Down imaging is a powerful feature on modern fishfinders that provides a detailed view of what's beneath your boat. One crucial aspect of down imaging is the beam angle, which determines the width of the area you're scanning. This blog post will delve into down imaging beam angles, specifically focusing on Humminbird units, and provide practical examples of how much bottom coverage you can expect at various depths.
Beam Angles Explained
The beam angle of a down imaging transducer affects the width of the scan area below your boat. A wider beam angle covers more area, while a narrower beam angle provides higher resolution but covers less ground. For fishfinders, there are typically two main beam angles used for down imaging:
 800 kHz: 45º cone angle
 455/1100 kHz: 75º cone angle
Coverage Examples
Let's look at how these beam angles translate into bottom coverage at different depths. For simplicity, we'll use the following depths: 20 feet, 50 feet, and 100 feet.
1. At 20 Feet Depth:

800 kHz (45º Cone Angle):
The formula for calculating the coverage width is:
$\text{CoverageWidth}=2\times \text{Depth}\times \mathrm{tan}(\frac{\text{BeamAngle}}{2})$For the 800 kHz beam angle:
$\text{CoverageWidth}=2\times 20\times \mathrm{tan}(22.5\text{\xba})\approx 2\times 20\times 0.414=16.56\text{feet}$So, at 20 feet deep, the 800 kHz beam covers approximately 16.56 feet of the bottom.

455/1100 kHz (75º Cone Angle):
For the 455/1100 kHz beam angle:
$\text{CoverageWidth}=2\times 20\times \mathrm{tan}(37.5\text{\xba})\approx 2\times 20\times 0.769=30.76\text{feet}$At 20 feet deep, the 455/1100 kHz beam covers about 30.76 feet of the bottom.
2. At 50 Feet Depth:

800 kHz (45º Cone Angle):
For the 800 kHz beam angle:
$\text{CoverageWidth}=2\times 50\times \mathrm{tan}(22.5\text{\xba})\approx 2\times 50\times 0.414=41.40\text{feet}$At 50 feet deep, the 800 kHz beam covers approximately 41.40 feet of the bottom.

455/1100 kHz (75º Cone Angle):
For the 455/1100 kHz beam angle:
$\text{CoverageWidth}=2\times 50\times \mathrm{tan}(37.5\text{\xba})\approx 2\times 50\times 0.769=76.90\text{feet}$At 50 feet deep, the 455/1100 kHz beam covers about 76.90 feet of the bottom.
3. At 100 Feet Depth:

800 kHz (45º Cone Angle):
For the 800 kHz beam angle:
$\text{CoverageWidth}=2\times 100\times \mathrm{tan}(22.5\text{\xba})\approx 2\times 100\times 0.414=82.80\text{feet}$At 100 feet deep, the 800 kHz beam covers approximately 82.80 feet of the bottom.

455/1100 kHz (75º Cone Angle):
For the 455/1100 kHz beam angle:
$\text{CoverageWidth}=2\times 100\times \mathrm{tan}(37.5\text{\xba})\approx 2\times 100\times 0.769=153.80\text{feet}$At 100 feet deep, the 455/1100 kHz beam covers about 153.80 feet of the bottom.
Choosing the Right Beam Angle
When selecting a down imaging transducer, consider the following:
 800 kHz Beam: Provides a narrower, highresolution view, ideal for detailed scanning in shallower waters.
 455/1100 kHz Beam: Offers a wider view, making it suitable for deeper waters or when covering larger areas quickly is important.
Understanding beam angles and coverage can help you make better decisions about your fishfinder setup and improve your overall fishing experience.