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Wireless b & g networks are designed to give roughly 300 meters coverage maximum, as far as I understand it that's a physical limitation of how far & strong the transmitters are allowed to be on those radio ranges, else their signals are too 'loud' and can cause interferance on other ranges of operation ...

Now if you had clear line of sight I might suggest you have a go with one of those big signal boosters you can buy certainly for class B networks, (if I googled enough I'm sure there is such a thing now for g class) ... I've had up to 1.2 km with these, but thats on a farm, using the highest point as a broadcast base, with a few blades of grass in the way, in a city I think you are gonna be hard pushed to get your 300m !

Here I think no chance to fat chance certainly with this kit, its not designed for long distances, but for use inside a building