What kind of radiation pattern is desired?
This is the first major delineator of antenna selection. The only way to increase gain is to concentrate power in a narrower beamwidth. The narrower the beamwidth, the greater the gain of the antenna.
A good conceptual example of this concentration is the vertical omnidirectional antenna. It is often used for line-of-sight communications with mobile stations spread out in various directions usually restricted to the horizon. We don’t have to warm the clouds, do we! If greater performance is required, the antenna gain can be increased by using a collinear type of omni that decreases the vertical beamwidth and hence concentrates more power on the horizon where it will be most beneficial. Figure 1 shows the typical radiation pattern of a gain omni antenna.
View Figure 1. Typical radiation pattern of a high gain omni antenna
Likewise, panel antennas are used to optimize a sector (typically 90 to 120 degrees in azimuth) with a narrow elevation beamwidth. Yagis, parabolic dishes and patch arrays are usually designed for high gain and hence have narrow beamwidths in both the azimuth and elevation plane. A typical high gain Yagi antenna pattern is shown in Figure 2. The gain can be further increased (and the beamwidth narrowed) by mounting multiple identical antennas in an array and properly spacing the antennas to account for phasing.
View Figure 2. Typical radiation pattern of high gain Yagi antenna
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This information should be used as a guideline only to help you in the appropriate selection of an antenna.