frequency tunable response for both feed ports, and (d–g) comparison between the measured and simulated gain radiation patterns for 4 V bias voltage which corresponds to 1.36 GHz tunable band for horizontal linear, vertical linear, RHCP, and LHCP, respectively."/>
Figure 1.4 (a) Top view photograph of a frequency tunable concentric circular microstrip patch antenna along with varactor diode placement locations, (b) photograph of the fabricated control feed network, (c) measured frequency tunable response for both feed ports, and (d–g) comparison between the measured and simulated gain radiation patterns for 4 V bias voltage which corresponds to 1.36 GHz tunable band for horizontal linear, vertical linear, RHCP, and LHCP, respectively.
Source: Babakhani and Sharma [2].
Tunable antennas, mostly, frequency tunable can be designed by incorporating variable capacitors in a radiating element in suitable placement arrangements. Figure 1.4a shows photograph of a prototype frequency tunable concentric circular patch antenna where four varactors (Skyworks SMV 1234) are placed between the central patch and the outer ring patch. Two feed ports are selected so that both linear and circular polarizations can be obtained by suitably exciting feed points. Figure 1.4b shows photograph of the control feed network which provides polarization reconfiguration along with simultaneously frequency tunability. The control feed network uses single pole double through (SPDT) and single pole 4 through (SP4T) RF switches along with quadrature power divider, dual in package (DIP) switch, and control lines for biasing varactor diodes and RF switches. A DIP switch is included to apply DC voltage to control the state of the switches. To bias the varactors for tunability, the central patch is connected to the positive pole through the feed network circuit. For the radiation pattern measurements in the anechoic chamber, varactor diodes were biased with a Jameco Electronics DC–DC Boost Converter.
The frequency response with bias voltage variation is shown in Figure 1.4c. With 0 V bias, the antenna resonates at the lowest frequency (1.17 GHz). Similarly, with 12 V bias, the antenna operates at 1.58 GHz. Thus, this antenna provides frequency tunability between 1.17 and 1.58 GHz which corresponds to 30% tunability. By applying bias voltage to two varactors each along with one feed point, vertical and horizontal linear polarizations are achieved. Similarly, applying the same bias voltage to all the varactors and exciting both feed points in ±90° time phase difference, right hand (RH) or left hand (LH) circular polarization is obtained. Measured and simulated radiation patterns are shown in Figure 1.4d–g for 1.36 GHz (4 V bias) for all four polarization cases. Similar was the pattern response for all other frequency tunable bands but are not shown for the sake of brevity.
1.6 Reconfigurable Antennas
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 (εr = 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
|
