A lot of people don’t realize that high gain antennas achieve their gain by narrowing the vertical beam of the antenna. For instance the Laird FG8246 antenna is listed as having 6 dBi gain at the horizon, but it drops to no gain at +/- 20^{o} angle off of horizontal (AOH) and drops even more beyond that angle.

So, what does that mean in real life? Well, let’s imagine that you have that antenna mounted on a 100′ tower. The 20^{o} beam will touch the ground roughly 300′ from the base of the tower. The formula to calculate the horizontal distance from the base to the intersection is:

**Height * sin( 90 ) / sin( AOH )**

In the diagram above, the area below the blue triangle will be limited in connectivity. If there are radios in that area, they may be “overlooked” by a high gain antenna, especially if the antenna is mounted on a high tower on the top of a mountain. You can use the formula above to calculate the distance from the tower the antenna should be most effective.

## HALF POWER BEAM WIDTH (HPBW)

This is a specification you may see on some antennas. It is the angle where the antenna is at 1/2 of it’s maximum gain. So for instance, a 8.15 dBi antenna should have a HPBW of 19.5^{o} (or 9.75^{o} AOH). Basically, a 2.15 dBi antenna has a HPBW of 78^{o} and for every 3 dBi of gain, the HPBW is cut in half.

Maximum Gain | HPBW | AOH | 100′ Tower | 500′ Tower |

2.15 | 78^{o} | 39^{o} | 159′ | 795′ |

5.15 | 39^{o} | 19.5^{o} | 300′ | 1500′ |

8.15 | 19.5^{o} | 9.75^{o} | 590′ | 2950′ |

11.15 | 9.75^{o} | 4.9^{o} | 1,170′ | 5850′ |

## Where more is less….

Ok, now let’s look at a situation where having a higher gain antenna could SERISOUSLY hurt your coverage. Imagine a 100 story building in the middle of NYC. On top of that building they have a 11.15 dBi gain antenna on a 20′ mast. Assuming an average of 14′ per floor * 100 floors + 20′ = a ‘tower’ height of 1420′. Plugging that into the calculation, the HPBW would touch ground over 3 miles away from the building.