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I maintain a work network that at one point bridges a gap between two buildings wirelessly, while also descending from the 4th floor to ground level. The AP is at the top and connects to the internet connection, and the client bridge at ground level extends the network into the second building. My estimate of the distance from the high-up AP to the client bridge router is about 25m, with perfect uninterrupted line of sight. Please dig this fabulous illustration I have scanned:

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"NO" represents units in the first building which don't need to receive any wireless from my network. I don't care if they do, but it isn't necessary to attempt to provide it to them. The little joystick-looking things are the current positions of the two wifi routers described in this question.

They both run dd-wrt and while the upstairs AP is responsible for both providing wireless to the unit it is in and also providing it to the client bridge downstairs, the downstairs client bridge immediately forwards the connection to a wired lan so it has no rebroadcasting responsibilities; it only has to maintain a good wireless connection with the AP upstairs. Therefore the upstairs AP has two antennae, one oriented towards the interior of the unit it is in and one oriented towards the downstairs client bridge across the gap. Both antennae are just the little standard antennae that routers ship with. The client bridge downstairs has a single (cheap, waterlogged and in need of replacement) outdoors antenna of the patch form factor with its flat surface approximately oriented towards the upstairs AP.

Currently, the network link between the upstairs AP and the downstairs client bridge maxes out at around 18Mbit/s on a good day. It seems to me that with the correct antennae and orientation they could do 54, especially if I bring the second AP antenna outside and use antennae with a little more directionality.

What is the optimal antenna form factor for creating this 25m line-of-sight link, without overkill? Please include details such as how to know if a particular model is sufficiently weatherproofed since they both should be outdoor antennae. Please be very specific, if you don't mind.

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closed as off topic by Zoredache, HopelessN00b, Michael Hampton, mdpc, Tom O'Connor Dec 12 '12 at 23:39

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Does the box acting as the client bridge, have two separate radios on separate channels?? If not, then your bandwidth is going to be halved. The client bridge uses some of its capacity for the link to the AP and some of the capacity to serve the local clients. The best I would expect you could get out of it is about ~24mb/s – Zoredache Dec 11 '12 at 21:22
No, the box acting as the client bridge doesn't rebroadcast, it only forwards the connection to a wired LAN in the downstairs building. It doesn't serve local clients wirelessly, local clients all connect to the wired LAN that connects to the client bridge via a wired connection. The upstairs AP does have two radios because it connects to the client bridge and serves local clients near to it. – Halle Dec 11 '12 at 21:24
"especially if I bring the second AP antenna outside and use antennae with a little more directionality." DO BOTH OF THESE NOW. Until that happens, there's not much point in any of the other suggestions you may get here. This may be all you need to solve the problem. – Joel Coel Dec 11 '12 at 21:26
It would probably be easier/cheaper to just buy 802.11n gear if you need more bandwidth rather then playing around with antennas. – Zoredache Dec 11 '12 at 21:26
My question is about what kind of outdoor antenna. I don't have an appropriate outdoor antenna for either router and I'm asking what the correct form factor for this application is. – Halle Dec 11 '12 at 21:27

1 Answer 1

As Joel Coel said, you really need to put a proper directional antenna on both ends of your system if you're having problem getting your signal (indicated signal strength) at the far-end of the link to meet your requirements.

Zoredache is also correct: Switching to 802.11n will probably improve things (There are range and bandwidth improvements with 802.11n.)

You are not required to run 802.11n over the 5GHz band, but if you elect to do so to avoid interference you will probably have no problem with 20-30M transmission distance.
If you do wind up with low signal strength a directional antenna (and possibly an RF amplifier) at both ends of the connection may help, and you're going to have to do that anyway, so we're just talking about a slightly different antenna design (5GHz or 2.4GHz directional)

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I guess I don't really understand why your and Joel's answer to "what is the correct kind of antenna type for this situation" is "you really need to first put a proper directional antenna on both ends of your system". I am trying to improve one link, not an entire network, which is line of sight and currently known to be badly designed. I am asking a very specific question and I have the impression that despite your mutual upvoting, you and Joel and Zoredache didn't read the title or the content of the question. – Halle Dec 12 '12 at 9:12
Even if I wanted a new 802.11n network, which isn't a big priority, why would I want to invest in that and then use it over a known-funky link which degrades its performance? If I have 802.11n, I also would like the full benefits of that network, not whatever bandwidth happens to get to the second building with an not-that-directional antenna. What is so out-of-bounds/bothersome about asking Server Fault for the antenna design that is the best match for this situation? – Halle Dec 12 '12 at 9:19
@Halle I read your question. I answered it (did you read my answer?). If you want a fully engineered design with specific model numbers you have three choices: build a "cantenna", experiment with commercial antennas, or hire a radio engineer to conduct a site survey and recommend a specific antenna design. There is no possible way anyone can reasonably recommend you a specific antenna/amplifier model without a site survey (and such a thing would be a product recommendation, which we don't do on Stack Exchange. – voretaq7 Dec 12 '12 at 16:32
The reason that I didn't think you read my question is that you restated as your answer a comment from someone who couldn't have possibly read it. It was surprising to me that as a mod, your interest was in bolstering the authority of that comment. Zoredache's first comment is a clanger as is the one that was deleted, sorry. Your last comment is an answer to the question I asked: that it is out of scope and that there are too many unknown environmental factors for someone here to know. That's unexpected (aren't there ready-made commercial antenna designs?) but it's an answer, so thank you. – Halle Dec 12 '12 at 17:16
"If you want a fully engineered design with specific model numbers" I asked for types of antenna. I guess this is impossible due to the large amount of variability in clear line-of-sight 25m distances that already get 18mb/s, but I was hoping someone would say something like "directional yagi 19dbi gain on both ends should improve things, here's how you can check their weatherproofness rating". – Halle Dec 12 '12 at 17:31

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