CHARACTERIZATION

The presented novel dielectric substrate material was derived from organic substances. The substrate material was manufactured using a controlled composition of recycled carton paper and banana pulp. It comprised of 75% carton paper and 25% banana pulp fiber. The carton paper was the major portion to identify the material properties while the banana fiber was added as a bonding component to enhance the stress and strain properties of the material. The material was produced in the form of thin sheets and later merged together to achieve a suitable substrate height for a microstrip patch. It was then passed through several rolling and pressing stages to get rid of moisture content.

In order to determine the dielectric properties of the proposed substrate material, characterization was done using a broadband material characterization technique. A dielectric probe was used to characterize the material for X-band frequency range. The complete material characterization set-up is shown in Figure 1. It consisted of a Speag 3.5 mm probe connected to a Rodhe & Schwarz 14GHz network analyzer. The probe was controlled remotely by a software platform installed on the PC.

Figure 1. Dielectric material characterization set-up The whole set-up was thoroughly calibrated for set-up errors using air, copper strip and water as open, short and load respectively. A firm contact between the probe face and the sample under test was established using a movable fixture stand. The results of the dielectric characterization are shown in Figure 2. The characterization results presented the dielectric permittivity and the loss tangent values of the material.

ASM Science Journal, Volume 12, Special Issue 2, 2019 for Malaysia in Space

49 The material showed minor variations of permittivity and loss tangent values over the entire band of operation. The proposed substrate material offered a relative dielectric permittivity of 1.63 along with a loss tangent of 0.046. The values were evaluated as a mean over the entire X – band frequency range.

Figure 2. Dielectric material characterization results for the proposed organic substrate material

III. SIMULATION & MODELLING

In order to analyze the behavior of the proposed substrate material as a dielectric, unit microstripreflectarray elements were modeled using a full-wave analysis technique based on the finite integral method. The simulated model was based on unit reflectarray element with perfect electric and magnetic walls. The simulated model is shown in Figure 3. The principle was based on assuming unit reflectarray element placed inside conductive walls, where the walls acted as mirrors for the cell. Thus the unit reflectarray element could be tested for the same mutual coupling and array effects as it experienced in an infinite array. The proposed dual resonant element configuration offered two distinct resonances in X – band operation. This was made possible by inserting two U – slots of variable sizes in a reflectarray rectangular patch element as depicted in Figure 4.

Figure 3. Simulation model of unit reflectarray element with boundary conditions

Figure 4. Proposed dual U - slot element configuration In order to establish a mathematical relationship between the different element dimensions and the resonant frequency, a thorough parametric study was carried out for the proposed element configuration. The selected dimensions were La, L1 and L2 along with the

dielectric permittivity εr, since they played important roles in controlling the distribution of the charge carriers on the patch surface. The simulations were then performed to analyze the effect over the resonant frequency. The analysis was done over the X – band operation in order to reduce the complexity. The effects of different parameters over the resonance are presented in Figures 5 – 8.

50 Figure 5. Parametric study - La

Figure 6. Parametric study –εr

Figure 7. Parametric study – L1

Figure 5 presents the effects of the length of the microstrip patch element on the resonant frequency. It could be seen that the element length affected both the upper and the lower resonant frequencies. Dielectric permittivity of the material was also taken in consideration. It could be seen from Figure 6, that the

dielectric permittivity, εr was varied from 1.8 to 2.3. The

variation of dielectric permittivity resulted in an equal

Figure 8. Parametric study - L2

effect on both the resonances. As the dielectric

permittivity, εr was varied from 1.8 to 2.3, the resonant frequency decreased from 9.2 to 8.35GHz and 11.7 to 10.55GHz for the lower and upper resonances, respectively. The decrease in resonant frequency was observed to be uniform for both resonances. The effects of the arms of U – slot were also monitored. Figure 7 shows the effects of the arm length L1 over the resonance behavior. It could be seen that with the increase in arm length from 2.5 to 3.2mm, the resonant frequency changed from 8.9 to 8.5GHz and 11.25 to 10.50GHz for the lower and upper resonances, respectively. This showed that L1 dimension significantly affected the upper resonance than the lower resonance. The last parameter taken into consideration was L2, the arm length of the smaller U – slot. The effects of L2 on the resonance behavior are presented in Figure 8. It could be seen that L2 had no effect on the lower resonance. As shown in Figure. 8, when L2 was varied from 0.6 to 1.5mm, the upper resonance was shown to vary from 10.65 to 9.80GHz while the lower resonance remained constant at 8.55GHz. This happened due to the special element configuration with unequal U – slots. The current paths taken by the surface charge carriers could be manipulated using the dimension L2 which resulted in an independent tuning for both the resonances.

In order to establish a relationship between the stated parameters and the resonance behavior, a mathematical relation was developed. The model was developed by using the empirical data generated from the commercially available CST computer model. The developed model was established for X – band operation to estimate the two resonances for the proposed element configuration.

ASM Science Journal, Volume 12, Special Issue 2, 2019 for Malaysia in Space

51 The mathematical relations are presented below

𝐹1= 23.15 − 0.09 𝐿𝑎− 0.137 𝐿1− 0.9 𝜀𝑟 (1)

𝐹2= 28.6 − 0.48 𝐿𝑎− 0.17 𝐿1− 1.15 𝜀𝑟− 0.14 𝐿2 (2)

Where F1 is the lower resonance and F2 is the upper resonance of the dual U – slot element configuration. Equations (1) and (2) relate the resonant frequency of the proposed element configuration with the element dimensions and the dielectric material permittivity. It’s

could be noticed from equation (1) that the term for the arm length L2 was not present, which demonstrated the independence of the lower resonance from the arm length L2. Thus, with the known values of La, L1, L2 and εr both the upper and lower resonances of the dual U – slot

element configuration could be estimated.

IV. FABRICATION &