Unit Cell Optimization of Groove Gap Waveguide for High Bandwidth Microwave Applications

Abstract

Recently, groove gap waveguides (GGWs) have shown significant potential in power handling and bandwidth enhancement compared to conventional waveguides. In this research work, we designed and developed an innovative mushroom-unit-cell-based groove gap waveguide (MGGW) that has shown improved bandwidth compared to conventional GGW structures. The dispersion characteristics of the MGGW were analyzed through the eigenmode solver feature of Microwave Studio CST, which showed that the bandwidth was improved by 8% compared to conventional unit cells in the microwave spectrum. To validate our proposed method for the physical dimensions of unit cell structures, we developed an MGGW structure for the S band, which shows similar trends aligning with the simulation results. The measurement results are promising as a reflection coefficient of less than −20 dB was achieved over the entire band for the WR284 Electronic Industries Alliance (EIA) standard waveguide adapter. The proposed MGGW structure with improved bandwidth will open new doors for researchers to develop ultra-wide bandwidth microwave applications, i.e., filters, transmission lines, resonators, attenuators, etc.

1. Introduction

Waveguides are essential components of a microwave system. They are used for transferring energy to and from the components of a microwave system. To fulfill the demands of high data rates in modern microwave systems, waveguides must be able to handle wider bandwidths and more power [1]. The most common waveguide design that has been extensively studied and investigated is the rectangular waveguide (RWG) because it offers several advantages over conventional transmission channels, including high power, minimal attenuation, a simple structure, and ease of installation. However, the waveguide is unable to provide sufficient bandwidth for modern microwave applications [2,3]. Therefore, researchers have proposed various designs to improve the bandwidth while addressing the requirements of intended applications, such as wireless communication and satellite communication [3]. Several types of waveguides being investigated for microwave applications include coaxial lines, rectangular waveguides, microstrip structures, and substrate-integrated waveguides. Each design has its own set of benefits and limitations. Among these, coaxial lines provide better performance at low frequencies; however, in the microwave spectrum, due to reduced dimensions, power handling capability is limited, and losses caused by the skin effect in the inner conductor and outer dielectric are significant. Because of their pure metallic construction and greater power handling capabilities, rectangular waveguides are considered to perform better in the microwave range. However, rectangular waveguides’ primary drawback is the expensive and challenging task of creating accurate metal junctions for smaller dimensions in the microwave spectrum to prevent leaks. Microstrip lines have a simple construction and are made of a ground plate, a dielectric substrate, and a metal strip which is typically utilized in inexpensive printed circuit boards. Significant losses result as the substrate’s dielectric constant decreases and approaches that of air in the microwave spectrum. Moreover, these waveguides are susceptible to crosstalk and have the potential to radiate parasitically [4]. Substrate-integrated waveguides have the combined benefits of both a microstrip and rectangular waveguides. They are compact, easy to integrate, and have more power-handling capability. However, their performance is significantly affected in the microwave spectrum due to dielectric losses [5]. To overcome the challenges faced by various types of transmission waveguides, gap waveguides are considered as one of the most promising candidates in the microwave spectrum.

A gap waveguide is an engineered, rectangular structure for the enhancement of the bandwidth [6,7]. A groove gap waveguide (GGW) confines energy in a rectangular propagation channel using a periodic structure of square metal pins. This structure creates an electromagnetic bandgap effect on both sides of the channel, ensuring efficient wave propagation without the need for metal contact between the lid and the main body. This effect restricts certain frequency components from propagating, effectively enhancing the bandwidth of gap waveguides compared to RWGs. The formation of a waveguide between two parallel metal plates with a small gap between them is known as a gap waveguide phenomenon. A significant advantage of the gap waveguide, compared to traditional waveguides, is its two independent metallic pieces, which can be combined to form a waveguide structure. Therefore, it eliminates metal-to-metal contact issues, leading to better performance and remaining completely invariant to the re-assembling process. Gap waveguides’ design eliminates the need for metal contacts between two parallel metal plates, relying solely on sidewalls to support the upper metal plate and maintain it at a sufficient, consistent height above the bottom plate. Consequently, the manufacturing of gap waveguides is regarded as more beneficial compared to the manufacturing of traditional waveguides in any frequency spectrum for desired applications. Additionally, the employment of solely metallic plates and patterned surfaces makes gap waveguides superior compared to microstrip or coplanar transmission lines (CPWs), which require the use of dielectric materials. The electromagnetic bandgap (EBG) effect, modular fabrication, and absence of metal contact make gap waveguides an attractive choice for achieving a wider bandwidth compared to traditional RWGs.

Currently, gap waveguides are frequently researched for commercial utilization for a wide range of products, including transducers, the packaging of components, transitions, filters, couplers, antennas, and transmission waveguides [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]. Figure 1 shows a sketch of three different gap waveguide setups to illustrate the gap waveguide idea. In the literature, various periodic configurations have been reported to develop EBG structures for gap waveguides, e.g., mushroom EBG configurations for microstrip lines, spring-like arrays, patterned zigzag conductors, periodically arranged conical pins, and structures with an inverted pyramid geometry [27,28,29,30,31,32,33]. Among these EBG configurations, the bed-of-nails structure is the most common and widely researched structure due to its ease of fabrication and its wider bandwidth applications. The periodic structure is augmented with a complete design of the desired model, including the antenna, feed network, filter, divider, etc. For a specific user application, the bandwidth of a completed structure is reduced significantly when multiple passive components like the feed network, filters, splitters or dividers, and antenna are designed simultaneously for a completed structure [21,24,25,32,34,35]. Unit cells with wider bandwidths are always considered effective for the design of any passive component or complete structure. To attain a wider bandwidth, it is important that the air gap separating the top metallic plate from the EBG structure is very small [36]. In conventional fabrication methods, the entire length of the design demands accurate surface finishing of both the upper metallic plate and the EBG pattern, which is an expensive and time-consuming operation. Additionally, the design also requires a customized feed to achieve a wide bandwidth [21,25,35].

In this research, we propose a new mushroom-type GGW configuration that significantly improves the bandwidth of the gap waveguide design for microwave applications. The design of the GGW was simulated in the eigenmode solver of Microwave Studio CST. The results from the simulations indicate that the bandwidth was improved by 8% compared to traditional unit cells in the microwave spectrum. To validate our proposed method for the physical dimensions of the unit cell structure, we practically developed an MGGW structure for the S band which shows similar trends to those of the simulations. The measured results are promising, with a reflection coefficient of less than −20 dB attained over the whole range of the Rs-261-B [37] standard waveguide adopter. The proposed MGGW structure will provide new opportunities for researchers to develop ultra-wide bandwidth microwave applications, i.e., filters, transmission lines, resonators, attenuators, etc. The research presented in the subsequent paragraphs will follow the following sequence: the design of the new unit cell, MGGW design for the microwave spectrum, an evaluation of existing designs based on the bed-of-nails unit cell concept, the fabrication of the design, and an analysis of the MGGW prototype’s results.

2. Materials and Methods

2.1. Design of Unit Cell

The design of the proposed unit cell structure was simulated in Microwave Studio CST. The goal was to achieve a wide bandwidth of transmission lines using simple periodic structures. To achieve this objective, the bed-of-nails unit cell structure was thoroughly studied and redesigned to improve the bandwidth of the EBG structure. The nail structure was modified in the shape of a mushroom-unit-cell-based EBG design. In the GGW design, dielectric losses do not exist because of the pure metallic structure, and a mushroom-shaped design provides more bandwidth along with greater air gap independence. For a unit cell design, the physical dimensions of the standard conventional rectangular waveguide WR284 EIA standard were taken as a reference. The selection of the WR284 waveguide provided a flexible input option for the testing of the proposed waveguide.

To analyze the distinctive performance of the proposed new unit cell, a groove configuration of the gap waveguide was selected. The EBG structure was created on a lower metallic plate, and an upper metal plate was aligned parallel and placed at a right angle to the periodic structure. This model is termed a mushroom-unit-cell-based groove gap waveguide (MGGW). For the modeling of the proposed design, the width and height of the WR284 rectangular waveguide adapter were used as a reference because we needed to practically connect it for testing. Its operating frequency range is specified in

Table 1. Standard working parameters and constraints of WR284.

Cut-Off Frequency of Lowest Mode (GHz)	Cut-Off Frequency of Upper Mode (GHz)	Recommended Frequency Band (GHz)
2.078	4.156	2.6~3.95

.

To develop an innovative unit cell for groove gap waveguides, dispersion properties were explored through the eigenmode solver of Microwave Studio CST. The dispersion diagram provides details about the cut-off band [38]. To begin with, the bed-of-nails EBG structure was analyzed using the same physical dimensions (width and height) as WR284, and its dispersion characteristics were parametrically optimized with a consistent air gap maintained between the upper metal plate and the top surface of the EBG structure. An analysis of the dispersion curve depicts that maximum optimized bandgap is achieved in two instances: first, when the air gap is selected as 4.036 mm between two surfaces, and second, when the air gap is selected ≤0.05 mm. However, to create such a narrow air gap precisely across the entire design is a challenging task as it requires a costly and expensive fabrication process. The dimensions of the bed-of-nails unit cell periodic EBG structure that were studied and evaluated are shown in Figure 2a. The dispersion characteristics of periodic configuration based on a bed-of-nails unit cell are depicted in Figure 2b. The first two dominant propagating modes, mode 1 and mode 2, experience a bandgap of 2.309 GHz from 1.91533 GHz to 4.2251 GHz. The energy will propagate along the periodic cells for the observed bandgap and will not be dispersed through pins.

To increase the bandwidth of a unit cell, the dispersion properties of various configurations were analyzed. In this research, a shape analogous to a mushroom-type EBG structure was employed to redesign a bed-of-nails unit cell for microwave frequency applications. In the literature, mushroom-type structures have been explored for low-frequency applications [39,40], utilizing printed circuit boards (microstrip) only. We modeled mushroom unit cells purely with metal structures for microwave applications. To model the new design of the unit cell, a parametric refinement was conducted on the proposed unit cell, incorporating a large air gap (4.036 mm) and minimizing the mushroom head to a very small dimension in relation to the unit cell’s entire length, in contrast to the mushroom structure that is employed for microstrip lines, in which size of the mushroom head is comparatively very large in relation to its tail that joins the ground surface. Thus, the size ratio between the head and tail of the proposed mushroom-like unit cell is easy to fabricate with good physical strength for commonly used metals like copper, aluminum, and brass, and it can perform equally well for high-power applications.

In the proposed mushroom-like EBG unit cell, a mushroom head is modeled additionally with a bed-of-nails unit cell. The dimensions of the mushroom head mirror each other precisely on both sides of a straight pin. The bandgap, in the lower cut-off frequency region, is increased by adding a small head to the straight pin. Figure 3a illustrates the physical dimensions of the proposed unit cell, while Figure 3b depicts its attained dispersion characteristics.

The bandwidth enhancement was analyzed from dominant propagating modes in dispersion characteristics. A broader bandwidth was exhibited by the mushroom unit cell’s flattened dispersion curve that was achieved by slowing down the waves’ group velocity through the engineering of the unit cell structure. The compression of the dispersion curve was accomplished by adding more resonances and couplings. Consequently, the waveguide’s bandwidth increased, permitting it to accommodate a greater range of frequencies with reduced attenuation and distortion of the signal. The dispersion results indicate that the proposed unit cell has a considerably large bandgap as compared to the prior bed-of-nails unit cell. The bandgap results are summarized in

Table 2. Dispersion profile comparison of unit cells with an air gap width (g1) of 4.036 mm.

Category of Unit Cell	Bandgap between the First and Second Propagation Modes
Bed-of-nails-based EBG	First mode cut-off frequency = 1.91533 GHz Second mode cut-off frequency = 4.2251 GHz Bandgap = 2.309 GHz
Proposed mushroom-unit-cell-based EBG	First mode cut-off frequency = 1.6049 GHz Second mode cut-off frequency = 4.1056 GHz Bandgap = 2.50 GHz A significant increase of 191 MHz (8%) is achieved for the proposed new unit cell

.

From the results, it can be observed that the proposed mushroom-unit-cell-based EBG design provides 191 MHz extra bandwidth in comparison to the bed-of-nails-based EBG structure. The proposed unit cell design results suggest that it is suitable for wide-band microwave applications as it significantly increases the bandwidth. Additionally, it provides ease of fabrication for the proposed design as it can be fabricated using conventional milling tools.

2.2. Design of Mushroom Groove Gap Waveguide (MGGW)

After the refinement of the unit cell parameters, the design of the MGGW was modeled, targeting S-band frequencies while adhering to the standard dimension of the WR284 waveguide adapter. To analyze the behavior and effect of bends for the MGGW structure, unit cell structures were parametrically optimized for a straight structure and single right-angle bend (90°) and double right-angle bend transitions. These bend transitions were added to observe wave propagation and losses for the S-band frequency spectrum. The transitions of 90° bend were achieved using only a single pin structure for both types of simulated MGGW structures. In the literature, such transitions are achieved by using multiple pins or shaping the physical geometry of the structure [21]. A pictorial view of the proposed MGGW designed with straight and bent configurations is shown in Figure 4a,b, respectively. The cross-sectional view of straight MGGW is depicted in Figure 4a. The waveguide was designed for an electrical length of 5λ for the center frequency, and it had a period of 20 for unit cells. The dimensions of the periodic cells were kept the same as mentioned in Figure 3a. In Figure 4b, the top view of single 90$°$ and double 90$°$ transitions along the waveguide is shown. The electrical lengths of the waveguides were taken as 5λ and 6.5λ for the two waveguide designs, respectively.

2.3. Fabrication of MGGW

To validate the simulation results, the proposed unit-cell-based MGGW structure was fabricated. In the fabrication process for testing and feeding the MGGW, we selected the EIA standard waveguide adapter WR284 physical dimensions as a reference with a recommended frequency range of 2.6~3.95 GHz. The air gap between the top face of the unit cell and the parallel metal plate was taken as 4.036 mm. The Alumina 6061-T6 material was used for fabrication because it has better tensile strength and better corrosion resistance as compared to T4. Additionally, it is low cost as compared to copper or other equivalent metals with similar electrical properties. The fabrication was carried out using computer numerical control (CNC) milling operation with an accuracy of 50 microns (to achieve tolerance, a 1 mm T-slot cutter was used for the fabrication of heads in the unit cell structure). The MGGW was fabricated with four independent components that include a bottom surface with the metallic EBG periodic structure, front and back metal walls, and a top metal cover plate. The waveguide was fabricated with independent components to provide a high level of flexibility for quick fabrication and ease of assembly. To achieve the precise assembly of components and alignment of waveguide adapters for measurement of results, 1 mm deep metal engraving was carried out for all four independent components. The assembly of the components was completed by using metallic screws. Details of the fabricated model are shown in Figure 5.

The geometric dimensions of the fabricated MGGW are presented in

Table 3. Geometric dimensions of fabricated MGGW for S band.

Geometric Parameter	Dimension	Geometric Parameter	Dimension
h2 (WR284, EIA standard)	34.036 mm	w1	10 mm
g1	4.036 mm	w2	15 mm
h1	25 mm	p	20 mm
h3	05 mm

. The MGGW is designed to cover 1.5λ (electrical length) of a signal at a center frequency of 3 GHz for measurement and analysis. The electrical length of the fabricated model was kept at 1.5λ only to minimize fabrication cost. The results of the fabrication design are also compared with simulations for analysis in Section 3.

3. Results and Discussion

In this section, three optimized MGGW designs (straight waveguide and with transitions) in the S band are analyzed based on simulations performed in CST Microwave Studio. Subsequently, a comparison between the measured and simulation results of the proposed straight MGGW is presented. The results are studied based on the S parameters (S11 and S21) of these designs.

First, the simulation results for the proposed MGGW with a mushroom-unit-cell-based periodic structure are presented in comparison with the GGW with a bed-of-nails-based periodic structure. The two waveguides were designed and optimized with similar dimensions and turns (single 90° and double 90° bends). The bandwidth analysis of all three designs was carried out for MGGW and GGW. The bandwidth results achieved via CST simulation are tabulated in

Table 4. Bandwidth comparison of simulated S parameters, reflection and transmission coefficients (S11 and S21), for the groove gap waveguide and proposed MGGW design.

GGW Simulated with	Bandwidth of Straight Structure (Figure 6)	Bandwidth of Single 90° Bend (Figure 7)	Bandwidth of Double 90° Bend (Figure 8)
Bed-of-nails GGW	2.248 GHz @20 dB	1.136 GHz @20 dB	0.766 GHz @20 dB
Proposed MGGW Design	2.632 GHz @20 dB (14.5% greater)	1.368 GHz @20 dB (16.9% greater)	1.136 GHz @20 dB (32.5% greater)

. The simulation results depict that the MGGW has a wider bandwidth as compared to the GGW with a straight-pin-based periodic structure. The bandwidth of designs is compared at a −20 dB reflection coefficient value. Figure 6, Figure 7 and Figure 8 graphically represent an S-parameter (S11 and S21)-based comparison of the simulation results of the straight, single right-angle bend, and double right-angle (90°) bend GGW with the proposed MGGW.

The results of the proposed unit cell are promising for the designed MGGW. In the case of straight waveguide design, the proposed MGGW provided a bandwidth enhancement of 14.5% compared to bed-of-nails GGW. Similarly, the bandwidth achieved for the proposed MGGW is 16.9% and 32.5% greater than that for the bed-of-nails design for the single right-angle 90° bend and double right-angle 90° bend GGW designs, respectively. For practical evaluations, these results have been obtained for an MGGW design with actual waveguide face dimensions that match the EIA standard WR284 waveguide adapter dimensions.

The examination of the simulated results suggested that the newly designed unit-cell-based MGGW exhibited broader bandwidth when compared to the conventional bed-of-nails unit cell presented in the literature. The proposed design has also been studied for two transitions, and the observed bandwidth is greater than that of GGW. Since a bed-of-nails structure has been used in the literature for a variety of design applications with customized feed networks, the outcomes of these simulations cannot be compared to any existing study findings.

It is important to mention that the proposed work in this study is evaluated by maintaining a fixed air gap between the upper metal plate and the top surface of the unit cell. This consistent fixed air gap is essential to facilitate the propagation of energy through the MGGW channel. To improve the bandwidth capabilities of the MGGW, further optimization of the bed-of-nails structure can be performed. However, this enhancement will lead to a reduction in the size of the air gap between the upper metal plate and the top surface of the unit cell. As the waveguide structure is comparatively large for application within the microwave spectrum, it is either very challenging to attain a refined surface finish through established fabrication techniques like conventional milling or contemporary 3D printing methods such as direct metal laser sintering (DMLS), or this accuracy and precision involve a high fabrication cost. The proposed design of the unit cell is specifically designed to ensure a large air gap is maintained between the upper metal plate and the top surface of the unit cell for achieving improved performance as well as quick and easy manufacturing of products.

Evaluation of Fabricated Design Modeled at 3 GHz Spectrum

For the fabrication of an MGGW for the S band, a straight MGGW was modeled at the center frequency of 3 GHz, and the optimized MGGW model was successfully fabricated. Following this, the prototype device was tested using a Keysight Vector Network Analyzer (VNA), model E5063A. The waveguide adapters utilized were WR284 EIA standard rectangular plane adapters. These adapters were calibrated prior to the prototype waveguide testing. The calibration of adapters was carried out independently by using an S-band rectangular waveguide termination of 100 watts and collectively by connecting adapters directly with each other. During the calibration, the RF cables were first calibrated as one port, and a two-port calibration procedure was adopted using the electronic calibration (ECAL) kit of the VNA. In the second step, waveguide adapters were connected to the 100-watt WR284 rectangular waveguide termination individually, and their response was observed. Waveguide adapters were then connected with each other directly, and their response was again verified as satisfactory. The prototype MGGW was finally connected between waveguide adapters and RF cables to observe a response using the VNA. A pictorial representation of the test setup is displayed in Figure 9, which shows the prototype device connected to a calibrated VNA utilizing proper radio frequency cables equipped with an N-type RF connector. The measured results of the prototype MGGW were analyzed for a frequency range from 50 KHz to 8 GHz. These results are very promising in the band of interest and are in good agreement with the simulated results, as their comparison in Figure 10 shows. The achieved reflection coefficient is less than −20 dB for the entire band of interest.

From the results, it can be concluded that even though fabrication was performed with a 50-micron milling tolerance, the entire fabricated MGGW’s length had a good surface finish, which produced the intended measured results because there is enough of an air gap (4.036 mm) between pin structure and top metal plate. As we discussed earlier in Section 2, an air gap of ≤0.05 mm is very difficult to achieve through conventional fabrication, and for the complete length of the design, the air gap may be reduced to zero, which will cause a disturbance in the wave propagation through the waveguide channel. The measured bandwidth of the fabricated model is 1.576 GHz for a passband ripple of 1 dB with a reflection coefficient of −20 dB. The results achieved are in good agreement with the simulated model. As we have used WR284 rectangular waveguide adapters for validation, the measured results match the simulated results in the WR284 recommended frequency band of 2.5~3.95 GHz. We have also observed that due to the WR284 rectangular waveguide adapters, the measured response beyond 3.95 GHz starts to degrade with a higher passband ripple (>1 dB) even though the reflection coefficient is still below the −20 dB threshold. Therefore, we have presented the recommended useable bandwidth of the prototype design model of MGGW, considering only up to 1 dB passband ripple, which extends up to 3.824 GHz.

It can be observed that the bandwidth is 15.7% greater compared to the recommended bandwidth of 1.3 GHz (2.6~3.95 GHz) for the EIA standard W284 rectangular waveguide. Based on the analysis of S parameters, it is inferred that the proposed mushroom-unit-cell-based EBG periodic structure provides a wider bandwidth towards lower cut-off frequency regions, which are often points of interest for practical fabrications.

4. Conclusions and Future Work

In this paper, we demonstrated a new design of a unit cell and a gap waveguide termed MGGW for applications in the microwave spectrum. A performance comparison of the proposed unit cell design was carried out with a conventional widely used unit cell, that is, a bed of nails. Based on dispersion characteristics, the proposed unit cell achieves an 8% bandwidth enhancement when compared to a bed of nails while keeping a constant air gap between the top metal plate and the top surface of the unit cell. The MGGW was then designed for a straight structure and single right-angle 90° bend and double right-angle bend transitions, and it was compared with the GGW periodic structure for performance evaluation. The CST simulation results show that the proposed MGGW provides a wider bandwidth towards a lower cut-off frequency spectrum as compared to the GGW, which is always an important aspect for practical design consideration. The fabrication of the MGGW was carried out using CNC milling operation, and the prototype has provided a reflection coefficient of less than −20 dB for the entire band of interest. The proposed design is suitable for developing ultra-wide bandwidth microwave applications, i.e., filters, transmission lines, resonators, attenuators, etc. The proposed unit cell can be explored for usage in mm-wave spectrum designs, particularly for the sub-bands of 28 GHz and 60 GHz, and further applications, where the fabrication of complex structures is challenging and expensive as well.