Cutler feed
A Cutler feed is a type of antenna feed used in radar systems operating at microwave frequencies. It consists of a coin-shaped resonant cavity mounted at the end of a waveguide, with radiating slots cut into the face of the cavity that direct energy toward a parabolic reflector. The design was widely used in airborne fire control radars from World War II through the 1960s, particularly in conical scanning systems where mechanical robustness and compact size were essential.
Design and operation
The Cutler feed comprises a disc-shaped resonant cavity attached to the end of a waveguide. Typically two slots are cut into the side of the chamber facing the parabolic reflector, through which microwave energy radiates. The cavity is tuned to resonance using a small adjusting screw. The waveguide feeding the cavity is tapered in its non-critical dimension to fit between the slots, and the resonator itself can be designed with either a circular or square cross-section.The radiation pattern produced by a Cutler feed exhibits an approximately sinusoidal power distribution due to the central waveguide. This characteristic makes it well-suited for illuminating parabolic reflectors in radar applications. A significant mechanical advantage of the design is that the waveguide itself serves as the mounting structure for the radiating element, eliminating the need for additional support rods or struts that would obstruct the antenna aperture.
Use in conical scanning
The Cutler feed is almost always found in conical scanning radar systems, where the antenna beam is offset slightly from the boresight axis and rotated to trace a conical pattern. This scanning technique allows the radar to determine a target's angular position with high precision by comparing signal strength as the beam sweeps past the target.To perform conical scanning with a Cutler feed, either the disc-shaped resonator alone or the entire feed assembly can be rotated about the antenna's axis. When the disc is rotated independently, it causes the beam to nutate while the waveguide remains stationary, which has the advantage of preserving a constant polarization. This is known as a nutating feed configuration. When the entire assembly rotates, the polarization rotates with it.
Advantages and limitations
The primary advantage of the Cutler feed is its mechanical robustness. The compact disc shape, only slightly larger than the waveguide diameter, combined with the waveguide's role as the structural support, makes the design highly resistant to vibration and mechanical stress. This durability was particularly valuable in aircraft radar systems, where equipment was subjected to severe environmental conditions.However, the design has several limitations. Only simple linear polarization is achievable, which restricts its use in applications requiring circular or dual polarization. Additionally, impedance matching constraints limit the usable bandwidth to approximately 3 to 5 percent of the center frequency, making it unsuitable for wideband applications.
History and applications
The Cutler feed was invented by Cassius Chapin Cutler, an electrical engineer at Bell Labs in New Jersey. Cutler developed the feed during World War II as part of Bell Labs' work on aircraft radar systems. In an oral history interview, Cutler recalled that the feed was used extensively in aircraft radars, many of which were mounted in streamlined pods hung beneath aircraft wings. These self-contained radar assemblies, typically about two feet in diameter and four to five feet long, contained the transmitter, receiver, and a small parabolic antenna with the Cutler feed, which swept a pencil beam across the surrounding airspace or terrain.Cutler was also known for inventing the corrugated-waveguide filter and differential pulse-code modulation, contributions that remained classified for many years. He received over seventy patents during his career and was awarded the IEEE Edison Medal in 1981 and the IEEE Alexander Graham Bell Medal in 1991.
Cutler feeds were popular in military radars from the 1950s through the 1960s. Notable applications included the Westinghouse AN/APQ-120 fire control radar used in the F-4E Phantom II fighter aircraft. In these systems, the antenna performed a nutating scan pattern to enable automatic target tracking. The technology was eventually superseded by monopulse radar, which offered improved accuracy and resistance to jamming, and later by flat-plate phased array antennas, which eliminated mechanical scanning entirely.