Towards a auto-aiming servo controled directional wifi antenna
May 14, 2011
I'm planning a project to get an auto-aiming directional wifi antenna working to provide a long-distance repeater for a boat at anchor connecting to access points we don't control.I think this is a robot, but a simple one. I think it's a robot because it senses its environment and responds to it. Sadly it won't be very pretty but it might be useful. It will definitely be fun ;)
The reason it's worthwhile thinking about this is that the boat rotates through 360° on the anchor line. This means most people use an omni antenna, but the range is not always sufficient and there is a ton of interference; worse the omni is pumping out even more interference to the world and that makes me sad . Lots of interesting background at Wikia and best description of the issues here.
Since this is a repeater situation (ie multiple client wifi devices onboard) this is a 2 radio situation. Currently that means a commercial router making a local network onboard, linked over ethernet to the picostation. I looked hard at platforms with single radio/two antenna but they are almost always diversity antenna (and therefore won't work well as repeaters) or you get one antenna at a time and I can't figure out how to bridge and switching antennas at high speeds!
Inspired by the UAV homebrew antennas I think it's possible to have a high-gain directional antenna (most likely a panel) on a rotating platform, driven by a (single) servo motor (and some simple re-aiming algorithm based on monitoring the signal strength in software). I'm a little concerned about side-to-side chop so maybe one day this will be a pan-and tilt setup, but currently I'm planning just a panning setup.
As a basic platform I considered commercial routers (e.g. ASUS 520GU) connected via USB to a servo control unit (e.g. Pololu micro) but since we need 2 radios anyway (ie one long distance and one for low-power local network) I'm attracted the the Ubiquiti RouterStation platform. It has three miniPCI slots, USB, serial and a few GPIO connectors. It's also powerful enough to run one end of a VPN tunnel (which is a nice to have requirement) and not too expensive at ~$62.
I'd be very interested to hear if anyone has an opinion on whether this can be used to directly drive a servo; RouterStation full specs here. There's a good description of doing this from the Linksys WRT GL that gives me hope (lost the link right at the moment).
Currently I'm thinking of sticking everything into a StationBox which can have an integrated 14dbi antenna (for $23 box and antenna) and hanging it from the top of a pole, using the servo to "creep" around the pole. Lots to think about there, might also do it just with a rotating platform and the antenna on top.
One challenge is how to cope with the rotation for the power supply; I found this site through that actually, searching for slip-rings and seeing the suggestion about the headphone plug. I might use that or I might use a cable-wrap and just handle the unwrapping in software (when network activity is low).
Any thoughts welcome!