A Multiple Resonant Frequencies Circular Reconfigurable Antenna Investigated with Wireless Powering in a Concrete Block

A novel broadband reconfigurable antenna design that can cover different frequency bands is presented. This antenna has multiple resonant frequencies. The reflection coefficient graphs for this antenna are presented in this paper. The new proposed design was investigated along with RF MEMS switches and the results are also presented. Investigations were carried out to check the efficiency of the antenna in the wireless powering domain. The antenna was placed in a concrete block and its result comparison to that of a dipole antenna is also presented in this paper.


Introduction
Reconfigurable antennas [1] are preferred in modern wireless communication systems as they provide a single antenna that can be used for multiple application systems.By using switches, the basic characteristics of a reconfigurable antenna such as the operating frequency, bandwidth, polarization, and the radiation pattern can be tuned to a specific application.These basic characteristics are changed by increasing or decreasing the electrical length of the antenna.A frequency reconfigurable antenna [2] also has low isolation problems.
A novel Circular Reconfigurable Antenna is presented in this paper.Its reflection coefficient, radiation pattern, and gain results are also presented in this paper.The application of this antenna is extended to the wireless powering domain.Investigations were carried out placing the Circular Reconfigurable Antenna and a dipole antenna inside a concrete block.In separate instances, a dipole antenna and another Circular Reconfigurable Antenna were placed above the concrete block and the currents on the surface of the antennas were measured.All results presented in this paper were obtained using HFSS.

CRA
The Circular Reconfigurable Antenna (CRA) shown in Figure 1 contains a circular patch of radius 18.6 mm at the center.Three concentric circular rings are placed around the circular patch.Each concentric circular ring is connected to the next circular ring through four more switches.The concentric circular rings have a radius of 13 mm at a distance of 1 mm around it.This antenna was placed as shown in Figure 1 on top of a substrate of dimensions 148.8 mm × 148.8 mm and thickness 1.5748 mm.
The design equations [3] of a Circular Patch Antenna are given below: where is the radius of the Circular Patch Antenna.
The CRA was designed with the idea that if the radius of the antenna could be increased using switches then the resonating frequency of the antenna could also change.The CRA has four iterations.In the first iteration, only the circular patch is radiating and all the switches are open.In the second iteration, the first set of four switches are on.Thus, the first concentric circular ring and the circular patch are radiating.In the third and the fourth iterations, the third and the fourth set of switches are on.Thus, the third and the fourth concentric circular rings are also radiating.Using the above design equations, the central circular patch can be designed for any frequency band (MHz, GHz, or THz) and the concentric rings can be used to vary the frequency of resonance.

CRA with RF MEMS Switches
In [4], the three-dimensional geometry of shunt capacitive RF MEMS switches was analyzed performing a full wave analysis.The CRA was analyzed in the fourth iteration by using the equivalent lumped circuit [4] model shown in Figure 2 for different geometries.A switch of dimensions [4] 280 m × 120 m is used along with the CRA as shown in Figure 3.The equivalent circuit values [4] are inductance of 5.03 pH, capacitance of 9.31 pF, and resistance of 0.034 ohm in the ON state.

Concrete
The permittivity of a concrete block [1,5] shown in Figure 4 has been modeled assuming that it is lossy dielectric and that a slab has a real part and an imaginary part.The permittivity of a concrete block shown in Figure 4 can be written as where   is the real part of complex permittivity of a concrete block and   is the imaginary part of permittivity of a concrete block.By modeling a concrete block as a Debye material [1,5], its frequency dependent complex relative permittivity obeys the following equation: The concrete model used in these investigations contains a moisture content of 12%.A CRA antenna and a dipole were placed inside a concrete block and the surface of the concrete block was excited using another CRA antenna and a dipole antenna.

Current Consumption of Typical Sensors
A temperature sensor [1,6] consumes 300 A ( = Micro = 10 −6 ) for 50 Sec for a stable reading every five seconds, and a humidity sensor consumes 2.8 mA for 150 msec for a stable reading every thirty seconds.The radio frequency energy incident on the receiving antenna has to be converted to electrical energy [6] to enable wireless powering.

CRA Results
While simulating the CRA, open circuit was used when the switches were off and the patches were shorted when the switches were on.Figures 5 and 6 show the input reflection coefficient  11 for the CRA iterations 1, 2, 3, and 4. The VSWR plots for the CRA iterations 1-4 are shown in Figures 7, 8, 9, and 10.The VSWR values for certain frequencies are very high, so the maximum value of 10 was chosen on the axis.This is the reason the VSWR values which are more than 10 are not visible in Figures 7, 8, 9, and 10.The antenna has a number of resonating frequencies from 3 to 9.5 GHz.The resonating frequency is given at every peak in the input  changing.The resonating frequencies for all the iterations are given in Table 1.

CRA with RF MEMS Switches Results
The input reflection coefficient  11 for the fourth iteration using RF MEMS switches shown in Figure 3 was plotted.This simulations result shown in Figure 22 was obtained using HFSS and is almost the same as the input reflection coefficient   11 for the CRA in the fourth iteration from 6 to 10 GHz.A resonant frequency of 4.27 GHz is however missing.These results confirm the theory that circuit elements can be used in HFSS to simulate RF MEMS switches [4,7] in the ON state (Figure 23).
As stated earlier, the concrete block was simulated in HFSS as a Debye model [1,5].The attenuation of the concrete block is very high at high frequencies [8,9].The dipole used was designed to resonate at a frequency of 500 MHz and the best resonant frequency of 6.64 GHz (iteration 4) shown in Figure 6 was used for the CRA.An input peak current of 10.75798A was provided to the transmitter using HFSS.The currents measured using HFSS on the surface of the receiver antenna are shown in Table 3.
Even though the CRA in Case 3 is being excited by a very high frequency of 6.64 GHz, the received current shown in Table 3 is almost the same as Case 1.The received current was shown to be more than sufficient to power sensors.The use of the CRA antenna is allowing typical wireless sensors   shown in Section 5 to be powered at high frequencies which in earlier publication results [1,9] was shown to be very difficult.

Conclusions
The CRA antenna can be designed for any frequency band.The gain and the radiation pattern of the CRA antenna can be changed by using the switches and changing the iterations.The input reflection coefficient  11 was shown with RF MEMS switches and the ability of the antenna was verified.The CRA antenna can also be used to power sensors at low as well as high frequencies.The currents received in a concrete slab by the CRA have been presented for the first time in this paper.
International Journal of Antennas and Propagation

Figure 11 :
Figure 11: Directivity of the CRA for the first iteration at 3.04 GHz.

Figure 12 :Figure 13 :
Figure 12: 3D gain of the CRA for the first iteration at 3.04 GHz.

Figure 14 :
Figure 14: Directivity of the CRA for the second iteration at 6.55 GHz.

Figure 15 :Figure 16 :
Figure 15: 3D gain of the CRA for the second iteration at 6.55 GHz.

Figure 17 :
Figure 17: Directivity of the CRA for the third iteration at a frequency of 8.5 GHz.

Figure 19 :
Figure 19: 2D gain of the CRA for the third iteration at 8.5 GHz.

Figure 20 :
Figure 20: Directivity of the CRA for the fourth iteration at 8.44 GHz.

Figure 21 :Figure 22 :
Figure 21: 3D gain of the CRA for the fourth iteration at 8.44 GHz.

Table 1 :
Resonating frequencies of the CRA.

Table 2 :
The three cases.
Figure 18: 3D gain of the CRA for the third iteration at 8.5 GHz.