Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
New Member

Distributed Antennas ?

Has anyone tried using multiple antennas for a single AP ? Why would you ask? For example, if a coverage area does not require high throughput requirements, however, coverage is critical, then more than 1 AP would still be needed. New solution - Use one AP, but split the antenna to multiple locations?

I was wondering what happens in overlapping areas. Would it be destructive for a AP/Client to see multipaths of the same signal from different antennas? Or constructive, if the system can use diversity to its advantage?

Thanks for your insight.

3 REPLIES
Cisco Employee

Re: Distributed Antennas ?

Hi

you can use two different antenna on one ap to cover two different areas ....

it is for diversity reason only

This document explains that Two Antenn on the AP is for the Diversity and not for covering two different areas .

I have Ap350E2R . I am installing external Antenna to the AP350 . Can I use two external Antenna to cover two different areas ? Area 1----Antenna1 ---AP --Antenna 2----Area 2

* Two Antenna connector is for the Diversity and multipath operations .

To seperate the two antennas on the AP to increase the coverage is

self defeating .

The receiver only listens to one antenna at a time , so while it is

listening to one antenna ,

it's deaf to the other , therefore no coverage area there , and

vice versa . In addition there is till loss in the rf cables which

reduces the effective range of the clients to the Ap

The best way to increase the coverage is to conduct the site survey to

determine the RF coverage and put the Ap in the appropriate areas .

* Distance is not critical . The purpose of the diversity is to overcome

multi-path reflections , therefore so long as the antenna are 1-8 feet

apart or so that would be fine

You don't want the antennas really close ( under 6 inches or so)

because they would tend to " play off each other " causing

the pattern to start to become directional when perhaps that's

not desirable

There is no "true optimum distance "

* Diversity means two different antennas covering the same area but from a

slightly different perspectives .

The angle of the antenna is what is covered by each antenna , but since

they are desinged to cover the SAME area .

http://www.cisco.com/warp/public/cc/pd/witc/ao350ap/prodlit/agder_rg.h

tm

New Member

Re: Distributed Antennas ?

I agree. Splitting a diversity antenna wouldn't do very much, however, if a 802.11 network was connected to a distributed antenna system (DAS), where each antenna is seperate, but connected to the same AP, theoretically you can cover an entire floor with a single AP. I was wondering if anyone has done this yet, because why throw up 4 APs or more, when you can throw 1 up just so that you are covered. You can increase capacity by adding more APs, but the APs can be centralized, making installation and maintenance easier.

Green

Re: Distributed Antennas ?

There may be a couple of reasons:

At 2.4 GHz, signal loss in cable is pretty severe. To buy cabling that would rduce the losses to acceptable limits (something like Andrew LDF4-50) would cost as much as another AP. Amplifiers for that freq range can also get pricey, depending on the gain (and noise figure).

Unless you use an "active splitter" (a multiport amplilfier), splitting the signal will reduce the signal levels by more than half (per split) because of the loss associated with the splitting device. You may have more antennas, but they're all putting out less than half of the signal thy used to ( the reception also suffers in the same way).

Another reason may be legalities. The (unlicensed) spectrum used by wireless devices (some of the 2.4G range, in this case) is restricted to certain maximum Effective Radiated Power (ERP) measured at the antenna(s). Exceeding the limit exposes you to potential Federal action (in the USA) - other countries regulate their spectrum differently.

The limit is imposed so that more people can use the spectrum. If you increase the coverage area by amplifying the signal (which would include an array of antennas) you are more likely to interfere (and be interfered with) with another party trying to use the same spectrum.

If you check, all of these systems are sold with specific part makeups or configurations: this antenna, with this cable, going to this AP. All have been tested for Federal compliance. If you replace the antenna with something bigger (more gain), or you use more efficient cabling (less loss = more power to the antenna), or insert an amp into the line, chances are you're out of Federal compliance.

I'm sure there are other reasons, these are the first two that came to mind.

FWIW

Scott

179
Views
0
Helpful
3
Replies
CreatePlease to create content