PC Pro has published a feature on fixing problems with Wi-Fi reception, promising to "find what's interfering with your Wi-Fi and then what you can do about it."
Beginning by saying that it's first necessary to find out what's wrong with the home or office Wi-Fi network before taking remedial action, PC Pro cite a number of free tools for discovering "how many other Wi-Fi networks you’re competing with."
Among the tools mentioned is HeatMapper which lets a user wandering around with a laptop taking point measurements in order to build up a picture of reception and potential congestion.
PC Pro goes on to discuss remedial action such as moving the channel of operation of Wi-Fi equipment, router placement and radio enhancement add-ons such as range extenders and external aerials.
Check out PC Pro's Fixing Wi-Fi Problems.