Proposed antenna element structure

The proposed modular subarray element structure is shown in Fig. 1. The antenna element consists of a slave radiation layer, dielectric layer 1, an air layer, a master radiation layer, dielectric layer 2, metal layer 1, dielectric layer 3, a network layer, dielectric layer 4, and metal layer 2, arranged from top to bottom. Dielectric layer 1 is made of FR4 (ε_r = 4.4, tan δ = 0.02), while dielectric layers 2, 3, and 4 are composed of TSM-DS3 (ε_r = 3.0, tan δ = 0.0011)

Figure 1.

Proposed antenna element with (a) 3D view, (b) perspective view.

The antenna element is a stacked microstrip patch antenna employing dual-feed points and electromagnetic coupling feed methods. It is composed of two parts: the antenna radiator and the dual-CP phase shift network. The antenna radiator part includes the slave radiation layer, dielectric layer 1, air layer, master radiation layer and dielectric layer 2. The master and slave radiation layer are rhombic patches with different side lengths, printed on the dielectric layer. Their side lengths are 6.9 and 5 mm, respectively. The master radiation antenna radiates LHCP and RHCP waves into space, while the slave radiation antenna broadens the bandwidth and enhances the gain.

The dual-CP phase shift network part employs a stripline structure, consisting of metal layer 1, dielectric layer 3, phase shift network layer, dielectric layer 4 and metal layer 2. Metallized vias are designed in dielectric layer 2 and dielectric layer 3 to connect the phase shift network and the master radiation antenna. As shown in Fig. 1(b), by reasonably arranging metalized grounding holes, a cavity-like structure can be effectively formed to block the coupling crosstalk between each antenna element. Additionally, connecting the upper and lower metal layers of the stripline can effectively eliminate the resonance, which can effectively improve the active VSWR of the dual-CP phased array modular subarray.

In addition, Fig. 2 shows the profile of the proposed multilayer antenna element. The antenna element consists of five metal layers, each with a thickness of 1 ounce, four dielectric layers and one air layer. Among them, the thicknesses of dielectric layer 1, air layer, dielectric layer 2, dielectric layer 3 and dielectric layer 4 are 1, 1, 0.51, 0.254, and 0.254 mm, respectively.

Figure 2.

Proposed antenna element with multilayer configuration.

The antenna element is driven by a miniaturized dual-CP feeding network, with each port responsible for generating either LHCP or RHCP. The introduction of the air layer and the use of the radiation patch effectively expand the bandwidth. Meanwhile, this low-profile design (with a profile height of 0.12λ) is highly conducive to vehicle conformability, which reduces the complexity of engineering applications.

Design of antenna element

The bandwidth of a single-layer microstrip patch antenna is relatively narrow. To enhance the bandwidth, a stacked microstrip patch configuration is employed. By adjusting the Interlayer dielectric thickness (h) and the patch dimensions (L₁, L₂), the resonance frequencies of the master and slave patches are brought closer together, resulting in a continuous wideband response. The resonant frequency is expressed as follows:

(1)\begin{equation}{f_n} = \frac{c}{{2\left( {{L_n} + \Delta L} \right)\sqrt {{\varepsilon _{r,{\text{eff}}}}} }},\end{equation}

(2)\begin{equation}\Delta L = 0.412h\frac{{({\varepsilon _{r,{\text{eff}}}} + 0.3)(W/h + 0.264)}}{{({\varepsilon _{r,{\text{eff}}}} - 0.258)(W/h + 0.8)}},\end{equation}

(3)\begin{equation}{\varepsilon _{r,{\text{eff}}}} = 0.5\left[ {{\varepsilon _r} + 1 + \left( {{\varepsilon _r} - 1} \right){{\left( {1 + \frac{{10h}}{W}} \right)}^{ - 0.5}}} \right],\end{equation}

where L_n is the length of the patch in the nth layer, $\Delta L$is the edge extension correction, ɛ_{r, eff} is the effective dielectric constant, W is the patch width, and ɛ_r is the relative permittivity of the substrate.

The total bandwidth after stacking is given by the following relationship between the bandwidths of the primary and secondary patches:

(4)\begin{equation}{B_{{\text{total}}}} = \sqrt {B_1^2 + B_2^2 + 2k{B_1}{B_2}} ,\end{equation}

where B₁ and B₂ are the bandwidths of the master and slave patches, respectively. k is the interlayer coupling coefficient, which is dimensionless and represents the strength of electromagnetic coupling between the layers. It typically ranges from 0 (no coupling) to 1 (maximum coupling), with stronger coupling leading to a wider total bandwidth. Increasing the interlayer dielectric thickness h can reduce interlayer coupling k ( $k \propto 1/h$). During design, a balance must be struck between bandwidth and interlayer dielectric thickness.

As shown in Fig. 3, the proposed antenna element attains optimized performance through careful tuning of the resonant frequencies of its master and slave patches, as well as the integrated phase-shift network. The VSWR of both LHCP and RHCP ports remains below 2 within the frequency range of 10.04–13.5 GHz (29.3%). The antenna element achieves a gain exceeding 5.3 dBi over 10.0–13.5 GHz (29.8%), while maintaining an axial ratio (AR) below 2.5 dB within 10.17–13.06 GHz (24.9%). Furthermore, it can be observed from the result diagram that the simulation results for both LHCP and RHCP are perfectly consistent, a result attributed to the complete symmetry of the antenna radiation layer and the 3-dB bridge network structure. This indicates that the antenna element has great potential for application in a wide range of scenarios.

Figure 3.

Proposed antenna element with (a) VSWR and (b) gain and AR of LHCP and RHCP with frequency.

Design of phase-shift feed network

The miniaturized 3-dB bridge-type phase-shift network is shown in Fig. 4, and its size is shown in Table 1. The key to achieve dual-CP is that the miniaturized 3-dB bridge phase shift network provides the amplitude and phase required for CP. Port 1 and port 2 (labeled as P1 and P2 in Fig. 4) correspond to RHCP and LHCP feed ports in Fig. 2 respectively. Feed ports 1 and 2 provide a phase difference of 90° and −90° with respect to ports 3 and 4 (labeled as P3 and P4 in Fig. 4).

Figure 4.

Proposed 3-dB bridge phase shift network.

Table 1.

Size of phase shift network

The two ports of each antenna element are independently connected to one radio frequency (RF) channel, which can independently control the amplitude and phase of the excitation. To achieve simultaneous dual-CP, FDMA technology is employed, allowing the antenna element to excite signals of different frequencies at port 1 and port 2 simultaneously.

Since the feeding network and radiation patch are inseparable components of the stacked microstrip antenna, impedance matching between them is crucial. Therefore, in this communication, an impedance matching method is employed to adjust the matching characteristics at the connector. By introducing high and low impedance lines at the positions labeled as P5, P6, P7, and P8 in Fig. 4, the matching performance at the connector ports is significantly enhanced.

The feeding network designed on this basis has excellent VSWR, amplitude and phase in the band. It can be seen from Fig. 5 that in the frequency range of 10.23–12.77 GHz (22.08%), the isolation is less than −13.2 dB, the VSWR of port 1 and port 2 are less than 1.71, the VSWR of port 3 and port 4 are less than 1.58, the amplitude difference is less than 0.35 dB, and the phase difference is less than ±4.1° in the frequency range of 10.23–13.8 GHz (22.08%).

Figure 5.

Proposed 3-dB bridge phase shift network with (a) isolation, (b) VSWR, (c) insertion loss, and (d) phase.