Study of Ground Plane Effects on Monopole Antenna Performance

Abstract

With the continuous development of the IoT, compact wireless communication modules have become indispensable components, and their antennas are gradually being developed from external devices into onboard integrated devices. The serpentine antenna, a variant of the monopole antenna known for its small size and easy integration, is often applied to engineering practices. However, its performance has always been closely affected by the size of the surrounding grounding plane. By conducting a characteristic mode analysis (CMA), this study explored the variation patterns in the ground plane size and the resonant frequency. Based on the simulation results, it was clear that when the ground plane size is less than a quarter of the working wavelength, the ground plane will have a significant effect on the antenna’s resonant frequency. Thus, this study further analyzed a serpentine antenna with a grounding branch, and through analysis of the basic law of the influence of grounding structure on the antenna’s performance, we found that by adjusting the branch length, the matching performance of the antenna can be effectively improved. Furthermore, by changing the size of the ground plate, the antenna’s resonant frequency can be adjusted. Such a conclusion will hopefully provide a reference for future designs of integrated antennas in engineering applications.

1. Introduction

As technologies have developed rapidly in recent years, wireless networks have been applied to many fields, including families, enterprises, and public places [1]. Both standards IEEE802.11a [2] and IEEE802.11b [3] take wireless channels as communication media for WLANs while acknowledging their contributions to the wireless transmission of data and video signals [4]. Wireless communication modules are both the fundamental link in IoT application communications, and a key link in IOT application sensing and network layers. They are currently widely applied in vehicle monitoring, telemetry, small wireless networks, wireless meter reading, access control systems, industrial data collection systems, wireless tagging, identification, contactless RF smart cards, fire safety systems, bio-signal acquisition, hydro-meteorological monitoring, robot control, digital audio, digital image trees, etc. As a key device for sending and receiving electromagnetic waves, antennas play a vital role in wireless networks, with their performance directly determining the quality of a wireless communication network [5,6] and attracting significant attention from scholars. Following the miniaturization requirements of wireless communication nodes, antennas are also being developed into small-sized devices that are easy to integrate [5,7]. The constant development of the IoT has prompted greater demand for product miniaturization. Against this backdrop, integrated antennas are currently one of the main developments anticipated to meet the demand for miniaturized and high-performance products. Therefore, the printed monopole antenna has become a main WLAN-related research topic thanks to its significant advantages, including its small size, low cost, and high gains [8,9].

The serpentine antenna is currently the most widely used monopole antenna structure and is found commonly in Bluetooth, Wi-Fi, and Zigbee devices, as well as in other fields. People are attracted to serpentine antennas due to their various advantages, such as their smaller size, lower cost, long durability, and easy whole-machine integration [10]. Although their bending structure benefits miniaturization, their gains may be reduced, causing more losses. Therefore, the optimal way in which to balance the size and gains of these antennas has become a main concern of many designers. A common practice is to use HFSS to design a serpentine monopole antenna structure of a specific area size. However, in terms of the relationship between the ground plane size and the monopole antenna wavelength, only general conclusions have been drawn stating that the impedance, center frequency, and gains of the antenna are impacted by the ground plane size; for example, the ground plane size and shape influence the impedance, resonant frequency, gains, and radiation direction of the antenna [11]. Some articles have also pointed out that as the ground plane size becomes smaller, the lower frequency limit of the antenna rises, resulting in a narrower bandwidth [12]; however, researchers have not yet conducted in-depth analyses of why such influences exist.

Taking the Bluetooth serpentine antenna as an example, this study applied characteristic mode analysis (CMA) to first analyze the grounding plane size’s influence on resonant frequency changes and then analyze its impact on the amplitude of the surface current value. Lastly, HFSS was used to explore the relationship between the ground plane size and the impedance, center frequency, gains, and wavelength of the antenna in depth. Based on such findings, we can conclude that the matching performance of the antenna can be significantly improved by adjusting the length of its grounding branch. Such results will provide practical guidance for antenna designers.

The innovations in the research findings have practical engineering significance. For example, by adjusting the ground plate size, the antenna resonance frequency can be changed, and by adjusting the branch length, both the antenna’s matching performance and bandwidth can be significantly improved. Such results will provide useful references for relevant engineering applications.

2. Effect of the Ground Plane on Monopole Antennas

2.1. Factors That Affect Monopole Antenna Characteristics

Melvin M. Weiner [13] proposed three main parameters that affect the operating monopole antenna’s characteristics, including length, radius, and ground plane. For monopole antennas with determined sizes, the ground plane will be the main influencing factor, mainly because when the antenna works, the feed point incident wave will produce omnidirectional diffraction at the outer edge of the ground plane, and such diffraction will affect its surface wave. The stronger the outer edge diffraction, the larger the surface wave value. With the change in the antenna ground plane, the strength of the outer edge diffraction also varies. The smaller the ground plane is, the stronger the outer edge diffraction will become; conversely, the larger the ground plane, the smaller the outer edge diffraction [14].

Therefore, compared to the infinite ground plane, the finite ground plane will affect the operating characteristics of the antenna, including center frequency, input impedance, and gains, because of its size.

2.2. The Serpentine Monopole Antenna Structure

Early monopole antennas were applied to an upright ground plane that was considered “infinite” and could be used to analyze the antennas’ performance using the principle of the method of images. However, when they are later placed on a finite metal plane (such as a train roof, car roof, airplane surface, etc.) and form a radiation body with the finite metal body, the metal ground plane will generate a vital impact on the antennas’ radiation characteristics (reflection coefficient, radiation pattern, etc.). There are currently two commonly seen serpentine antenna models, including that in Figure 1a, a serpentine antenna with no grounding branches, and that in Figure 1b, a serpentine antenna with grounding branches.

Taking a Bluetooth serpentine antenna with a grounding branch as an example, this study analyzed the effective range of the ground plane of the monopole antenna, using the commercial software HFSS 15.0 [15]. The simulation results showed that the designed antenna met the application requirements of the system [16,17].

2.3. The Serpentine Antenna Model

Figure 2 illustrates the specific structure of the serpentine antenna. In terms of its structure, here, the 1-mm-thick plate medium was made of FR4, and the whole antenna length was between 0.25 λ₀ and 0.25 λg [18] (λ₀ = 30 mm, λg = 15 mm; λ₀ is the free space wavelength, λg is the guided wave wavelength in the dielectric layer; and εr = 4.4). SMA connectors with a characteristic impedance of 50 Ω were used for feeding.

Table 1. Initial dimension parameters of the serpentine antenna (unit: mm).

W	H	D	L1	L2
0.5	7.5	0.9	3.5	6
SY1	SY	SX1	SX	R
9	37.5	11	28	0.15
SX2	L	H2	SY2
11	2	2.5	3

shows the initialization parameters of the serpentine antenna.

Figure 3 shows the reflection coefficient of the antenna S11 = −29 dB; the center frequency point was 2.45 GHz, and the bandwidth was between 2.38 and 2.50 GHz (the Bluetooth antenna bandwidth ranges from 2.402 to 2.482 GHz). The gain in Figure 4 was 1.17 dBi, and the antenna was omnidirectional and met the design requirements.

2.4. CMA of the Serpentine Antenna

This study demonstrated the performance of the CMA of the serpentine antenna with CST. The ground plane SY value on the Y-axis was changed, and the resulting resonant characteristics can be seen in Figure 5a. If its size falls within the range of less than a quarter wavelength, the ground plane will significantly affect the antenna resonant frequency, and when it exceeds the range, the ground plane will then have a gradually decreasing effect on the resonant frequency.

Similarly, the value of SX (set SY value as 37.5 mm) was changed, and the resulting resonant characteristics can be found in Figure 5b. If the SX falls within the range of less than a quarter of the wavelength, the ground plane length will seemingly affect the antenna’s characteristics; if the length is more than a quarter of the wavelength, the effect of the ground plane length change on the antenna’s resonance characteristics shall be small.

According to Figure 5c,d, when the SY and SX values are relatively small, their MS values are also small, meaning that the antenna cannot reach the resonant state.

To summarize, when the length of the ground plane is less than a quarter of the wavelength, it will greatly impact the antenna’s resonant frequency on both the X-axis and Y-axis.

2.5. Serpentine Antenna Surface Current Analysis

With CST, the surface current of the serpentine antenna is analyzed. Figure 6a,b shows the schematic diagrams of the surface current of the serpentine antenna with the variation in the X- and Y-axes. The maximum surface current amplitude that occurs when varying the SY value of the ground plane on the Y-axis is shown in Figure 6c. Within the range of less than a quarter of the wavelength, the influence of the ground plane size on the surface current is large; outside of this range, the influence of the ground plane size on the surface current becomes small, and the amplitude of the surface current trends gradually toward a certain value.

Similarly, as is shown in Figure 6d, if the size falls into the range of less than a quarter of the wavelength, the ground plane will significantly affect the surface current, and if greater than this range, the ground plane will have a small effect.

2.6. The Effective Ground Plane Range of the Serpentine Antenna

The length of copper laying in the ground plane on the Y-axis was varied. The initial value was set to SY = 9 mm; in fact, the length of the ground plane on the Y-axis was zero by then (SY − SY₁ = 0). As is shown in Figure 2, SY represents the whole length of the Y-axis, and SY₁ represents the distance between the starting point of the Y-axis and the ground plane. SY was increased every 3.75 mm on the Y-axis, and the maximum value of SY was taken to be 69 mm. The simulation results can be seen in Figure 7.

When the SY value ranges from [9 mm, 69 mm] (the Y-axis length of the ground plane ranges from [0, 60 mm]), the center frequency of the antenna decreases as SY becomes larger. However, when SY approaches a certain critical value, the center frequency grows with the increase in SY. In Figure 8, the ground plane Y-axis length starts from 0, and the antenna impedance shows the capacitive reactance, which also decreased as the SY length grew. When the ground plane Y-axis length reaches approximately 0.25 λ₀, the capacitive reactance is 0, and the impedance is pure resistance, approximately 50 Ω. After that, the antenna impedance shows inductive reactance, the impedance increases, and the antenna impedance matching becomes worse. Figure 9 shows that the antenna gain also increases with the increase in the SY value, before gradually converging to a constant value.

Additionally, from the HFSS simulation, it is clear that when the Y-axis length of the plane is approximately equal to 0.25 λ₀, the omnidirectional characteristics of the antenna will be excellent, as shown in Figure 10a. When the Y-axis length of the ground plane exceeds 0.5 λ₀, multiple lobes will appear on the radiation pattern, and the antenna gain will decrease, as in Figure 10b.

According to the filtering equation $f_0\propto \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\sqrt{LC}$}\right.$ [19], the capacitive reactance formula ${\times }_c=\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2\pi wc$}\right.$, and the inductive reactance formula $X_l=2\pi wl$, the then L₂ of the antenna is connected to the ground plane. With the ground plane, this may be seen as one radiating body. Changes in the ground plane also lead to changes in the $L$ and $C$ of the antenna. The antenna and ground plane are seen together as a filter, while the center frequency varies with the changes in the $L$ and $C$ values. At first, the impedance of the antenna shows larger capacitive reactance, and capacitance $C$ and capacitive reactance ${\times }_c$ are shown to be inversely proportional; as the absolute value of capacitive reactance decreases, the capacitance $C$ increases gradually. At this time, the center frequency decreases, and impedance shows pure resistance at approximately 0.25 λ₀ on the Y-axis of the ground plane. By then, S11 is the smallest, and the bandwidth is the largest. After that, the impedance encounters inductive reactance and gradually increases, at which time $L$ also increases, while the center frequency $f_0$ decreases. However, due to the pure resistance in the impedance and inductive reactance (which are also increasing), the antenna will generate reflection coefficient, which eventually leads to a gradual increase in S11 and a narrowing of the bandwidth. Later, the antenna shows capacitive reactance and gradually increases; meanwhile, the antenna bandwidth becomes narrow, and the center frequency $f_0$ grows. It is thus clear that an increase in the ground plane size in the direction of the Y-axis will affect the impedance of the entire radiating body of the antenna and ground, and such changes also affect the center frequency and bandwidth. At the same time, the increase in the ground plane’s size will also enlarge the radiation range of the antenna, thus changing the antenna gains and allowing these gains to converge to a certain extreme value before decreasing.

To conclude, the Y-axis length change in the ground plane directly affects the resonant frequency, bandwidth, impedance, and gains of the antenna.

The Y-axis length of the ground plane was set to 0.25 λ₀ (the then impedance of the antenna was the best match), and an increase in the length of the X-axis was simulated; meanwhile, the antenna’s impedance, bandwidth, S11, and gain requirements were comprehensively considered. It is clear that the antenna meets the design requirements only if the X-axis length falls within the range of 0.25 λ₀ to 0.5 λ₀. When the X-axis length becomes larger than 0.5 λ₀, the antenna shows multiple lobes. In Figure 11a, SX = 30 mm, and the X-axis length is approximately 0.25 λ₀; in Figure 11b, SX = 60 mm and the X-axis length is approximately 0.5 λ₀. The gains of both antennas are larger than 1 dBi. In Figure 11c, SX = 80 mm and the antenna appears to have multiple lobes.

In Figure 12, the range of SX is within 0.25 λ₀ and 0.5 λ₀, and all impedances meet the design requirements. When the SX value is between 0.25 λ₀ and 0.5 λ₀, the gain constantly decreases, and when SX exceeds 0.5 λ₀, the gain shows an upward trend (as shown in Figure 13).

Based on the simulation analysis of the length of the ground plane of the serpentine antenna along the X-axis and Y-axis, it can be concluded that the antenna’s ground plane size has a critical effect on the antenna’s characteristics, including the reflection coefficient, gains, impedance, and bandwidth. Only when the ground plane is within the effective range can the antenna demonstrate optimal performance. Beyond the effective range, the antenna will not work properly.

2.7. The Influence of the Grounding Branches of the Serpentine Antenna on the Antenna’s Performance

As is shown in Figure 2, the antenna branch L₂ is connected to the ground plane through an area consisting of both SX₂ and SY₂. In Figure 14, it can be seen that variation in the length of the parametric antenna branch L₂ mainly affects the impedance matching characteristics of the antenna. Figure 15 shows the relationship between variation in the length of parameter SX₂ and the reflection coefficient, thereby proving that by changing the size of SX₂, the impedance matching of the antenna can be changed. Figure 16 shows the relationship between changes in parameter SY₂ and the reflection coefficient, proving that changing the size of SY₂ can effectively adjust the resonant frequency of the antenna. Later, with the increase in SY₂, the location plane gradually approaches the serpentine antenna and the coupling effect becomes even stronger, eventually leading to the antenna’s gains becoming smaller and the directionality worse, as shown in Figure 17.

Based on the above discussion, the antenna branch length L₂ and grounding length SX₂ have been proven to be the two main factors affecting the antenna’s impedance matching, while the length change in SY₂ is the main factor affecting the antenna’s resonance frequency.

3. Physical Test of the Antenna

According to the simulation parameters of the simulation software, the aforementioned Bluetooth serpentine antenna was processed with PCB technology, and the model was tested using a multi-probe test system. A piece of 0.05-mm-thick double-sided copper foil was used to change the dimensions of the metal ground. During the design test, the variation in the metal ground was simulated by pasting lengths of copper foil on the surface of the dielectric plate. The design length of the copper foil laying in the Y-axis direction was 7.5 mm. The length of the PCB was 70 mm, and its width was 28 mm. The parameters are shown in

Table 2. Length of the ground plane.

PCB board no.	1	2	3	4	5	6	7	8
Ground plane length SYY/(mm)	7.5	11.25	15	18.75	22.5	26.25	30	33.75
PCB board no.	9	10	11	12	13	14	15
Ground plane length SYY/(mm)	37.5	41.25	45	48.75	52.5	56.25	60

.

A 1.8-mm-diameter IPEX to SMA coaxial cable (RG178) with 50 Ω impedance was selected. The core was welded to the feed point of the Bluetooth serpentine antenna, and the outer surface of the shielding layer was welded to the PCB ground plane. To ensure solid contact, each PCB board and copper foil connection was soldered and tested with a multimeter to ensure that the connection resistance value was close to zero. The effects can be seen in Figure 18 (PCB board no. 1, 4, and 12, from left to right).

Part of the results of the laboratory tests using the network analysis instrument (model Agilent E8363B) and the corresponding simulation parameters are shown in Figure 19; they are shown to stay near the central resonant frequency point of 2.45 GHz. This proves that different ground planes can lead to different S11 performances. A ground either too large or too small will affect the presentation of the best S11 value, thus affecting the antenna’s performance. Our results showed that with the increase in the Y-axis length SY of the ground plane, the frequency of the antenna started to drop, after which the frequency of the antenna rose again with the increase in SY. The actual test parameter (S11) data curve deviated slightly from the simulation results, but the overall trend in its center frequency was consistent with the previous simulation results. Due to the better matching performance of the coaxial adapter cable used, its measured S11 value was also smaller.

The antenna’s characteristics were tested in the microwave laboratory, as shown in Figure 20a,b. Figure 21 shows a comparison between the maximum gain value simulated by HFSS and the experimental test value; the tested gain value was seemingly larger than the simulated value. Therefore, the parameter of coaxial cable was added to the simulation, and the results showed that the resulting gain value was larger than the previous simulated value. Additionally, experiments in which the metal ground plane below the coaxial line was wrapped in wave-absorbing materials have been performed, meaning that, theoretically, the coaxial cable would no longer radiate the electric field. The measured results are shown in Figure 21 (the measured curve denoted as using wave-absorbing materials); the antenna’s gain value was less than the simulation value. Therefore, we noted that the antenna’s gain due to the electric field radiated by the coaxial cable was greater than the simulation result [20].

Figure 22a–c represents the 2D patterns of the simulated and measured antenna results (SYY = 30 mm). The simulated and measured results were basically the same in trends, and the differences between them were caused by the coaxial cables.

Figure 23 shows the simulated and measured antenna radiation efficiency graph (SYY = 30 mm) at a center frequency of approximately 2.45 GHz. The measured efficiency of the antenna was closest to the simulation result. As the serpentine antenna has a narrow bandwidth, and in the actual test, the antenna transmission line and the dielectric plate would incur a great loss to the antenna, the measured antenna efficiency attenuation was higher than that in the simulation.

This study provides a new approach to analyze the effect of the ground plane. Firstly, CMA analysis was used to analyze the variation law of the ground plane size and resonant frequency. This study further analyzed the serpentine antenna with a grounded branch, and reached the conclusion that the matching performance of the antenna can be effectively improved by adjusting the length of the branch, and the resonant frequency can be adjusted by changing the size of the ground plane. This method has practical implications for antenna designers, especially when applied to PCB board antennas, effectively saving designing time.

Table 3. Comparison of the previous literature along with proposed work.

Ref	Antenna Type	Antenna Size	Gain (dBi)	Radiation Efficiency (%)	Innovation Points
[21]	Monopole antennas	Diameter = 30 mm	/	/	Spiral ground plane design
[22]	Elliptical planar monopole antennas	R1 = 12 mm R2 = 9 mm	/	90	The effects of the dimensions of the rectangular ground plane
[23]	Multiband monopole antenna	20 × 18.6 mm	2.0	/	The ground plane influence has been minimized by adjusting the feed line
[24]	Printed multiband monopole antenna	6 × 4 mm	6.4	85	An air gap was set between the proposed antenna and system ground
Proposed work	The serpentine antenna	10.9 × 7.5 mm	1.17	75	The impact of ground plane size by CMA analysis, and the effects of the branch length.

shows the performance of the antenna designed in this study, compared with that in similar studies.

4. Conclusions

To summarize, the ground plane of the serpentine antenna, as part of the antenna, has a huge influence on its performance; it cannot simply be regarded as a monopole antenna device, but as a variation of the monopole antenna. Based on such a hypothesis, this study, taking the serpentine antenna as an example, applied the theory that the ground plane and the antenna will together constitute the radiating body; it also adopted the CMA to first analyze the relationship between the size of the ground plane of the serpentine antenna and its resonant frequency, followed by a discussion of the relationship between the size of the ground plane of the serpentine antenna and its surface currents. Finally, this study used HFSS to discuss the effects of ground plane size and grounding branch length on the antenna’s characteristics in detail, while also verifying simulation results through processing tests. These research results have critical and real engineering significance; we found, for example, that by changing the ground plane size, the antenna’s resonance frequency can be adjusted, and by changing the branch length of the antenna, the antenna’s matching performance and bandwidth can be effectively improved. Such findings will hopefully provide a reference for future engineering designs.