question;

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.

From this good doc;

Multipath and Diversity

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

Hope this helps!

Rob

Green

Re: Cisco 1250 dipole antenna spacing

Cool, thanks Rob!

(+5)

Take care

Scott

Hall of Fame Super Red

Re: Cisco 1250 dipole antenna spacing

Hi Scott,

You are most welcome my friend! I always remember Diversity conversations based on your Ballpark analogy :) The best info I've read here!

Take care,

Rob

Cisco Employee

Re: Cisco 1250 dipole antenna spacing

Let's talk a bit about Cisco diversity..

When a conventional Access Point such as an AP-1240 is in operation, it starts off favoring the right (primary) antenna, this is the antenna the Access Point typically uses to communicate to the 802.11a or b/g client. When the client starts to get out of range and retries result, the Access Point then samples the secondary antenna and if the signal is better it will use same. A client out in the very fringe area could benefit by as much as a 3 dB gain in certain null or problematic fringe areas.

Lets talk about how the 1250 Diversity works...

With an 802.11n Access Point such as the AP-1250 (unlike the older conventional Access Point) each antenna has a dedicated radio so there is no delay when it comes to switching antennas (to determine which one is the best). An added benefit when the signals are OFDM (as is with 802.11g and 802.11a clients) the Access Point can hear the signal on all antennas at the same time and then correlate or compare them all at the same time to enhance readability (that's essentially what MRC does). This has the benefit of hearing the signal much better in noisy or bad multi-path environments where a single antenna (receiver) would be problematic.

To put it into more simple terms... Think of a conventional (non-802.11n) AP much like an old fashioned television with "rabbit ear" antennas... when you are watching and the picture is not so good you can see "ghosts" images slightly off from the image that you are viewing. To fix that "multi-path" problem that is caused by the TV signal reflecting off of objects, you physically move the rabbit ear type antenna so as to minimize reflections. The Access Point basically does the same thing but rather then physically moving the antenna (like in the case of the TV) it electrically moves the antenna by switching to the other antenna on the Access Point.

Now with 802.11n (instead of this electrical switching of the antenna) think of several antennas with a method that dynamically "puts" all of these antennas together (without having to choose just one) for best performance. This is why all clients including older 802.11a/g clients benefit from this method.

So when it comes to antenna placement, you can look at multiples of a given wavelength but it's not nearly as critical as you might think. Basically you don't want the antennas so far apart that they cover different cell sizes - and they could (but shouldn't be) any closer then they are on the AP-1250 AP which is approximately 2 & 3/4 inches.

If you put them much closer then that, you run the risk that the Dipole elements will play off each other and perhaps take on additional directional properties.

Fred Niehaus N8CPI

TME Cisco WNBU.

Hall of Fame Super Red

Re: Cisco 1250 dipole antenna spacing

Hi Fred,

Thanks so much for this excellent info (+5) :) I love the "Rabbit Ear" reference, this will help me remember this going forward. You may have to explain the reference to some of our younger members ;-)

Take care,

Rob

sdt
New Member

Re: Cisco 1250 dipole antenna spacing

Thanks Fred and other respondents. I think the take-away here is that the Cisco 1250 antennas should be minimally about one wavelength apart, but no wavelength dependent spacing beyond that.

thanks again

Scott

New Member

Re: Cisco 1250 dipole antenna spacing

I have a question...

Say I've got a 1230 AP, and I'm going to use two AIR-ANT2506 antennas in diversity configuration. The antenna's will be seperated by more than 4.92 inches, but do they have to be seperated by multiples of 4.92 inches, or can I mount the antenna 7 inches apart and still have a good diversity deployment?

For example... I have an outdoor AP enclosure, with an AIR-ANT2506 mounted on the left-hand side of the enclosure and another on the right. Now I'm not sure what the spacing between these two antennas is, but I know it's more than a wavelength. Is there a problem with this type of deployment, or should I be keeping the antennas seperated by multiples of 4.92 inches?

Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

JolietJake,

As long as you are between the minimum and maximum distances you should be okay. I have multiple deployments even with the 1252's that require 3 antennas and I have kept the distances the same for 2.4 and 5 GHz with no issues at all.

-Scott
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New Member

Re: Cisco 1250 dipole antenna spacing

fella5,

Thanks for your reply.

So the antenna's don't have to be spaced by multiples of the length of a radio wave, just so long as the antenna's are at least 1 wavelength and no more than 4 wavelengths apart?

So for example, I could space my antenna 1.5 wavelengths apart, and still have a good diversity configuration?

Sorry for being pedantic, but I'm going on-site next week to install 10 APs with diversity AIR-ANT2506s and just want to be sure.

Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

You are correct. As long as you don't use any multiple of the wavelength you will be fine. The wavelength fro channel 2412 is 4.896766169 the wavelength for channel 2462 is 4.797319253. So as long as you stay above 4.90, you should be fine.

-Scott
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Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

Don't be too paranoid.... you will end up messing something els up :)

-Scott
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Re: Cisco 1250 dipole antenna spacing

Hi,

Please mark the Question as Answered if the provided information is correct and issue is fixed.

thanks,
Vinay

Thanks & Regards
New Member

Re: Cisco 1250 dipole antenna spacing

Hi,

I know this thread is a bit old and for the most part it is clear. But I'm a bit thick!

My question is pertaining to the AIR-AP1262n and using 3 (three) AIR-ANT1728 antennas.

I understand by this thread that the antennas should be spaced about 5 inches apart.

So does this mean that I should have one transmit antenna on the left, and then 5 inches over to the right of that antenna the middle receive antenna, then 5 inches to the right of the receive antenna should go the right side transmit antenna? Basically I should emulate the placement of the connectors on the 1262 itself?

Hope this question makes sense!

Thank you in advance.

Joe

Hall of Fame Super Silver

Re: Cisco 1250 dipole antenna spacing

Yes you are correct.

Thanks,

Scott Fella

Sent from my iPhone

-Scott
*** Please rate helpful posts ***
New Member

Re: Cisco 1250 dipole antenna spacing

Thank you Scott for the fast response.

Take care.

Joe

New Member

Re: Cisco 1250 dipole antenna spacing

I'm like Joe and I know this thread is rather old. But be that as it may here is the situation I have (83) AIR-LAP-1231G-A-K9

the client is LXE VX6 terminals (they connect one at any given time) Here is the odd part: the APs have two mobile mark OD9-240 omni directional antennas 9 dBi beam width 14 degrees. Since the 4400 controllers did not have a drop down for that antenna they used a AIR-ANT24120 drop down. I'm receiving (65%) CRC errors and I'm assuming it is some kind of interference.

I plan on changing out the antennas: which ones would you recommend? Also do you think the antenna issue is what is causing the CRC errors?

Thanks in advance for your help

Tom

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