Design of Omnidirectional Antennas Using TM₂₂ and Quasi-TM₁₁ Modes with Characteristic Mode Analysis

Abstract

This study presents the design of two high-gain omnidirectional antennas with minimal pattern ripple. Antenna I is based on a conventional microstrip patch structure, while Antenna II integrates a modified design with four metal probes. Characteristic mode theory (CMT) was applied to analyze the far-field radiation patterns of both antennas, with a focus on the distinct radiation modes. The analysis revealed that Antenna I operates in the TM₂₂ mode and Antenna II in the quasi-TM₁₁ mode, both exhibiting omnidirectional radiation characteristics. A comparative investigation of four different feeding techniques was conducted to ensure equal amplitude and phase excitation at each port, resulting in a low pattern ripple for both designs. A 1:4 power divider was implemented to validate the designs, and the performance of Antennas I and II was experimentally assessed. The measurement results showed that the −10 dB operating bandwidths of Antennas I and II spanned 2.42–2.50 GHz and 2.34–2.57 GHz, respectively, with corresponding peak gains of 8.0 dBi and 4.55 dBi at a frequency of 2.45 GHz.

1. Introduction

Omnidirectional antennas, which can transmit and receive electromagnetic waves uniformly in all directions, are crucial in wireless communication systems such as WLAN, Bluetooth [1,2,3], and other fields of MIMO [4,5,6]. Traditional omnidirectional designs primarily using monopole antennas have high-profile structures that impede integration with low-profile systems. While recent advancements leverage high-order modes in circular or ring-shaped structures to achieve omnidirectionality, these designs face limitations in gain performance.

For instance, Economou designed an omnidirectional antenna with a circular structure. The antenna, with the TM₀₁ mode and surface current, presents a symmetrical distribution and achieves an omnidirectional pattern similar to that of the monopole antenna [7]. Al-Zoubi designed an antenna with a center-fed circular ring structure that allows the antenna to operate in a high-order mode and realize an omnidirectional pattern. At a working frequency of 5.8 GHz, the antenna’s gain is 5.7 dBi [8].

Yunliang designed several omnidirectional antennas, mainly by etching slots in the antenna structure and loading pin probes such that the antenna operates in the TM₀₁ and TM₂₀ modes to achieve an omnidirectional pattern, resulting in an antenna gain of 6 dBi at an operating frequency of 2.45 GHz [9,10].

Xiaohan Zhai proposed a broadband omnidirectional antenna based on the TM₀₁, TM₀₂, and TM₀₃ modes, realizing low-profile vertical polarization characteristics [11]. Similarly, Oh. J designed a broadband vertically polarized omnidirectional antenna by utilizing the coupling effect of parasitic structures [12]. Dong designed a low-profile vertically polarized omnidirectional antenna using the TM₀₁ and TM₀₂ modes of a microstrip antenna, achieving a gain of 2–5 dBi within an operating bandwidth of 1.7–2.79 GHz [13].

Despite these efforts, all the aforementioned omnidirectional antennas achieve omnidirectional characteristics through the high-order mode of the antenna; however, the gain values of the antennas are not very high. Array-based approaches have been explored to enhance gain, but they introduce impractical size constraints [14,15,16,17,18,19,20]. For example, Sun’s 2 × 2 bent-dipole array [14,15] and Yu’s dipole-parasitic unit array [16] achieved omnidirectionality with gains of 2.4 dBi, yet their large footprints limit applicability in compact systems [17].

Characteristic mode theory (CMT) has emerged as a powerful tool for effectively analyzing the different working modes of antennas and has been widely used in the design process of broadband, circularly polarized, and omnidirectional antennas [21,22,23,24]. Yang [21] applied CMT to design a metamaterial-based antenna with a 16.6% operating bandwidth, while Shuai [22] utilized CMT to achieve broadband omnidirectionality in a dipole array. However, these works did not address the dual challenges of high gain, low profile, and minimal pattern ripple simultaneously.

In this study, we propose two novel low-profile, high-gain omnidirectional antennas operating at 2.45 GHz. Antenna I employs a conventional microstrip patch structure operating in the high-order TM₂₂ mode, while Antenna II integrates four metal probes to excite a quasi-TM₁₁ mode. Using CMT, we analyzed the far-field radiation patterns and surface current distributions of both designs, ensuring symmetric excitation and low pattern ripple. A 1:4 power divider was designed to enable equal amplitude in-phase feeding, and both antennas were experimentally validated.

2. Design Process of Antenna Unit Structure

Figure 1 depicts the evolution of the antenna structure designed in this study. Figure 1a illustrates a conventional microstrip patch antenna, while Figure 1b shows a pin-loaded microstrip antenna structure. Previous studies have demonstrated that shorted pin-loaded microstrip antennas can achieve higher gain values compared to conventional microstrip antennas of the same size [25,26,27,28]. To realize high-gain omnidirectional radiation characteristics, a pin-loaded microstrip antenna structure was developed in this work.

The conventional microstrip antenna and the pin-loaded microstrip antenna have the same dielectric substrate and size. The dimensions of the antennas are as follows: lp = 104.4 mm, wp = 104.4 mm, lp1 = 52.2 mm, wp1 = 52.2 mm, lp2 = 62 mm, wp2 = 62 mm, lp3 = 36 mm, wp3 = 36 mm, rp = 1 mm, h = 3 mm.

We employed the characteristic mode analysis (CMA) tool in the CST electromagnetic simulation software [29] to investigate the radiation patterns and mode distributions of the antennas. The structure of the antenna in the electromagnetic simulation software CST is shown in Figure 2. The area around the antenna was set as the radiation boundary condition, and the antenna dielectric layer was set as a lossless substrate material.

Figure 3 presents the far-field radiation patterns of the antennas in various working modes. Figure 3a shows the radiation patterns corresponding to five modes for the conventional microstrip antenna, while Figure 3b illustrates the patterns for three modes of the pin-loaded antenna structure. The conventional microstrip antenna exhibits omnidirectional radiation characteristics in modes 1, 2, and 5. Specifically, under mode 1—that is, when the operating frequency was 2.45 GHz—the microstrip antenna operated in the TM₂₂ mode. Similarly, the pin-loaded antenna structure achieves omnidirectional radiation when operating in mode 1, corresponding to the Quasi-TM₁₁ mode.

Figure 4a illustrates the different mode distributions of the conventional microstrip patch antenna, while Figure 4b shows different mode distributions of the pin-loaded antenna.

It can be further obtained from Figure 4 that the reference microstrip antenna has five modes within 1–3 GHz, and its working frequencies are 1.2 GHz, 1.75 GHz, 2.45 GHz, 2.5 GHz; meanwhile, the antenna II has three modes with 1–6 GHz, and the working frequencies are 2.45 GHz and 3.6 GHz respectively.

3. Array Antenna I

The structure of the antenna is illustrated in Figure 5. The antenna consists of three layers: the radiation layer, the intermediate ground layer, and the feed signal layer. The dielectric substrate of the radiation layer and the power-division network layer are both made of S7136 material with a dielectric constant of 3.6, a loss tan of 0.0035, and a thickness of 0.508 mm. An air dielectric layer with a thickness of 5 mm is placed between the radiation layer and the ground layer of the power-division network. The structural dimensions of the antenna are as follows: wg = 220 mm, lg = 220 mm, lp = 110 mm, lf = 36 mm, lf1 = 18 mm, lf2 = 19 mm, wf1 = 1.1 mm, wf2 = 0.24 mm, h = 5 mm, and h1 = 0.508 mm.

The surface current distribution of the antenna is shown in Figure 6. When the excitation ports are of equal amplitude and the phases between adjacent ports have the same phase, the surface current of the antenna exhibits central symmetrical and omnidirectional characteristics.

A power divider with transmission line theory was adopted for the feed network, as shown in Figure 5c. The input port was located in the center (port 1), with output ports (ports 1, 2, 3, and 4) arranged around it (1:4 power divider).

The simulated reflection coefficient (S₁₁) and transmission coefficient (S₂₁, S₃₁, S_41, and S₅₁) of the 1:4 feed network are shown in Figure 7. From Figure 7a, one can see that for a bandwidth of 2.0–3.0 GHz, the S₁₁ of the input ports is below −10 dB, and the amplitudes of the output ports are approximately equal. At 2.45 GHz, the insertion loss is approximately 0.07 dB. Figure 7b shows that the phases of each output terminal are identical, meeting the phase requirements of the output ports.

The antenna designed in this paper has an omnidirectional radiation pattern. To investigate the influence of the metal ground plate size on radiation pattern and gain, the simulation results for different ground plate sizes are shown in Figure 8a. The XOZ-plane was selected to analyze the radiation pattern due to the high symmetry between the XOZ-plane and YOZ-plane. As shown in Figure 8a, increasing the ground plate size from 140 mm to 220 mm raises the peak gain from 7.6 dBi to 8.2 dBi at 2.45 GHz. The peak gain position also shifts from θ = ±30° to θ = ±25°, moving towards the center. Figure 8b shows the simulation results of the antenna’s radiation pattern with different air dielectric layer thicknesses. As the air layer thickness increases from 1 mm to 5 mm, the peak gain increases from 6.6 dBi to 8.2 dBi.

To verify the antenna’s performance, the antenna structure was fabricated, as shown in Figure 9. The S₁₁ of the antenna was measured using a network analyzer, and the radiation patterns were measured in a near-field microwave anechoic chamber. The measurement results for the array antenna are shown in Figure 10. The −10 dB bandwidth of the antenna is 2.42–2.50 GHz (80 MHz). At 2.45 GHz, the S₁₁ is −20 dB. The antenna exhibits omnidirectional radiation with a peak gain of 8.0 dBi at 2.45 GHz.

The radiation patterns of the antenna are shown in Figure 11. The antenna demonstrates omnidirectional patterns similar to those of a dipole antenna in both the XOZ-plane and YOZ-plane, as shown in Figure 11a,b. At θ = 28°, the peak gain is 8.0 dBi, as shown in Figure 11c. The measured gain is 0.2 dBi lower than the simulated value due to antenna processing, assembly, and measurement errors. The antenna’s symmetrical surface current distribution, resulting from the four-fed structure, ensures low pattern ripple at 2.45 GHz.

4. Array Antenna II

Figure 12 depicts Antenna II’s structure. The antenna comprises multiple layers: the upper radiation layer, intermediate dielectric layer, lower metal ground layer, feed network dielectric layer, and feed signal layer. The dielectric substrate of the radiation layer is also the S7136 material with a thickness of 0.508 mm.

The structural dimensions of the antenna are as follows: wg = 124 mm, lg = 124 mm, l2 = 62 mm, w2 = 62 mm, lp1 = 10 mm, lp2 = 18 mm, rp = 1.0 mm, h1 = 3 mm, h2 = 0.508 mm, wf1 = 1 mm, lf1 = 2 mm, lf2 = 1 mm, wf2 = 0.24 mm, lf3 = 6 mm, lf4 = 2 mm, and lf5 = 5 mm.

The current analysis of the antenna radiation patch structure is shown in Figure 13, from which it can be observed that when the amplitudes of each excitation port are equal and in phase, the current shows a central symmetric relationship such that the antenna has omnidirectional radiation characteristics.

To analyze the influence of structural parameters on antenna performance, the following parameters are investigated: the length of the radiation patch, and the radius of the metal probe. The simulation results are shown in Figure 14.

As shown in Figure 14a, the length of the patch significantly affects the antenna’s S₁₁. As the patch length increases, the resonant frequency shifts to lower frequencies. Figure 14b illustrates the influence of the metal probe radius on the antenna’s S₁₁. As the radius increases, the resonant frequency shifts to higher frequencies. Through the theoretical analysis of microstrip antenna, the antenna designed in this study mainly works in the high-order mode of the antenna. The increase of the metal probe is equivalent to the smaller size of the effective radiation patch so that the resonant frequency of the antenna changes to the high frequency.

The antenna structure was fabricated and its performance was evaluated, as shown in Figure 15. The S₁₁ of the antenna was measured using a network analyzer, and the radiation patterns were measured in a near-field microwave anechoic chamber. The measurement results are shown in Figure 16 and Figure 17.

Figure 16 shows the S₁₁ measurement results for Antenna II. The antenna exhibits a −10 dB bandwidth of 2.34–2.57 GHz, with an S₁₁ of −16.4 dB at 2.45 GHz.

Figure 17 shows the simulated and measured results for the radiation patterns of Antenna II. At 2.45 GHz, the antenna demonstrates omnidirectional radiation characteristics in both the XOZ and YOZ planes, with a gain of approximately 4.55 dBi. The cross-polarization ratios on the XOZ and YOZ planes are 49.3 dB and 50.1 dB, respectively. The antenna exhibits less than a 0.2 dB pattern ripple at 2.45 GHz. The peak gain occurs at θ = 36°, as shown in Figure 17c.

5. Performance Comparison

In this section, we compare the performance of the proposed omnidirectional antennas with those reported in the literature. As

Table 1. Comparison with other designs.

Reference		Bandwidth (GHz)	Polarization	Peak Gain (dBi) at 2.45 GHz	Size (λ03) λ0 at 2.45 GHz
[13]		1.7–2.79	vertical	4.5	0.26 × 0.26 × 0.086
[18]		1.7–3.54	horizontal	0.6	1.225 × 1.225 × 0.0086
[19]		1.62–2.75	vertical	4.7	0.62 × 0.62 × 0.77
[20]		1.67–2.73	horizontal	1.7	0.98 × 0.98 × 0.0058
[22]		1.7–5.0	vertical	3.2	0.36 × 0.36 × 0.49
This work	Antenna I	2.42–2.5	vertical	8.0	1.76 × 1.76 × 0.048
	Antenna II	2.34–2.57	vertical	4.55	0.99 × 0.99 × 0.028

shows, the peak gains of Antennas I and II at 2.45 GHz are 8.0 dBi and 4.55 dBi, respectively.

Antenna 1 mainly operates in the TM₂₂ mode, while Antenna II mainly works in the quasi-TM₁₁ mode. Antenna I is larger in size than Antenna II. According to antenna theory, if the radiation aperture of the antenna is larger, while the thickness of the antenna dielectric layer is greater, then the gain of the antenna will be higher. Therefore, compared with Antenna II, Antenna I has a higher gain. Considering the four-metal probe in Antenna 2, the bandwidths of Antennas 1 and 2 are slightly different.

These gains are competitive with those of similar omnidirectional antennas reported in [13,18,19,20,22]. Notably, Antenna II achieves a height of 0.028λ₀, which is significantly lower than that of other comparable omnidirectional antennas [13,19,22]. This low-profile design enhances the integration potential of the antenna in compact systems.

Both Antennas I and II are vertically polarized, making them suitable for deployment in metal grounding environments. This feature is particularly advantageous for applications such as wireless energy harvesting, where vertical polarization is often preferred for optimal performance.

6. Conclusions

This study employed characteristic mode theory (CMT) to analyze and design two distinct omnidirectional antennas for high-performance wireless communication applications. Antenna I, based on a high-order TM₂₂-mode microstrip structure, achieved a peak gain of 8.0 dBi at 2.45 GHz, while Antenna II, utilizing a pin-loaded TM₁₁-mode configuration, demonstrated a gain of 4.55 dBi at the same operating frequency. Both antennas were fed by a 1:4 power divider, ensuring uniform excitation and low pattern ripple. The proposed antennas exhibit several notable advantages. Antenna I offers high gain and a compact profile, making it suitable for applications requiring high directivity and efficient energy transfer. Antenna II, with its low profile (0.028λ₀) and omnidirectional radiation pattern, is particularly well-suited for integration into compact systems and environments with metal grounding. Both antennas are vertically polarized, enhancing their applicability in wireless energy harvesting and other wireless communication scenarios.