Figure: Primus 880 Beam Width, from Honeywell Primus 880 Pilot's Guide, figure 5-16.
Size matters. The larger the plate antenna on your radar the narrower the beam, and that helps you see more weather at longer distances. If you are flying a G450 you have a 24-inch plate antenna and your beam width is 4.2° — you should know that number. (The only place you will find the plate diameter is in the G450 Maintenance Manual §34-44-00, ¶3.A.)
What follows are quotes from the sources listed below, as well as my comments in blue.
Figure: Radiated beam width, from Honeywell Radar Training, Part 3, Slide 5.
The radar's energy is sent in a beam that is most intense in the center but tapers off away from that center.
Figure: Radiated beam width versus gain, from Honeywell Radar Training, Part 3, Slide 12.
The antenna gain, contrary to what is sounds like it should mean, does not refer to the intensity or energy of the beam. It refers to the focus of the beam. The higher the gain, the tighter the focus.
The gain on your radar should be calibrated to reflect agreed upon colors levels for certain levels of reflectivity. Changing the gain, which you may need to do, will distort those color levels. More about that: Procedures & Techniques / Radar - Weather Calibration.
Figure: Beam width versus Tilt, from Eddie's notes.
The radar paints more than just a point at the end of the beam, it includes everything in the width of the beam. In the case of a G450, you beam is painting this much:
|Distance (nm)||Beam Width (feet)||Beam Width (nm)|
Table: G450 beam width versus distance, from Eddie's notes.
Photo: From Eddie's cockpit.
In this particular shot, the ground is painting just beyond 100 nm. I should have brought the tilt down a degree for a better paint, had I done that, the width of the ground clutter would have been just over 7 miles.
What is more interesting is what we see at 50 nm. The beam width should be 22,238' at that distance, so those blotches of red would indicate the storm we were painting was at least 22,238' vertically developed at the red level. Was it? See the photo below.
Photo: A view from the right seat, from Eddie's cockpit.
Figure: Beam width level flight tilt, from Eddie's notes.
If you know the width of your beam in degrees, you can figure it in feet with a little math. More about that: Beam Width and Tilt, above.
In the case of the G450, we know our 24" plate produces a 4.2° beam that equates to 44,537' at 100 nm. Armed with this knowledge, if we were flying along at 44,537', we could adjust the tilt so the ground paints right at 100 nm. Raising the tilt 2.1° from that point will point the radar along level flight. With a little more math we could come up with the correct distance to paint the ground for various altitudes:
|Aircraft's Altitude (FL)||Distance to paint edge of ground clutter|
Table: G450 distance to paint ground clutter for level flight tilt adjustment, from Eddie's notes.
So let's say you are in your G450 at FL 350. Adjust your range scale to put 100 nm on the outer ring and adjust your tilt to show the ground clutter just starting at around 80 nm. Raise your tilt half the beam width, 2.1°, and know that the center of your beam is now point along level flight.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 28.
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Honeywell Airborne Weather Radar Training, Rev E, 12/09/02, Honeywell Inc. Commercial Flight Systems Group, Phoenix, AZ.
Honeywell Primus 880 Pilot's Guide, Pub. No. A28-1146-102-03, Revised January 2006, Honeywell International Inc. Commercial Electronic Systems, Glendale, AZ.