Wireless bridge

Tim Huffman Tim at bobbroadband.com
Thu Jun 18 16:27:28 UTC 2009

> The line of sight is all clear, no trees. Only one building along the way
> has a rooftop of similar height, but the antennas are extended far above
> the
> roofline. We have used a rifle scope to confirm line of sight is all clear
> at all angles.

Unfortunately, you can't necessarily rely on visual line of sight. At 800meters, the Fresnel Zone on your radio is about 14ft in diameter at the midpoint. You need to make sure that this is free of obstructions.

> Oh I know. Luckily it's located in an industrial area just on the
> outskirts
> of the city. There isn't a lot of other WiFi (in my opinion); 3-5 total
> SSIDs spread across 2 of the 3 physical channels (1,6,11) depending on
> which
> rooftop you measure from.

Make sure you're using the channel that doesn't have an AP on it!

> Bandwidth requirements aren't too picky. If it can handle minimum 9 Mbps
> full-duplex everyone will be happy. Of course, the faster the better.
> I don't know if it makes a difference or not but this is all taking place
> in
> Canada. I don't know of any regulations drastically different from the
> U.S's
> regarding frequency use here. The biggest problem I've ever had though has
> just been payment/shipping depending on the supplier (some don't ship to
> Canada or are very specific about payment methods!).

Canadian and US regulations are very similar in the unlicensed bands. I'd still pick 5.2GHz if you were replacing the radio. 

> Just to answer a few more questions I've been getting, the access points
> are
> located inside, connected to a small UPS. The antenna wire is a very thick
> coax up to the roof, BNC connectors to the access point and I'm fairly
> certain BNC connectors on the antenna end as well. I'll double check
> grounding on the poles but I'm somewhat afraid to turn it into a lightning
> rod. I'm fairly certain that the ground in the antenna wire is clean but
> again, something to double check.

How long is your cable run, and what kind of cable is it? It's probably LMR-400 (the most common) loses about 6.6dB of your signal for every 100 feet. Also, you should check the waterproofing on the connector at the antenna. We normally use a 'courtesy wrap' of electrical tape, followed by a thick layer of Mastic tape, followed by another layer of electrical tape. Also, check your cable for nicks or kinks.

> Rain/moisture doesn't seem to cause problems. In fact the connection is
> more
> reliable through the winter. The last 2 months here have been cold/warm,
> dry/wet and there's been no pattern to the stability issues. The only
> correlation between weather and stability that they have noticed there is
> lightning related.

Moisture in the cables doesn't necessarily show up during rain! That moisture can seep throughout the cable, and cause attenuation when it gets cool and the moisture condenses, for example.

You haven't said what kind of antennas you are using, but if they are yagi's, they probably have very poor back-to-front ratios, which means that you could be picking up interference from behind you, or on the sides, especially if the antennas are up above the tree cover. You might try horizontal polarization on the antennas (just rotate them 90 degrees, but make sure you do it on BOTH sides!) to see if that helps. Cross-polarization is usually good for about 20dB of noise rejection.

The fact that there doesn't seem to be any pattern to your loss means that it's probably either interference (somebody changing channels), hardware failure, or software failure.

Hope this helps.

Tim Huffman
Director of Engineering
Business Only Broadband, LLC
O (630) 590-6012
C (630) 340-1925
tim at bobbroadband.com

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