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If there is no other option such as a relay station or a higher antenna spot (danger from lightning, needs to be grounded and secured) you can only try another antenna- but if the signal is too weak there is not much you can do, even with a grid or dish antenna.
The most simple solution (and cheapest or easiest to build) are "cantennas" (NO pringles can, since they are suboptimal sizewise) and biquads.
Cantennas require a (smooth at the inside) can, a piece of wire, and a low loss Wifi Cable (H155 for example, since others will loose over 2dbm signal per meter!).
The biquad is rather simple too: Bend a copper wire from a electrical house wire, install a reflector behind it (tinfoil, old CD, anything) and again, coax/wifi cable.
http://images.google.com/images?q=cantenna
http://images.google.com/images?q=wifi+biquad
Also, adding a cone shape to the can will increase the signal gain about 3dbi or such:
http://noether.uoregon.edu/~sadofsky/cookie-cantenna.jpg
Also note that such an antenna will have 8-15dbi gain, depending on the model, build quality and cable length.
3dbi is the double signal strength.
In Europe 100mw/20dbm EIRP (not the output power but the directional strength compared to a theoretical 0dbi all-arround antenna) is legal maximum, I do not know about the US regulations, but I think I do remember its higher.
Most routers have an output of 50-70mW allready. With a 2dbi short rubber antenna thats about 100mW allready, so your scraping legal issues here- usualy no one will care unless you cause lots of disturbance and someone reports it.
Anyway, two more things to keep in mind:
-2,4GHz will most likely get reflected or absorbed by obstacles and rain.
-A antenna with more dbi does not make the signal stronger all arround but focus the "beam". You might get a connection to the other station but get weak reception behind the antenna in the house (and need to install a repeater/AP)
-Polarisation (turning the antenna 90 degrees) might help avoid other disturbances, but both antennas must be turned the same way!
-Helix-Antennas do have a good reception when there are many reflections
http://images.google.com/images?q=wifi+helix
-A different channel (+/-3 channels up or down) can decrease connection problems caused by other stations, wireless video transmitters, and other disturbances