Miniaturized patch antennas with circular polarization (CP) are widely used in present day wireless communication systems. CP antennas are able to attenuate multipath effects and allow the information transmission autonomous of the orientation of the receiver with reference to transmitter. In general, CP microstrip patch antennas can be divided into two categories: single feed type and dual feed type. The dual feed type CP microstrip patch antennas can provide wider CP bandwidth but they are large in size and require a complicated feeding network [1–3]. The single feed type CP microstrip patch antennas involves an orthogonal pair of modes with 90° phase shift, which usually requires a suitable feeding position and a small disturbance on the antenna structure to excite the orthogonal modes, such as slots, slits and truncated corners. Be that as it may, these customary procedures offer no size diminishment [4–6].

Metamaterials are the smart materials engineered to have properties that have not yet been found in nature. Novel antennas like dual band dual polarized antennas are designed by using metamaterials because of their artificial electromagnetic properties. Split-ring resonators (SRRs) and wires are the basic metamaterials. Composite right/left-handed (CRLH) structures are another type of metamaterials, which are used to achieve zero and negative order modes. Microstrip patch antenna loaded with mushroom unit cell (MUC) is able to excite the negative order resonances and provide decreased resonance frequencies, thus enabling the antenna miniaturization [7–12]. These materials have been used for multiband applications. The reactive impedance surface (RIS) is used to provide a zero reflection phase at a particular frequency [13, 14]. The compactness and circular polarization operation can be achieved by applying fractal curves to the edges of the square patch [15, 16].

In this paper, MUC loaded compact single probe-feed asymmetrical fractal patch antenna based on RIS is proposed. The detailed fractal antenna design, CP working principle and radiation characteristics are then presented. The measured results are in good agreement with the simulated results which are obtained from the HFSS simulation.

The structure of the proposed antenna which is fed by single coaxial probe feed is shown in the Fig. 1. It is a three layer structure where the top and bottom dielectric substrates are “FR4 epoxy” with a dielectric constant of 4.4 (Fig. 2). The RIS is sandwiched between these dielectric materials and is composed of 4 × 4 periodic planar array of metallic asymmetrical fractal patches. The optimized dimensions of the suggested antenna are listed in Table 1.

Fractals are analyzed by two important factors: iteration order (IO) and indentation factor. For semi-circle fractal curves, the indentation factor is indentation radius (IR). The proposed square patch length is L2 and rx and ry are IRs along the x- and y-axes. The CP operation is accomplished by deploying the fractal semi-circled curves. The four sides of the square patch antenna are etched with fractal curves of different IRs. Two orthogonal modes with 90° phase shift are introduced for CP radiation with the use of asymmetrical fractal curves along the sides of the square patch antenna.

Single fractal MUC is inserted into the fractal patch antenna to get the double band. To separate the RIS from VIA and feed, two separate holes are etched from the RIS so that VIA and feed do not touch the RIS. Desirable CP bandwidth can be obtained by adjusting the slot position on the patch and mushroom position. The patch and MUC are capacitively coupled by a small gap. Because of mushroom unit structure a lower resonance frequency band is occurred.

Half circled or semi-circled fractal curves are characterized by two parameters, IO and IR. Equal IR (rx1 = rx2 = ry1 = ry2) along four sides of the square patch antenna results in linear polarization at upper frequency band. Circular polarization radiation can be obtained by loading different IRs along each side of the patch (rx1 ≠ rx2 ≠ ry1 ≠ ry2) or (rx1 = rx2 = rx ≠ ry1 = ry2 = ry). High bandwidth can be obtained when IRs along same axis are equal and are not equal along the perpendicular axis (rx1 = rx2 = rx ≠ ry1 = ry2 = ry). The reflection phase characteristics of the RIS MUCs are shown in Fig. 3.

Operation of the proposed design can be understood by considering an evolution of patch antenna design shown in Fig. 4. Initially a square patch with 18 mm × 18 mm is chosen as reference which is implemented on different types of RIS for single-band operation. The RIS of Ant1 is composed of 5 × 5 planar array of square elements with each element size is 5 mm. Ant2 has 5 × 5 planar array of each element size is 6 mm. Ant3 has planar array of each element size is 7 mm. Ant4 has planar array of each element size is 8 mm. From the above four antennas Ant3 is selected for dual-band operation because of its high impedance bandwidth. Ant5 is designed with square RIS (all elements are square) and fractal MUC loaded square patch for dual-band dual-polarization operation. Ant6 designed by introducing asymmetrical fractal curves to patch in Ant5. Ant7 is obtained from Ant5 by introducing fractal curves to each element of RIS. Ant8 is designed by introducing fractal curves to both patch and each element of RIS. Finally Ant8 is the proposed antenna with dual-band dual-polarization.

The simulated return loss characteristics of single band antennas from Ant1 to Ant4 are shown in Fig. 5 and the 10-dB return loss bandwidth of each antenna is listed in Table 2.

The simulated return loss characteristics of all dual-band dual-polarized antennas from Ant5 to Ant8 are shown in Fig. 6 and the impedance bandwidth at two resonating bands of each antenna is listed in Table 3.

The proposed antenna (Ant8) is printed on FR4 epoxy material and is shown in Fig. 7. The experimental results of the fabricated CP patch antenna are compared with the simulated results. The return loss characteristics are shown in Fig. 8. The upper resonance frequency band which is occurred due to RIS and fractal patch results in wide impedance bandwidth that covers 10-dB return loss bandwidth of 8.48% (3.21–3.49 GHz). The lower resonance frequency band which is achieved due to MUC results in 10-dB return loss bandwidth of 2.59% (2.30–2.36 GHz). The axial ratio characteristics are shown in Fig. 9. The measured 3-dB axial ratio bandwidth at upper operating frequency band is 4.26% (3.21–3.35 GHz).

The measured and simulated radiation patterns of the proposed antenna at lower and upper resonating band are shown in Figs. 10 and 11, respectively. The measured and simulated gain of the antenna is given in Fig. 12.

Table 4 gives the comparison of the results of the proposed antenna with antennas which already exist in the literature. The proposed antenna is more compact and produces more bandwidth compared to remaining antennas listed in Table 4.

A novel miniaturized single probe feed asymmetrical fractal patch antenna loaded MUC has been studied and presented based on RIS and MUC for dual-band operation. The measured 10-dB return loss bandwidth of antenna prototype are 8.48 % and 2.59% at upper and lower resonance frequencies, respectively. The 3-dB axial ratio bandwidth of the antenna prototype is around 4.26% at upper band. The suggested antenna covers WLAN (2.4 GHz) and WiMAX (3.4 GHz) operating bands.