Netgear 9dbi Omnidirectional Antenna, Ant-2409 (Matlock DE4)

This is an indoor/outdoor antenna designed to enhance the range and performance of wireless (802.11 b/n/g) devices. It works by amplifying the signal strength of your wireless network, allowing you to extend the coverage and reliability of your network.

It comes with a cable to connect it to a wireless access point but nothing else (mounting kit, docs, etc), although documentation, installation details, etc can be found online.

I've never used it so can't guarantee what (if anything) it will do for you but if it might be of use, it's available for collection from Matlock. NB. I've seen these for sale on the interweb for over £100 but equally for as little as £25.