RFSS based on cross dipole or grid using PIN diode

This work presents an RFSS with two discrete states, corresponding to a pass‐band or stop‐band response depending on PIN diode biasing. The RFSS behaves as an array of cross dipole elements when all diodes are in the OFF‐state and as a grid when the diodes are in the ON‐state. The cross dipole patch array has a stop‐band filter response and the grid behaves as a pass‐band filter at the design frequency of 1.6 GHz. Simulation and measurements of insertion loss are presented, demonstrating the switchable band‐pass to band‐stop response.

[6] Xi T, Huang S, Guo S, Gui P, Huang D, Chakraborty S. Highefficient E-band power amplifiers and transmitter using gate capacitance linearization in 65 nm CMOS process. To

| I N T R O D U C T I O N
Typically, frequency selective surfaces (FSS) are twodimensional periodic arrays that behave as spatial filters. 1 The frequency response of an FSS will behave as a stopband if the single element of the array is a patch. Analogously, an aperture array element will result in a pass-band response. 2 Variables, such as unit cell geometry, dimension, and periodicity are important factors for determining the FSS frequency response. 3 An extension of the FSS is the reconfigurable frequency selective surfaces (RFSS). RFSS properties (such as resonant frequency and polarization) can be changed in real time, while passive FSS has constant characteristics. The ability to vary the frequency response can be achieved by introducing active elements into the device, such as the PIN diode, 4 which is commonly used as a switch. PIN diodes are placed as switches along the RFSS structure to provide reconfiguration. An external DC bias is applied to the diodes as forward or reverse bias to define the ON or OFF state of the PIN diodes, respectively. When a diode is forward biased, it creates a new path for current flow, which results in changes over the FSS's response. MEMS, 5 varactors, 6 and photodiodes 7 can also be used to reconfigure devices. This paper presents a RFSS whose array consists of cross dipoles connected by PIN diodes. By changing the state of the diodes, the RFSS's frequency response can be toggled between stop band and pass band. Both configurations operate in dual TE/TM polarization for an orthogonal incident angle. The simulation was held using CST Microwave Studio 2016 ® . Simulated and measurement results for insertion loss are presented here.

| Structure
A cross dipole is shown in Figure 1A. The cross dipole resonates when its length is a half-wavelength. 3 An FSS whose elements are cross dipoles is expected to have a stop-band characteristic response, since the cross dipoles behave like patches. Another possible FSS configuration is the grid configuration, shown in Figure 1B. This time, the FSS unit cell is a grid expected to produce a pass-band response, since its structure is composed by apertures.
The key to achieving the proposed RFSS structure, relies on PIN diode placement at the edge of the cross dipoles, as illustrated in Figure 1C. Notice that when the diodes are in OFF state, there is no current flowing between the cross dipoles. Therefore, the RFSS will behave as a cross dipole FSS (stop-band). However when the diodes are in ON state current flows through the dipoles. Consequently, the RFSS will behave as a grid FSS (pass-band). This simple configuration is remarkably interesting because of its simplicity and also because it allows FSS reconfiguration by a change of filtering function (ie, toggling between pass-band and stop-band responses).

| Circuit bias
The RFSS contains an array of N 3 N diodes. The cross dipoles are connected to each other by diodes placed at their edges. By choosing the appropriate diode orientation (see Figure 2) it is possible to switch all diodes with just two DC bias points in the structure (GND and Vcc). The diodes are connected in an orientation that goes from lower voltage to higher voltage, or vice versa, as shown in the Figure 2. The diodes are forward biased or reverse biased, all at once. In order to simplify simulations, the ON-state diode is replaced by a conducting printed strip on the substrate connecting the cross dipoles. Alternatively, the OFF-state diode is represented by a spacing between the cross dipoles, even though this model does not account for diode intrinsic properties, such as resistance and reactance, it provides a first approximation and allows demonstrating the reconfigurable bandpass to stop-band RFSS concept.

| R E S U L T S A N D D I S C U S S I O N
This section presents the measured and simulated results for the proposed RFSS considering a normal incidence of a vertically polarized plane wave. The simulation of the structure considering open and short circuits replacing the diodes is shown in Figure 3. The center frequency occurs at 2.21 GHz with S 21 values of 229 dB for the OFF-state and 20.6 dB for the ON-state, successfully demonstrating the stop-band and pass-band responses of the device, respectively.
In order to validate the simulations, a 4 3 4 array (ie, 16 unit cells) version of the RFSS was fabricated as shown in  Table 1 presents a comparison between simulated and measured insertion loss. A frequency shift between the simulated and measured results occurred due to the parasitic elements of the diodes.

| C O N C L U S I O N S
As shown in both simulated and measured results, the RFSS performs as expected, changing filtering characteristic when its diodes are toggled between ON and OFF states. Experimental and simulated results show a good agreement and demonstrate the versatility of the structure as a reconfigurable filter. However, a shift between experimental and simulated results is presented. This is believed to be due to the use of an ideal short circuit as a model for the diode ONstate. The RFSS behaves as a band-pass filter when the diodes are at ON state. At OFF state, the RFSS behaves as a band-stop filter. Switching Vcc between 0 and 5 V is sufficient to change the filtering characteristic of the RFSS, therefore, the structure can be controlled by ordinary low-power circuits. This structure can be applicable for devices such as adaptive and RFID antennas.  Abstract A method is proposed for designing a small dual-band slot antenna using capacitor loading. A capacitor is loaded at the right end of a slot antenna to form a parallel resonance using inductance generated by the slot end. As the impedance of the parallel resonator becomes capacitive at a frequency higher than the resonance, the slot antenna resonates at less than a quarter wavelength. To match input impedance in dual-band, a coupled feed is placed near the capacitor and a structure for frequency tuning is also

| I N T R O D U C T I O N
A slot antenna is made by etching a slot in a metal surface and its resonant frequency depends on the size and shape of the slot. As the parts or materials attached to the metal surface have no significant effect on the performance of the antenna, it is a preferred type of antenna for installation in an environment surrounded by metallic materials. Because the slot structure has an advantage in accommodating lumped elements such as inductors or capacitors, it is useful as a tunable antenna. In practice, varactor diodes, pin diodes, stubs and liquid metal Galinstan have been embedded in slot antennas in designing tunable antennas. [1][2][3][4] As a slot antenna uses the ground plane, its performance could be enhanced by integration with conventional antennas such as monopole or inverted F antennas. In a dual-band antenna, a slot is applied not only to create high frequency band resonance but also to enhance the impedance bandwidth. 5-7 A slot antenna can be used to advantage in designing the antenna for a portable device which has a metal case or metal frame. 8,9 But as the slot antenna has a structure with half-wavelength resonance, it is difficult to reduce the antenna size 10-14 and the slot size is fundamentally dependent on the surrounding equipment which creates difficulties with antenna design. A slot antenna with open end is proposed to reduce the antenna size. 15,16 However, in actual practice, this requires cutting the metal frame, which places constraints on the design options. This letter proposes a technique to reduce slot size of a mobile device antenna using capacitor loading along with a technique to utilize a dual-band design.

| L O A D I N G E F F E C T S OF TH E S L O T A N T E N N A
Transmission line models for a conventional slot antenna and the proposed antenna are illustrated in Figure 1. A conventional slot antenna with a half wavelength resonance can be modeled incorporating a transmission line with shorts on the left and right. When connecting the capacitor to the short at either end of the slot antenna, parallel resonance is formed