Design of a Wideband Loaded Sleeve Monopole Embedded with Filtering High–Low Impedance Structure

Abstract

In this paper, a compact wideband filtering monopole is presented for remote terrestrial omnidirectional communication systems. The presented antenna features a sleeve monopole structure integrating with two key components: the lumped parallel RLC circuits and an embedded high–low impedance structure within the sleeve section. The integrated high–low impedance structure enables the monopole to achieve excellent filtering characteristics while maintaining the monopole compactly. Meanwhile, the combination of the RLC loads and the sleeve monopole ensures wideband omnidirectional radiation performance. To validate the design, a prototype operating from 200 to 1500 MHz is fabricated and tested. The measurement results demonstrate that the monopole achieves a VSWR below 3 across the entire operating band and a measured gain exceeding 0 dB. Furthermore, the monopole exhibits satisfactory out-of-band rejection from 1700 to 4000 MHz, confirming its effective filtering capability.

1. Introduction

The increasing demand for high-quality terrestrial wireless omnidirectional communication has placed stringent requirements on antenna systems, including wideband operation, compact integration, and improved anti-interference capabilities [1,2,3,4]. A wideband monopole that can cover multiple frequency bands would significantly simplify multiband omnidirectional communication platforms by eliminating the need for multiple narrowband antennas. Moreover, the development of monopoles with large frequency ratios has become a critical trend to meet the growing demand for communication capacity in future systems [5,6,7]. As wireless communication technologies rapidly advance, the integration of RF front-end components has become a key focus in modern system design. However, conventional monopoles typically serve as basic electromagnetic energy converters, facilitating the transition between guided wave structures and free space. To enhance their functionality, incorporating filtering characteristics directly into monopoles presents a promising solution. This approach has already proven effective in other antenna designs, such as patch antennas [8,9,10]. By integrating filtering capabilities, monopoles can not only improve the electromagnetic immunity of communication systems but also contribute to overall size and weight reduction in terrestrial omnidirectional communication platforms [11,12,13].

Ordinary monopole, with its simple linear structure, is typically considered an open-end transmission line, exhibiting classic resonance characteristics. A standard quarter-wavelength monopole offers only about ten percent relative bandwidth for a VSWR < 2 near its resonant frequency [14,15], which is suitable primarily for narrowband applications. To meet the demands of wideband terrestrial wireless omnidirectional communication, it is essential to address the inherent resonant behavior of the conventional monopole. In wideband antenna design, both radiation and impedance bandwidths must be considered simultaneously to create a practical wideband front-end device for modern communication systems. To enhance the radiation bandwidth of the monopole, one effective approach is to load it with lumped elements. This technique alters the current distribution along the monopole, enabling wideband radiation characteristics while maintaining the monopole’s linear shape. By employing various loading strategies [16,17,18,19], the relative bandwidth can be expanded by several thousand percent. To improve the impedance bandwidth, the use of hat or sleeve structures proves to be a promising solution, especially for electrically small monopoles. Novel variations in these hat or sleeve structures have become crucial in advancing wideband antenna design [20,21,22,23,24,25].

Traditional monopole designs primarily focus on improving radiation characteristics, such as broadening radiation bandwidth or enhancing radiation efficiency [26]. However, these conventional monopoles lack filtering capabilities. Filtering technology plays a crucial role in suppressing interference and purifying the spectrum in the electromagnetic signal transceiver link, making it a key functional module in wireless communication systems. The evolution of filtering technology has consistently driven improvements in communication system performance. In traditional communication architectures, the filter and antenna are designed separately. The electrical connections between the antenna and filter relies on transmission lines. This architecture results in additional loss in signal receiving path. In addition, the combination of lossy transmission lines and filters increases the communication system size, weight, and cost.

To meet the growing demand for the integration of RF front-end devices, incorporating filtering functionality directly into monopole presents a promising solution for terrestrial wireless omnidirectional communication. The design of filtering monopoles can draw inspiration from other filtering antenna designs. One straightforward approach is to introduce a filtering structure at the antenna port, utilizing established filter synthesis methods to create controllable passbands and stopbands [27,28,29]. Although this method allows monopole to possess filtering capabilities, filtering and radiation structures remain relatively independent, resulting in a design that is not compact enough. To further reduce size and improve functional integration, fusion design approach can be used, where the antenna structure itself is leveraged to implement filtering functionality more effectively. In the fusion design method, eigenmode analysis of antenna is used to adjust radiation structure, thereby integrating filtering characteristics into antenna design [30,31,32]. While this method is easily applied to patch antennas or cavity-based structures, where having at least two dimensions facilitates mode distribution adjustments, it is more challenging for monopole designs, especially for whip monopoles that use wire or thin tube as a radiator. The linear shape of monopoles limits the ability to adjust mode distribution, complicating the integration of filtering functionality in filtering monopole design.

To achieve both filtering and wideband characteristics in a compact monopole design, this paper presents a novel wideband monopole that integrates filtering functionality using a structure-reuse technique. This approach combines sleeve and high–low impedance structures to achieve the desired filtering effect without compromising compactness. Section 2 details the design of the presented monopole, which starts with a simple linear monopole and gradually incorporates the sleeve structure, high–low impedance structure, and distributed lumped loads with minimal size expansion. Simulation results demonstrate that the presented monopole achieves both wideband and filtering characteristics in a compact form. In Section 3, a prototype of the designed monopole is fabricated for experimental validation. The measured results closely match the simulation, confirming the accuracy of the design. Section 4 compares the performance of the proposed monopole with other similar wideband monopole designs, showing that the presented monopole excels in both wideband performance and filtering capability. Section 5 concludes the paper, highlighting the potential of the presented monopole design as a promising solution for future terrestrial wireless omnidirectional systems.

2. Antenna Design and Configuration

2.1. Antenna Configuration

The overall design scheme of the presented monopole is illustrated in Figure 1. The initial structure is a familiar whip monopole with narrowband characteristics. To broaden its bandwidth without significantly increasing its size, a sleeve structure is introduced. The lower part of the initial monopole is converted into the inner conductor of the sleeve structure, while the outer conductor of the sleeve becomes a part of the radiator for the monopole. This modification creates a transmission line formed by the lower part of the sleeve monopole and the inner side of the sleeve. As a result, the feed point of the sleeve monopole is virtually elevated to the top of the sleeve, as illustrated in Figure 1. When the electrical length of the sleeve monopole changes from λ/4 to λ/2, the current at this virtual feed point experiences only slight variation. This behavior differs from that of the ordinary monopole, where the current magnitude at the physical feed point shifts significantly from a peak to a trough as the electrical length changes from λ/4 to λ/2. Consequently, the input impedance of the sleeve monopole remains relatively stable across a wider bandwidth, contributing to its enhanced broadband performance.

To incorporate filtering functionality, the lower part of the initial monopole inside the sleeve is replaced with a multistage metal structure as illustrated in Figure 1. In this configuration, the high-impedance transmission line is electrically equivalent to a series inductor, because thin inner conductor effectively increases inductive reactance in coaxial transmission line. The low-impedance transmission line corresponds to a parallel capacitor, because wide inner conductor effectively increases the capacitor between inner conductor and the inner side of the sleeve. A ladder network composed of alternating series inductors and parallel capacitors naturally exhibits a low-pass filtering characteristic for frequency-varying signals. Thus, this multistage structure, along with the inner conductor, forms a high–low impedance transmission line that provides a necessary filtering function for filtering monopole. Importantly, the introduction of the filtering function does not result in any significant size increase, as the filtering capability is achieved by simply modifying the inner conductor of the sleeve structure. As a result, the presented monopole retains its compact size while incorporating the filtering function.

After designing the filtering structure, the upper part of the monopole remains an ordinary whip radiator. The electrical length of an ordinary monopole increases with frequency, and as the structure becomes electrically large, reverse currents can occur. This leads to a degradation in omnidirectional radiation performance and results in a narrow radiation bandwidth. To enhance the monopole’s radiation bandwidth, parallel RLC circuits are introduced along the radiator. A parallel RLC load exhibits a trap effect at its resonant frequency, presenting very high impedance that effectively isolates the upper portion of the monopole from the lower portion. This creates a shorter effective electrical length at higher frequencies. As a result, the loaded monopole can maintain stable omnidirectional radiation across a wide frequency range. However, due to the non-negligible mutual coupling between adjacent radiating segments separated by the loads, careful tuning of both the resonant frequencies and positions of the RLC loads is required to achieve the desired performance. For the presented monopole, by optimizing the size of the embedded high–low impedance transmission line in the sleeve, as well as the values and positions of the lumped elements, both acceptable filtering and radiation characteristics are achieved. The detailed structure of the proposed wideband filtering monopole is shown in Figure 2. The monopole is fed via a standard 50 ohm coaxial port, similar to typical monopole feed networks.

For applying in remote terrestrial wireless omnidirectional communication, a working band ranging from 200 MHz to 1500 MHz is designed. To achieve a low VSWR around 200 MHz, the total length H1 of the monopole, as shown in Figure 2, is selected to be 30 cm. The sleeve, made from a hollow cylinder with a diameter D2 = 5 cm, encloses the lower part of the radiator and has a length H2 = 10 cm. The bottom of the sleeve is grounded. To reject the widely used 2450 MHz band and the 5G communication bands in the 3000–4000 MHz range above the working band, the monopole is designed with a low-pass filtering characteristic. Inside the sleeve, a multistage structure is designed to function as a high–low impedance transmission line with the required low-pass filtering characteristics. The specifics of this high–low impedance transmission line are discussed in Section 2.2. The upper radiator, which resembles a traditional monopole, is constructed from a hollow tube with a diameter D₁ = 1.5 cm. To further broaden the monopole’s bandwidth, three lumped circuits are loaded onto the radiator. These lumped circuits consist of parallel RLC elements that adjust the current distribution on the radiator, resulting in wideband omnidirectional radiation characteristics. The details of these distributed lumped loads are addressed in Section 2.3.

2.2. Design of Filtering Structure

The design of the high–low impedance transmission line begins with a fifth-order Chebyshev low-pass circuit, as illustrated in Figure 3a, which provides a rejection characteristic of below −20 dB from 1700 MHz to 4000 MHz. By applying Richards’s transformation and Kuroda’s identities [33], the low-pass circuit in Figure 3a is converted into a modified impedance transmission line, as shown in Figure 3b. To realize the low-pass circuit from Figure 3b in the sleeve structure shown in Figure 2, the high–low impedance transmission line is integrated into the sleeve as depicted in Figure 4a. In this design, the inner surface of the sleeve acts as the outer conductor of the high–low impedance transmission line.

As the filtering structure is located at the lower part of the monopole, it also functions as a support for the upper part of the monopole. Although the high–low impedance transmission line is made of metal, the high-impedance portion is too thin to provide adequate support for the upper monopole. While Teflon supports could be used to stabilize the structure, incorporating such supports would complicate the overall design of the filtering structure. To address the support issue, the high–low impedance transmission line is designed using a two-sided patch structure, as shown in Figure 4b. The dielectric substrate serves as a support for both the high–low impedance structure and the upper part of the monopole.

The detailed architecture of the high–low impedance transmission line, based on a substrate embedded in the sleeve, is illustrated in Figure 5. Two copper patches are etched onto a Teflon substrate with a relative permittivity of 2.1, and these patches function as the inner conductor of the high–low impedance transmission line. The patches are interconnected using multiple via holes, each with a radius of 1.5 mm, to replicate the filtering structure shown in Figure 4a. The width of the substrate W1 is 48 mm, which matches the inner diameter of the sleeve. The width of the etched low-impedance section W2 is 47 mm, slightly smaller than W1. This difference allows the substrate to support the inner conductor of the transmission line. The remaining dimensions, as depicted in Figure 5, are derived from Richards’s transformation [33] and calculated based on the low-pass filtering circuit in Figure 3b. The length of each low-impedance section is optimized to compensate for parasitic capacitance effects.

As shown in Figure 6, the optimized high–low impedance transmission line based on the substrate successfully achieves the desired low-pass characteristics, with a cutoff frequency around 1500 MHz and a stopband with S21 below −20 dB from 1700 MHz to 4000 MHz. Although the low-impedance line tends to resonate around 4000 MHz, causing a slight degradation in rejection from 3000 MHz to 4000 MHz, the rejection remains below −20 dB in this range, meeting the specified rejection requirements.

2.3. Wideband Monopole

Once the filtering structure is designed, the high–low impedance structure based on the substrate is integrated into the sleeve, as shown in Figure 2. To achieve satisfactory radiation performance, three lumped loads are strategically placed on the monopole, as depicted in the same figure. Each lumped load consists of a parallel RLC circuit. The monopole is simulated using the Method of Moments (MoM) to model its electrical characteristics. The positions of the loads and the values of the components in each load are optimized to achieve the desired electrical performance. The optimized parameters of the loads are summarized in

Table 1. Parameter of optimized load of the lower monopole.

Load Position (mm)	Resistance (ohm)	Inductance (nH)	Capacitance (pF)
178	100	25	3
225	1600	8.5	1
265	200	10	1.2

, with the load positions measured from the ground. These optimized values are practical and easily implemented in real-world designs.

The optimized monopole demonstrates an omnidirectional gain greater than 0 dB across the relevant frequency range, as shown in Figure 7. For comparison, the gain of an ordinary monopole and that of an ordinary sleeve monopole are also plotted in the same figure. Both of these monopoles exhibit narrower bandwidths due to the occurrence of a zero gain in the omnidirectional pattern. In contrast, the presented monopole avoids this issue thanks to the proper design of its lumped loads.

Figure 8 and Figure 9 show the E-plane and H-plane radiation patterns of the optimized monopole at several typical frequencies within the operating band. The monopole exhibits relatively uniform radiation characteristics in the omnidirectional pattern throughout the band. The radiation efficiency of the designed monopole is above 40% across the working band, as illustrated in Figure 10, which is acceptable for wideband applications. As shown in Figure 11, the optimized monopole achieves a VSWR of less than 3, which is an acceptable port characteristic for wideband operation.

3. Experiment and Result Analysis

A fabricated prototype of the designed monopole is shown in Figure 12, with the sleeve separated for clarity. The monopole is fed through a standard 50 ohm N-connector. The experimental setup was conducted in an outdoor open half-space environment, as shown in Figure 13. Figure 13a shows the presented monopole under test; Figure 13b shows the transmitting terminal; Figure 13c displays the test data collecting and control platform. To perform the simulation, the presented monopole is tested in an outdoor half-plane environment, rather than an anechoic chamber, to approximate a free space environment. A circle metal plane is placed below the fabricated monopole, providing a good near-field connection to the ground. The grass plane in the test environment is flat between transmitting and receiving antennas, providing an acceptable half-plane environment for the experiment. The transmitting and receiving antennas were positioned 20 m apart to measure the far-field performance at 200 MHz, based on the 10λ criterion for far-field assessment.

The measured and simulated VSWR of the presented monopole are compared in Figure 14. The results demonstrate that the monopole effectively covers a wide frequency range of 200–1500 MHz with a VSWR of less than 3. The agreement between the simulation and measurement is generally good, with any discrepancies likely arising from the practical fabrication of the load circuits, which may differ slightly in spatial arrangement compared to the point load used in the simulation. The measured gain and filtering characteristics of the monopole are shown in Figure 15, with satisfactory agreement between the measurement and simulation. The figure also compares the gains of an ordinary monopole and a standard sleeve monopole of the same size. The presented monopole not only offers a broader bandwidth but also demonstrates superior filtering performance, with a stopband below −20 dB from 1700 MHz to 4000 MHz.

4. Discussion

Table 2. Comparison of presented work with other works.

Ref.	Electrical Length (λmax) 1	Bandwidth (MHz)	Gain (dB)	Filtering Character
[34]	0.18	100–550	−4	No
[29]	1.8	5835–6100	5.5	Yes
[4]	0.3	900–5200	0.8	No
[36]	0.68	2500–3500	2.5	Yes
[35]	0.32	700–1700	0	No
[37]	0.25	3600–4600	5	Yes
This work	0.2	200–1500	0	Yes

¹ The electrical length of each monopole is its physical length normalized by the wavelength λ_max at its lowest operating frequency.

shows the performance comparison between the presented monopole and other reference monopoles, including their electrical length, bandwidth, gain and filtering character. Refs. [4,34,35] presented some designs with wideband characteristics, but their gains are lower. In addition, there are no filtering characteristics in those designs. In Refs. [29,36,37], the presented monopoles are all designed with filtering characteristics and higher gain, but they can only be used for narrow band application. In sum, wideband and filtering characteristics are achieved in the presented monopole, and the structure is simple and compact in size.

5. Conclusions

A wideband filtering-based loaded sleeve monopole is presented in this paper. By incorporating a high–low impedance structure in place of the inner conductor of the sleeve, filtering functionality is achieved without introducing additional components. This design effectively utilizes structure reuse, resulting in a compact form. The lumped load technique is also employed to further enhance the radiation bandwidth. The monopole was tested across a frequency range of 200 MHz to 4000 MHz, demonstrating stable gain above 0 dB in the passband from 200 MHz to 1500 MHz, along with a satisfactory stopband below −20 dB from 1700 MHz to 4000 MHz. With its combination of wide bandwidth, filtering capabilities, omnidirectional radiation, and compact size, this monopole is an excellent candidate for vehicle-based terrestrial wireless omnidirectional communication systems.