Main Reflector

The efficiency of a reflectarray is usually not that high as compared to a conventional reflector antenna. The reduction in efficiency results from the power losses in the stubs-attached patch elements and the phase and polarization errors due to mutual coupling between non-identical elements. Besides, in a folded type antenna, there are extra losses in the path because the reflected fields by the sub-reflector are no greater than the incident fields, and thus the efficiency would be lower. Although the typical values for reflectarrays range from 10% to 30%, efficiency up to 70 % has been reported [15]. The aperture efficiency (η) is defined as:

To get a maximum density of the array elements, the square patches are

arranged to form an equilateral triangular array with spacing S equal to 5 mm, or about 0.6λ, within a circular area with diameter D. In general, the aperture efficiency for a small reflectarray, even with reasonable good illumination, is not very good, but gets much better with increasing size and adapted illumination.

Figure 3.4 shows the calculated aperture efficiency of the folded reflectarray antenna as a function of D when illuminated by the same feed patch. As the antenna size increases, the efficiency first increases due to the fast growth of the antenna gain.

Then, when D is further increased, the efficiency slightly decreases. This is because that the illumination power decreased as the distance and angle from the feed patch increases; the received power of the square patch antenna decays rapidly as the patch moves far away from the feed antenna, and thus the gain of the whole reflectarray becomes saturated. A maximum calculated efficiency of about 25% could be achieved as D equals 140 mm. However, when D becomes larger than 100 mm, the efficiency growth by enlarging the array size is quite limited, since the normalized received powers of the square patches located outside the circular region of radius 50 mm are below –8.3 dB. Therefore, D is determined to be 100 mm in this work.

The same substrate material as that for the feed antenna was used to fabricate the main reflector. The main reflector comprises several hundreds of square patch antennas located inside a circular area of diameter D = 100 mm. The square patch antennas are oblique to the feed antenna with 45°. For field twisting and focusing, two microstrip open stubs, each with a λ/4 impedance transformer, are attached to two adjacent edges of the square patch, as shown in Figure 3.5. The square patch measures 2.3×2.3 mm². The widths of the stubs and the transformers are, respectively, 0.17 mm and 0.1 mm. The field incident on the antenna will be received by the two stubs, reflected at the open end, and then fed back to the antenna

for re-radiation. The difference between the open stubs’ lengths is designed to be λ/4. This will make the re-radiated field orthogonal to the incident field, as will be explained in the next paragraph. Besides, the absolute lengths of the stubs are determined according to the location of each square patch so as to compensate the phase delay due to path differences. Therefore, the antennas on the main reflector are excited with uniform phase and tapered amplitude distribution.

0 5 10 15 20 25

40 80 120 160 200

Diameter D (mm )

Efficiency (%) .

Figure 3.4 Calculated aperture efficiency, as a function of the aperture’s diameter D of the folded reflectarray antenna.

To see the field twisting effect, let us consider a vertical field (radiated from the feed antenna) incident on the stubs-attached patch. As shown in Figure 3.5(a), the incident field can be decomposed into two orthogonal components, that is, component A and component B. These two equal-amplitude components are separately received by the two orthogonal open stubs on the left and right sides. After reflected at the stubs’ open ends, these components are fed back to the antenna. Since the left stub is longer than the right stub by λ/4, component A experiences 180° more phase delay than component B in the round-trip tour. The resultant total re-radiation field is thus twisted to the horizontal direction, as shown Figure 3.5(b).

For better estimation of the folded reflectarray’s performance, scattering parameters of the individual square patch antenna was measured by extending the two open stubs as two ports. Around the design frequency of 38.5 GHz, the return loss (S11) and isolation between ports (S21) are less than –20 dB. Also, the co-polarization and cross-polarization patterns were taken with one port terminated.

The broadside gain and the 3-dB beamwidth of the co-polarization component are 5.6 dBi and 88° respectively. By putting the measured patterns of the individual square patch into (5), a co-polarization gain of about 26 dBi and a maximum cross-polarization gain (assumed the cross-polarization fields of the square patches were in-phase) of –4.8 dBi of the folded reflectarray were obtained.

(a) (b)

Figure 3.5 The field twisting effect of the 45° tilted square patch antenna with two open stubs.

Furthermore, derived from the full wave simulation result of the single square patch with appropriate periodic boundary conditions applied, the maximum cross-polarization of the folded reflectarray grows up from –4.8 dBi to about –1.5 dBi.

It is the mutual coupling effect by the close proximity of the square patches and the stubs. Actually, the cross-polarization component of the array is at least 27.5 (=26–(–1.5)) dB lower than the co-polarization component of the array, which is very small and will be blocked by the sub-reflector.

For more efficient use of the excitation power and suppressing the pattern ripple, the feed antennas for radar mode should be placed as near the center of the array as possible. Square patch antennas overlap the feed antennas are to be detached for the accommodation of the feed antennas. Therefore, since the effective aperture reduces, the main beam gains will decrease and the side lobes will increase. Assume that there are no square patch antennas within the circular area of diameter Df. The simulated patterns for various Df are shown in

Figure 3.6. When Df = 30 mm, the gain drops about 1.27 dB in comparison to Df = 0 mm. In reality, the feeds will occupy an area substantially less than that of the Df = 30 mm circle. The pattern will not be significantly affected.

It is also known from the simulations that the scattered fields reflected by the ground plane of the main reflector are relatively weak, in comparison to the fields re-radiated from the square patches. Moreover, the ground scatterings are haphazardly distributed; they do not lead to co-phasal behavior in any direction and thus were ignored in the design.

-10 0 10 20 30

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

Angle (degree)

Gain (dBi) .

Df= 5 mm Df= 10 mm Df= 15 mm Df= 20 mm Df= 25 mm Df= 30 mm Df= 35 mm Df= 40 mm Df= 45 mm Df= 50 mm

Figure 3.6 Calculated H-plane patterns of the folded reflectarray antenna for various Df at 38.5 GHz.