126 6. ReconfigurableReflectarrayDemonstrators
6.2. Reflectarray withTwoDimensionalControlCapability 127
thus bringing all patches to a common potential. This reflectarray works therefore, as the previous two, only in linear polarization.
A further novelty in the realization of this demonstrator, compared with the one pre-sented in the previous chapter is the use of SU8 structured spacers. As already pointed out in the previous section, the use of soft RF substrates as spacers is not practical, since it makes it very difficult to accurately control the cavity height. SU 8 is an epoxy resin that can be spin coated on a substrate to very precise layer heights, than cured and struc-tured with photolithographic techniques. Technological details are given in Appendix A4.
The SU8 was structured on the TMM3 substrate supporting the patches (see Fig. 6.13).
Columns with a diameter of 4 mm were disposed circularly and placed at the edge of the LC cavity. Thinner columns, of only 1 mm in diameter were uniformly distributed inside the LC cavity, in between the patches, in order to keep the cavity height as uniform as possible.
The choice of the spacer arrangement has been influenced by the need to subsequently spin the polyimide film. Hence, columns have been preferred to a continuous frame in order to prevent the accumulation of the polyimide at the edges in the spinning process.
The design height of the SU8 columns was 110μm. After the structuring process, the spacer height was acquired with a surface profiler. The average height was 116μm with fluctuations of ±14μm. The bonding of the two parts was made by placing small glue drops on the 4 mm SU8 columns and then pressing the two component substrates to-gether under a custom-made press.
grid via
Figure 6.12: Photograph of the ground plane with etched grid for two dimensional volt-age adressing. On the right, the back side of the ground plane is shown, with connectors for applying the control voltage.
128 6. ReconfigurableReflectarrayDemonstrators
LC-cavity
Figure 6.13: Photograph of the SU 8 spacers structured on the TMM3 substrate.
6.2.2 Experimental Results
With this demonstrator, capacitance measurements were conducted as well, in order to estimate the height of the LC cavity (Fig. 6.14). The measured capacitance values and by extension the derived cavity height show once again a decreasing slope from thefirst to the last row. The outlier at row 14 must be a measurement inaccuracy, since such a large height variation (≈60μm) between adjacent rows is improbable.
(a) Measurement of capacitance of each row of
patches. (b) Cavity height derived from measured
capaci-tance.
Figure 6.14: Capacitance measurement for the estimation of the cavity height for the reflectarray with 2D control capabilityfilled with MDA-05-893.
6.2. Reflectarray withTwoDimensionalControlCapability 129
Next, the cavity wasfilled with the liquid crystal MDA-05-893 with the dielectric proper-ties presented in section 4.3. In the anechoic chamber the reflectarray has been measured following the same procedure as for the previous one. Practical issues, such as the large calibration time or the very small change in recorded power when only one element
(a) Focussing of the beam.
(b) Beam pointing to different steering angles.
Figure 6.15: Pattern measurements
130 6. ReconfigurableReflectarrayDemonstrators
is driven, did not allow the verification of the beam steering both planes (2D control).
Therefore, the pins visible in Fig. 6.12 at the back side of the ground plane were con-nected together for each row, reducing the functionality of the reflectarray to steering in one plane, like in the previous section.
In Fig. 6.15(a), a comparison is shown between the patterns recorded with all rows connected to 15 V and the voltage configuration that maximized power toward 0◦. One can observe the distinct focusing of the beam, but a SLL of only 4.2 dB could be achieved.
In Fig. 6.15(b), two more patterns are shown, corresponding to voltage configurations that maximize power toward -35◦ and 25◦ respectively. For these steered cases the SLL improves to about 6 dB. The relatively poor SLL is attributed to the same reasons as in the case of the 1D demonstrators: high amplitude variations, randomly distributed across the aperture, high phase errors, low taper of the feed pattern at the edges, the calibration algorithm, yielding maximum power in a direction but not necessarily the best SLL.
A gain of about 20.3 dBi was measured for this reflectarray, with the beam pointing at 0◦. From the two 360◦-cuts, one in the E-plane and one in the H-plane with the main beam pointing toward 0◦, an approximate directivity of about 24 dBi is calculated. This yields an antenna efficiency of 42,6%. Despite the very high peak losses of the cells, the overall loss on the reflectarray is thus only about 4 dB. This is due to the calibration trying to maximize the reflected power, thus preferring states with lower losses but higher phase errors.
Properties of the 1D reflectarrayfilled with BL006 and of the 2D reflectarray filled with MDA-05-893 are compared in Table 6.1. It can be concluded that their performance is relatively similar, with a slightly higher gain for the 2D reflectarray, which can be attributed to a better realization of the cavity with SU 8 spacers.
Table 6.1: Summary of the realized LC-reflectarrays with beam steering capability Electronic steering in E-plane only possible in both planes
Liquid Crystal BL006 MDA-05-893
Spacer type RT Duroid 5880 h=127μm SU8 columns h=110μm SLL in E-Plane -4.5 dB to -6 dB -4.2 dB to -6 dB
SLL in H-Plane -14 dB -14 dB
Directivity 24 dB 24 dB
Measured Gain 19.5 dB 20.3 dB
Efficiency 35.4% 42.6%