This article looks at one of the most fundamental topics in understanding weather radar – Antenna and Beamwidth. A firm grasp on this subject allows pilots to better understand other essential building blocks, enabling them to use radar more effectively.
Most people think about the radar beam as being a nice triangle or cone shape. Even though it’s not, it is easiest to think of it that way for the purpose of this explanation.As a pilot it’s important to grasp the size of the radar beam in order to understand what happens to the energy it transmits, and how to point the antenna to detect and analyze weather. This information is in the Pilot’s Guide, but here’s a useful exercise that will help to visualize the beam, understand some basic radar principles, and provide a tool to help perform storm cell analysis.
Radar Beam Exercise
Before pilots had GPS and all the fancy navigation equipment available today, they had do a lot more math. When navigating using VOR’s, they used the 1-In-60 rule (i.e. if an aircraft is 60nm from a VOR and 1° off-track, the aircraft is 1nm off-track). But there is a more generic form of the rule that is useful for radar. Just take the radar range and add two zeros to find out what one degree equals.
In the example shown here, add two zeros to 60nm. This means that one degree at 60nm equals 6,000 feet, which is very close to 1nm. At 40nm from the VOR station and 1-degree off-track, add two zeros to 40nm to see that one degree equals 4,000 feet.
Knowing what 1-degree is at different ranges makes it easy to visualize the size of a radar beam by multiplying it by the beamwidth. Using a 30-inch air transport antenna with a three degree beamwidth in this example, look at the picture below to visualize several things about the beam and how it can be used to detect weather.
At long range the beam is extremely large, almost 16nm tall and wide. The narrow pulse of energy transmitted by the radar gets spread out over this entire area. This is why long range weather detection isn’t ideal on most systems.
At short ranges the beam is extremely narrow. At high cruise altitudes the narrow beam intersects the storm cell entirely in the frozen layer where there are less-reflective frozen ice crystals. The very dramatic narrowing of the beam is also why pilots must tilt down to track a storm cell as it gets closer to the aircraft, or the cell can disappear under the beam.
The final concept to visualize from this picture is how to analyze a storm cell. If there is a cell at 40nm and a crew wants to measure its height, they use the formula discussed earlier to learn that 1 degree equals 4,000 feet. If they raise the beam 4 degrees from the surface and the top of the cell disappears, they know its top is at 16,000 feet (4 degrees x 4,000).
Beam Size vs. Range
The image below shows an example of the beam intersecting a storm cell at long range. Here the beam is hitting the more reflective raindrops. The cell shown at 80nm has fairly strong green and yellow reflectivity. As the aircraft gets closer to the cell (bottom picture), the beam is narrower and intersects it at cruise altitudes where the frozen storm tops are less reflective to the radar. Although this storm had enough energy to lift moisture over 37,000’ up into the atmosphere, it only shows up as light green on the display.
The chart below shows approximate beamwidth size vs. range for different antenna sizes. The term approximate beamwidth is used because antennas aren’t always perfectly round or exactly the size stated. For example, one manufacturer’s air transport antenna is 30 inches vertically but only 27 inches horizontally, giving it a 3.2 degree beamwidth vertically and 3.5 degree beamwidth horizontally. In many cases the antenna may not come close to resembling a circle. The sides may be straight allowing it to fit into and scan within the swept volume of a very narrow radome. For most purposes these minor differences are insignificant and engineers round up or down to the nearest even number to keep the math simple. But check the applicable Pilot’s Guide to find the antenna size for a specific radar.
The next Radar Corner will look at one of the most misunderstood radar topics – reflectivity and gain.
Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached at Stephen.Hammack@Honeywell.com