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Choosing an Antenna 

Any omni-directional antenna generates a radiation pattern that looks for all intents and purposes like a doughnut. However, the particular shape of the doughnut can vary from antenna to antenna. Some doughnuts are tall (thick), but with a small diameter, and other doughnuts are squashed flat, but have a very large diameter. The shape of the doughnut is related to the dB rating of the antenna. Here are some pictures of the cross-section of the doughnuts to make this clearer:

4dB Antenna (Beam Width = 50 degrees)

6dB Antenna (Beam Width = 28 degrees)

8dB Antenna (Beam Width = 15 degrees)

15dB Antenna (Beam Width = 8 degrees)

The 15dB antenna has a very wide range--many miles wide, but the doughnut shape is very flat.  The 4dB antenna has broad vertical coverage, but the range is more limited.  The flatness or thickness of the doughnut is called vertical beam width.