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Wireless Bridge between two building using AIR-BR1310G-A-K9-R bridges and AIR-ANT1949 antenna’s

Posted on 2012-04-11
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Last Modified: 2012-08-13
I have two Cisco AIR-BR1310G-A-K9-R Bridges and two AIR-ANT1949 antenna’s left over from another project.  I recently have been asked to setup a wireless bridge between two buildings that are about 25 meters apart from each other.  I don't want to purchase any equipment if I can use what I already have.  Are the AIR-ANT1949 antenna’s to much antenna for a link of only 25 meters?  Do you think I can get this to work?  Thanks.
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Question by:denver218
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by:JT92677
ID: 37833071
What's nice about the Cisco antennas is they look clean, have high gain, aiming looks very easy.

However, two 14 dB antennas aimed at each other at 25 meters seems a bit like overkill.

If it were me, and it's not, but if I were setting this up, I'd use the two expensive, high gain
antennas and get the rest of the bridge details worked out. Then change one of the antennas
to something less expensive, and look at the signal quality/strength, maybe even replace
both antennas with something with less gain, that meets the requirements.

Nice antennas. I've used dish and panel antennas for similar projects, but they cost a
lot less than the cisco variety.

Jeff
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Author Comment

by:denver218
ID: 37833123
I already have the two 14dB antennas, they are already paid for, just sitting on a shelf.  I know its overkill, but do you believe it would work? I've used these antenna's before, but at a farther distance.  What do you think?
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by:JT92677
ID: 37833173
Sure. At these frequencies, the real losses are in the coax cable length. But at the distance you're trying to bridge, you should have no problems.

I just looked at a couple of cheap antennas that would also work. For less than 25 for a pair !!

http://www.ebay.com/itm/160777050766

Hope it goes up for you easily, it should. As these are yagi antennas, the polarization will be affected by the way they are mounted, I'm sure you know that. Aimed at each other, and with the same polarity.

Jeff
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by:Darr247
ID: 37833470
I agree with JT92677's initial assessment in http:#a37833071

Here's a recommendation: RE09P-RTP 8dBi patch unit from L-Com. ~$30/each.

Those have 4' cables with Reverse Polarity Threaded Neill-Concelman (RP-TNC) plugs on them, so should fit your APs without adapters, for direct replacement of the AIR-ANT1949.
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by:JT92677
ID: 37833622
Darr247 - The nice thing about WiFi connector standards is that there are so many to choose from !! <<grin>>

Here's an inexpensive adapter for RP-SMA to RP-TNC

http://www.amazon.com/TRENDnet-RP-TNC-RP-SMA-Adapter-TEW-AMBA/dp/B000NZNTVE/ref=sr_1_1?s=electronics&ie=UTF8&qid=1334162542&sr=1-1
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by:Darr247
ID: 37833686
Count on every connection losing 0.5 dB of gain... that's why I try to avoid adapters/pigtails whenever possible.
(or adding -0.5 dB, whichever way you want to look at it.)
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by:JT92677
ID: 37833724
But 1/2 dB in a world of 20 dB gain antennas, and high cable losses is a small price to pay for a solution that works.

Avoiding whenever possible is what I do too, but sometimes, ya gotta get 'er done !!

10 feet of quality cable at 2.5 Ghz has a loss of .68 dB, just for comparison.
Lower quality cable is even more lossy at these frequencies.
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by:denver218
ID: 37833752
Thanks for all the advice, but we are all in agreement that the two AIR-ANT1949 that I already have sitting on the shelf, although they are overkill, will do the job even with the distance only being 25 meters apart from each other.  The problem is the customer all ready bought these antennas on their own like six months ago, without consulting an IT Professional first, and they can't return them as the vendor only has a 90 day return policy.  I told them these antennas would work, as I have used them before, but I didn't know until I went on site that the buildings were so close together only 25 meters.
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Assisted Solution

by:JT92677
JT92677 earned 250 total points
ID: 37833808
Denver218 -

Yes, agreement on the antennas. They're fine.

Jeff
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Craig Beck earned 250 total points
ID: 37833956
I would use the two antennas you already have.  Sure, they're 13.5dBi but so what?!  Over 25m they'll provide a solid and reliable link.  Just remember to bring the power down on the bridges so they're not too strong.  With a 13.5dBi antenna at each end, over 25m, 1mW should be more than enough power to sustain 54Mbps.

Sometimes adding a little cable-loss is a good thing, especially if you have antennae which are 'overkill'!
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by:Darr247
ID: 37834265
Their signal wedge is probably about 40 degrees to get that amount of gain, which would make the coverage cone about 18m wide by the time it travels 25m.
So aiming won't be too critical... I don't think the 2nd fresnel zone even needs to be empty at that range.
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by:JT92677
ID: 37834424
I'm sure we can add more dimensions to this discussion. The yagi polarity affects the cone shaped pattern idea, and if the receiving antenna is setup for vertical polarization, and the sender is on horizontal polarization, there are added losses there too.

And if the gain is really high, the signal could go out into space and be picked up by a spy satellite and compromise security.

But all that said, I think all he wanted to know was whether he could use the antennas, and I think that was answered in the first response.

May the force be with you.

Jeff
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Expert Comment

by:Darr247
ID: 37834718
The asker probably already found this but here are the specs for the AIR-ANT1949.
Its signal 'wedge' is 30 degrees on both axes, not 40 degrees as I guesstimated... so the spread will only be about 13m at 25m... or plus/minus 6.7m up/down and right/left.
And yes, attention should still be paid so their polarization is aligned (just orient the coax so it exits the radome the same on both... either both on the bottom or both on either side... even though the radome has weep holes, I would not mount them with the coax coming out the top... your mileage may vary).

And I have never had my midi-chlorian count checked.  :-)

Darr
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by:JT92677
ID: 37834911
Darr - Wow, not 40 huh? But 30? That's pretty good eh? Unfortunately, none of these details will have any bearing whatsoever on the asker's initial question, but maybe he'll follow-up with something requiring some really arcane detail like how will the temperature of the buildings impact the propagation during a rain storm? That might be good to know, maybe even critical, ya think? <<grin>>
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by:Darr247
ID: 37835617
How many wolverines does it take to change a light bulb?

just 1 - they hang on to the lamp and the world revolves around them.
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by:JT92677
ID: 37838029
Darr247 -- Kindly do not write to me directly about your credentials. I'm not impressed, sorry.

Here's what he wrote to me:

"Hi Jeff,

Heh... Though I've been a state-licensed electrician for going on 30 years now, networks (wired *and* wireless) are about all I do for a living anymore.

I don't see your name in that list over there on the right, no matter which tab I click... when it is, feel free to question what I think is relevant.  :-)

seven threes,
Darr"

Clearly you've spent time investigating me, why? My credentials are far more impressive than yours, but I don't go around touting them as you do. And points on EE are not a measure of expertise, they are measure of spare time, and apparently you have a lot of spare time to be on EE.

Get a life, and leave me out of it.
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Author Closing Comment

by:denver218
ID: 37839044
I appreciate the help.  I will go ahead and use these antennas.  It sounds like my question started a war:)  Let's play nice.
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Author Comment

by:denver218
ID: 37882943
craigbeck, where do I go to bring the power down on the bridges?  I don't see a setting for that.
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Expert Comment

by:Craig Beck
ID: 37883228
You need to go to the Radio Interface Settings page.

This guide will direct you, and also tell you how much power you can use in your regulatory domain...

http://www.cisco.com/en/US/docs/wireless/access_point/channels/lwapp/reference/guide/1300_chp.html
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by:Darr247
ID: 37890326
unmonitored
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