Corner reflectors are often used to reflect radar signals back toward the transmitter. Their efficiency as reflectors makes them useful for testing radar systems or to increase the radar visibility of objects. Common corner reflectors include dihedral and trihedral designs constructed from metal plates intersected at right angles to each other.
Reflections from dihedral reflectors are sensitive to the aspect angle. They may be used when circularly polarized (CP) radar signals are involved and co-polarized reflections are desired. Each “bounce” from a reflecting surface reverses the handedness of CP signals, a result of the change in direction. Dihedral corner reflectors produce co-polarized reflections of CP signals while trihedral corner reflectors produce cross-polarized reflections of CP signals. Linearly polarized signals retain their polarization for either reflector type.
It's possible to construct trihedral reflectors that produce co-polarized reflections of CP signals, or cross-polarized reflections of linearly polarized signals. Such “depolarizing” trihedral reflectors often have one or more reflecting surfaces that are ‘tuned’ to produce a specific phase shift using corrugations or fins. The tuned surfaces provide reflections with a frequency-dependent phase shift that is different from that of a flat conducting surface.
For any corner reflector, its “effective area” indicates how much of the reflector’s cross-section is used to redirect an incoming signal back toward the source. For a trihedral reflector constructed with square reflecting surfaces (Fig. 1), the cross-section presented to an observer is given by:
where α is the side-edge length of the reflecting surfaces.
Figure 1 – A trihedral corner reflector with square reflecting surfaces
It can be shown that for a trihedral corner reflector constructed from square plates, the plate surfaces are entirely “self-illuminating.” That is, when a ray coming from the boresight direction strikes any point on one of the plates, the reflected ray reaches both of the other plates and is reflected back in the boresight direction. The effective area of the reflector is thus equal to the cross-section presented to an observer.
Many trihedral corner reflectors are constructed with plates that are triangles instead of squares (Fig. 2). Such designs have triangular cross-sections. Triangular corner reflectors are often preferred because they are mechanically more stable.
Fig. 2 – A typical trihedral corner reflector has a cross-section in the shape of an equilateral triangle, with front-edge lengths equal to α√ 2
The cross-section presented to the radar signal source is given by:
Only two-thirds of the reflector surfaces are self-illuminating. Incoming rays that strike the plates near the outside corners do not strike all three plates and thus are not reflected back to the signal source. The effective area of the reflector is thus given as:
Using geometric optics, it can be shown that the theoretical radar cross-section (RCS) of a perfectly constructed trihedral reflector may be calculated as:
Where λ is the wavelength of the signal.
Equation 4 is based on the assumption that all of the reflected signal sums coherently (no appreciable diffraction effects and no phase or amplitude errors). It also assumes that the wavelength is much smaller than one-tenth of the side-edge length. In practice, the RCS may be reduced by field distortions at the edges of the reflector, as well as surface imperfections or deviations in the plate orientations from orthogonal. As a result, the actual RCS may be significantly less than the maximum value given by Equation 4.
The calculated maximum RCS of a triangular trihedral corner reflector with a front-edge length of 10 inches (side-edge length of 7.07 inches) is shown in Fig. 3. The effective area of the reflector is about 0.02 m2. The RCS of a corner reflector is a frequency-dependent quantity, and is typically hundreds or thousands of times greater than its effective area.
Fig. 3 – Maximum RCS calculated for a triangular trihedral corner reflector. Units are in square meters (blue) and dB relative to one square meter (red).
The RCS of a corner reflector can be thousands of times greater than its effective area. An explanation for this can be found in the definition of RCS and how it is used to compute the reflected power received by the radar. An object with an RCS of 1 m2 that is illuminated by an incident wave with power density equal to 1 W/m2 would appear to the radar as if it were re-radiating 1 W of the radar signal equally in alldirections. By concentrating most of the reflected signal back toward the radar system, a corner reflector produces an RCS many times greater than its effective area.
Eravant offers a selection of triangular trihedral corner reflectors as standard products. They feature rugged aluminum construction with a durable chemical-film finish. Front-edge lengths range from 1.8 to 16 inches. Custom reflector designs are also available.