All antennas are measured against what is known as an isotropic antenna, which is a theoretical antenna. This is the basis for all other antennas, as shown in Figure . Isotropic antenna coverage can be thought of as a balloon that extends in all directions equally. When an omnidirectional antenna is designed to have gain, coverage is lost in certain areas.

Imagine pressing in the top and bottom of a balloon. This causes the balloon to expand in an outward direction, which covers more area in the horizontal pattern. It also reduces the coverage area above and below the balloon. This yields a higher gain, as the balloon, which represents the antenna, appears to extend to a larger horizontal coverage area. This can be seen in Figure .

Remember the higher the gain, the smaller the vertical beamwidth.

Some important antenna types include the following:

• Isotropic antenna - This is a hypothetical antenna that radiates or receives energy equally in all directions. Isotropic antennas do not exist physically, but they represent convenient reference antennas for expressing directional properties of physical antennas
• Dipole antenna - This is usually a straight, center-fed, one-half wavelength antenna, which is pictured in Figure .
• Antenna array - This is an assembly of antenna elements with dimensions, spacing, and illumination sequence arranged in such a way that the fields for the individual elements combine. This combination produces a maximum intensity in a particular direction and minimum field intensities in other directions.