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Reconfigurable antennas can be frequency reconfigurable, pattern reconfigurable, or polarization reconfigurable or combination of these types. Once again, reconfigurable antennas can be categorized as a multifunctional antenna in general like the one discussed in the Section 1.5. Reconfiguration is obtained by varying the antenna structure with the help of RF or optical switches. One such antenna demonstrating frequency reconfiguration is shown in Figure 1.5.

The geometry of the proposed PIFA element is shown in Figure 1.5a and b. This miniaturized antenna is able to achieve consistent high band coverage while reconfigurable lower frequency bands are maintained. The antenna is matched across all the 4G/LTE lower reconfigurable bands and simultaneous higher consistent wireless bands. The antenna miniaturization and impedance matching are possible by exploiting the meandering and mutual coupling between the different parts of the antenna structure. It employs ground plane edge effect for its optimum performance.

The reconfigurable antenna element (Figure 1.5a, b) is designed by using Ansys HFSS on a tablet size ground plane of L₄ = 180 mm and W₄ = 150 mm [3, 4]. The corresponding antenna dimensions are: length arm, L₁ = 73 mm and width, W₁ = 2.3 mm to the first PIN diode switch, and second ground extension, L₂ = 20 mm with end width, W₂ = 7 mm, coupling grounding gap, g = 5.6 mm and g₁ = 3.8 mm designed on a FR4 (ε_r = 4.4, tan δ = 0.021) substrate with thickness, h = 0.8 mm, W₄ = 150 mm, L₄ = 180 mm. The antenna clearance and area required are W₃ = 48.5 mm and L₃ = 20 mm. Four PIN diodes are used to generate frequency switching for the 4G/LTE lower communication bands (704–960 MHz) while it maintains simultaneous consistent higher frequency bands between 1710 and 2690 MHz. The bias network (Figure 1.5c) was used for each PIN diode (Microsemi MPP4203) to prevent DC and AC signal getting influenced by the power supply. Also, it prevents the power supply line from becoming part of the antenna. The 47 nH inductor choke and 560 pF DC blocking capacitor were used as a bias network to prevent RF signal distortion. Photograph of the fabricated antenna on the tablet size ground is shown in Figure 1.5d.

Figure 1.5 (a) Geometry of the proposed reconfigurable PIFA radiating element along with design parameters, (b) location of the PIN diodes on the radiating element, (c) bias network used for PIN diodes, and (d) photograph of the fabricated radiating element on the tablet size ground plane.

The designed antenna can operate in LTE bands 13, 14, 17, EGSM, GSM (lower frequency) and LTE bands 4, 7, DCS, PCS (higher frequency) with near omnidirectional radiation patterns for each band. There are a total of five switching states to reconfigure lower frequency as given in Table 1.1. The first state is when all the PIN diode switches are in the OFF state. The second state occurs when the first PIN diode switch is activated in the ON state while the other diodes are in the OFF state, which allows tuning of the center frequency from 930 to 850 MHz. The third state (780 MHz) occurs when the first and second PIN diodes are switched in the ON state while the remaining diodes are in the OFF state. The fourth state (750 MHz) occurs when the first, second, and third PIN diodes are switched in the ON state while the fourth PIN diode is in the OFF state. The fifth state (720 MHz) is obtained when all the PIN diode switches are in the ON state. While we manage these PIN diodes from the first state to the fifth state, we simultaneously maintain the higher frequency bands between 1710 and 2690 MHz.

Table 1.1 PIN diode states for reconfiguring the lower frequency bands while the higher band is consistently maintained.

Switch table list
	LTE 17 (0.704–0.746 GHz)	LTE 13 (0.746–0.787 GHz)	LTE 14 (0.758–0.798 GHz)	GSM 850 (0.824–0.894 GHz)	EGSM (0.880–0.960 GHz)
First switch	ON	ON	ON	ON	OFF
Second switch	ON	ON	ON	OFF	OFF
Third switch	ON	ON	OFF	OFF	OFF
Fourth switch	ON	OFF	OFF	OFF	OFF

Current distribution (Figure 1.6) shows current flow and hotspot for radiating element at the lower frequency end (780 MHz, Figure 1.6a) and the upper frequency end (1880 MHz, Figure 1.6b) for the proposed reconfigurable PIFA. Low and high bands have individual hotspots which can tune each band separately. Different sections of the radiator employ mutual coupling to obtain better matching and bandwidth.

Simulated and measured reflection coefficient magnitudes for the lower reconfigurable bands are compared in Figure 1.7a. It can be observed that the antenna is matched well below −7 dB between 704 and 960 MHz which includes the reconfigurable states of LTE 17, 13, 14, GSM, and EGSM. Similarly, Figure 1.7b shows the simulated and measured matching for the high‐frequency band. Once again, it can be observed that the antenna consistently maintains the matching level between 1710 and 2690 MHz better than −7 dB for all the switch states.

The 3D radiation patterns and total antenna efficiency were measured using the Satimo chamber. The total efficiency for the reconfigurable stages at the lower frequency bands and the consistent higher band is shown in Figure 1.8a and b, respectively. This efficiency takes care of all the possible losses in the antenna such as mismatch loss, Ohmic and dielectric losses, and losses due to the PIN diodes and bias components. The antenna efficiency is above 50% for all the switching states in the lower band and the higher band. The simulated and measured efficiencies agree reasonably well except toward edges of the bands.

Figure 1.6 Surface current distribution for (a) 780 MHz in lower frequency band and (b) 1880 MHz in upper frequency band.

The simulated (Figure 1.9a, c, e, g) and measured (Figure 1.9b, d, f, h) 3D radiation pattern for the lower reconfigurable bands for the selected 720, 770, 850, and 910 MHz is shown in Figure 1.9. Similarly, simulated (Figure 1.10a, c, e, g) and measured (Figure 1.10b, d, f, h) 3D radiation pattern for the higher consistent bands for the selected 1710, 1880, 2100, and 2600 MHz is shown in Figure 1.10. From these figures, it can be observed that the patterns are near omnidirectional except toward the higher frequency end, where patterns show slight multilobes and nulls. In all the cases, realized peak gain is positive and around 2 dBi for both simulated and measured cases. Further, the patterns toward the higher end show some directionality due to the ground plane effect which tends to push the radiated energy in a direction causing diversity that is helpful when the MIMO antenna system is implemented.

Figure 1.7 Comparison of the simulated (solid lines) and measured (dash lines) reflection coefficient magnitudes against specifications of the (a) lower 4G LTE frequency reconfigurable bands and (b) consistent higher frequency bands.

Figure 1.8 The simulated (solid lines) and measured (dash lines) total antenna efficiencies for (a) the reconfigurable states of the lower 4G bands and (b) the consistent higher frequency band.

Figure 1.9 Simulated (a, c, e, g) and measured (b, d, f, h) near‐omnidirectional 3D radiation pattern at (a, b) 720 MHz, (c, d) 770 MHz, (e, f) 850 MHz, and (g, h) 910 MHz.

Figure 1.10 Simulated (a, c, e, g) and measured (b, d, f, h) near‐omnidirectional 3D radiation pattern at (a, b) 1710 MHz, (c, d) 1880 MHz, (e, f) 2100 MHz, and (g, h) 2600 MHz.