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Poster: steffenyount Date: Nov 25, 2003 2:20pm
Forum: sflan Subject: 4+ 802.11b/g radios sharing a single site...

You guys have been able to point me to so much expert advice, Thanks.

I hope you'll be able to help me find answers for the following questions too:

I was reading that the 802.11b/g devices only really have 3 non-overlapping channels 1, 6, and 11.

Does this mean that one can only co-locate a maximum of 3 radios on a site? Each using a different one of these channels? So as to avoid interference between them?

I believe that I have LOS to sflan 14, 7, and 11. In addition, IIRC, I believe that I have detected nodes 12, 18, and 17 from my place.

(BTW: It would be great to have all the sflan nodes shown on the sflan homepage's map...)

I was looking at some antennas for possibly building me a super sflan node on top of my place, and I was wondering: is it feasible to use more than three radios by having each employ directional antennas?

If so, how far apart do I have to place them for use on the same channel, so that they won't cause each other interference problems or mutual bandwidth degredation? How much will this required distance be affected by having the antennas oriented with different polarities?

What would be the distance needed between placing two of the 16dBi Vagis listed below? The distance needed between two of the 24dBi grid Parabolics? The distance needed between one of the vagis and one of the grid parabolics? Is it possible to shield the co-located antennas from eachother, is it neccessary?

When the specs say that the Vagis have a beam width of 30 degrees, does that mean that I can co-locate two of these Vagis aimed at an angle >= 30 degrees apart from eachother while tuned to the same channel and not experience bandwidth interference from a third remote source centered in the angle between the two? If not, how wide of an angle would I need to have between them?

Here's what I've been looking at for antennas:

A 12dB Omni with a 3 degree down tilt
http://www.pacwireless.com/PAWOD24-12PF_install.pdf

A 16dB Vagi
http://www.pacwireless.com/PAWVA_Data_Sheet.pdf

and A 24dB Parabolic
http://www.pacwireless.com/PAWDCA-24.pdf

Also I've been looking at the following radios:

802.11b EL-2511MP PLUS Mini PCI
http://www.netgate.com/Library/Spec2511PCI.pdf

802.11b/g 3054 ARIES MP Mini PCI
http://www.netgate.com/Spec/802.11G/3054MPAries.pdf

Ideally I think I'd want to get cards with the Atheros AR5004 chipset. But I haven't seen any for sale yet.
http://www.atheros.com/pt/index.html

Your hints and suggestions are appreciated,

-S

Reply [edit]

Poster: Drew @ alamedawireless.org Date: Nov 26, 2003 3:07am
Forum: sflan Subject: Re: 4+ 802.11b/g radios sharing a single site...

There was actually a study done awhile back that said you could colocate 4 antennas, all on different channels, with little interference. Granted, some of the channels will be overlapping slightly, but I think the theory was that most of the power is concentrated into the center of the channel, so it's OK. I don't have the URL for this study, sorry.

As for distances to not interfere, I don't think your rooftop is large enough. Even if you just had 2 dipoles from the parabolic dishes, they'd be able to reach each other from across a rooftop. Also, Vagi's have side/back lobes to worry about. I think the best you can do is minimise the interference, but not completely eliminate it. That said, there are plenty of AP's in the city with lots of interference, and they're doing fine.

Beamwidth specs usually mean the width at which the signal is half the strength it was in the center. So in your example of 30 deg. with the Vagi's, they're still spewing signal at 31 deg., but it's at -3dB gain.

I also want some cards based on the AR5004, if anyone has any info on who is releasing gear when, let us know. :)

Reply [edit]

Poster: John Berry Date: Dec 1, 2003 4:07am
Forum: sflan Subject: Re: 4+ 802.11b/g radios sharing a single site...

The standard sflan node has two radios and two antennas. We could put multiple nodes on a roof if we are careful about sectorizing the antennas and selecting channels. You can see examples on most cellphone towers.

Panel-type antennas design to be used as a sector should work better than yagis designed for point-to-point. Their radiation patterns are more uniform over the sector and the good ones have minimal sidelobes. They often have reflectors on the back to minimize the rear lobe.

There are some nice graphics that show how even non-overlapping channels overlap somewhat.

http://www.smallnetbuilder.com/Sections-article59-page3.php

Reply [edit]

Poster: steffenyount Date: Nov 26, 2003 3:23pm
Forum: sflan Subject: Re: 4+ 802.11b/g radios sharing a single site...

Yeah,

my roof isn't that wide, I could probably only get a maximum spacing of a little more than 50ft between two masts...

How would I go about calculating the expected interfering signal strength between multiple antennas?

Is there an sflan18 or was I wrong... It seems that 18 was the only number missing from the map maybe all the nodes are already included?

-S

Reply [edit]

Poster: drydiggins Date: Nov 28, 2003 10:44am
Forum: sflan Subject: Re: 4+ 802.11b/g radios sharing a single site...

A couple of recollections from my days in 2-way radio. (30 - 800 MHZ kind).

Vertical separation (on the same mast) gives much better isolation (antenna to antenna loss) than horizontal separation foot-for-foot.

A corner reflector antenna using a solid sheet of metal as the reflector probably gives the best front-to-back ratio and side-lobe response. The trade-off is higher wind loading than a grid reflector.

George Lowry
Placerville, CA