You will mislead yourself by attempting to "strengthen" a Wi-Fi signal. All base stations already use the maximum power permitted by regulatory agencies.
The above KB article explains how to add one or more AirPort base stations to extend the range of another AirPort base station. Extending the effective area of your wireless network is the goal... not increasing the wireless signal you already have, which will prove fruitless.
Read it carefully. The best solution is called a "roaming network":