Some wireless related info on the standards page

802.11 Power Level: 1-200mW

Speed of light = 300,000,000 meters per second
Wavelength (m) = 300,000,000m/s / frequency Hz
Frequency (Hz) = 300,000,000m/s / wavelength m
Wavelength of a 2.4Ghz signal = 12.5cm
Wavelength of a 2.4Ghz signal = 6cm

Antennas (metallic element) length that match wavelength get  the best reception. 2nd best is 1/2 or 1/4 of wavelength

Waves 180 degrees out of phase will cancel each other out. Waves in phase will create a stronger signal. Waves slightly out of phase will either strengthen or weaken each other.

Between buildings
Semi-directional or highly directional antennas.

Central point with a omni-directional antenna, points with semi-directional antennas