Antenna Polarization
Free-space radio waves propagate in the TEM, or Transverse Electric Magnetic mode.

This means that the electric field and the magnetic field are both perpendicular to the direction of travel.

If the transmitting antenna is a simple electric dipole, the electric field is set up parallel to the dipole. This electric field is resolved onto the receiving dipole as cos(theta), where theta is the angle between the two dipoles. 

So if the dipoles are lined up, theta=0, and the reception has gain=1. 

If the receiving dipole is inverted, theta=pi, and the gain is -1. 

If the receiver is at right angles, theta=pi/2, and the gain is zero.