The Corner Reflector Antenna

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To increase the directivity of an antenna, a fairly intuitive solution is to use a reflector. For example, if we start with a wire antenna (lets say a half-wave dipole antenna), we could place a conductive sheet behind it to direct radiation in the forward direction. To further increase the directivity, a corner reflector may be used, as shown in Figure 1. The angle between the plates will be 90 degrees.

corner reflector antenna

Figure 1. Geometry of Corner Reflector.

The radiation pattern of this antenna can be understood by using image theory, and then calculating the result via array theory. For ease of analysis, we'll assume the reflecting plates are infinite in extent. Figure 2 below shows the equivalent source distribution, valid for the region in front of the plates.

equivalent geometry for reflector antennas

Figure 2. Equivalent sources in free space.

The dotted circles indicate antennas that are in-phase with the actual antenna; the x'd out antennas are 180 degrees out of phase to the actual antenna.

Assume that the original antenna has an omnidirectional pattern given by radiation pattern. Then the radiation pattern (R) of the "equivalent set of radiators" of Figure 2 can be written as:

radiation pattern for corner reflector antenna

The above directly follows from Figure 2 and array theory (k is the wave number. The resulting pattern will have the same polarization as the original vertically polarized antenna. The directivity will be increased by 9-12 dB. The above equation gives the radiated fields in the region in front of the plates. Since we assumed the plates were infinite, the fields behind the plates are zero.

The directivity will be the highest when d is a half-wavelength. Assuming the radiating element of Figure 1 is a short dipole with a pattern given by , the fields for this case are shown in Figure 3.

radiation pattern for corner reflectors

Figure 3. Polar and azimuth patterns of normalized radiation pattern.

The radiation pattern, impedance and gain of the antenna will be influenced by the distance d of Figure 1. The input impedance is increased by the reflector when the spacing is one half wavelength; it can be reduced by moving the antenna closer to the reflector. The length L of the reflectors in Figure 1 are typically 2*d. However, if tracing a ray travelling along the y-axis from the antenna, this will be reflected if the length is at least spacing from  feed antenna to reflector. The height of the plates should be taller than the radiating element; however since linear antennas do not radiate well along the z-axis, this parameter is not critically important.

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