Via-Wall Confinement: Why SIW Beats Microstrip at Millimeter-Wave

SIW eliminates the dominant insertion loss mechanisms of microstrip — radiation loss and surface-wave excitation — by replacing the open guiding structure with a fully enclosed planar waveguide. Rows of metal-lined through-hole vias between top and bottom copper cladding on a dielectric substrate recreate the field confinement of a rectangular metallic waveguide while remaining fabricable by standard PCB processes. According to the comprehensive review from Multimedia University (2020), SIW achieves compact dimensions, low insertion loss, and a high quality factor by combining the advantages of classical bulky waveguides within a planar structure — a direct consequence of this via-wall confinement.

The surface-wave problem is particularly severe at millimeter-wave frequencies. A comparative study from Universiti Teknologi Malaysia (2020) explicitly demonstrated that the SIW-implemented antenna on Rogers RT5880 (loss tangent 0.0009) achieved superior return loss and radiation pattern at 28 GHz compared to a conventional microstrip patch on the same substrate, attributing the improvement to reduced surface-wave loss. A separate study from Universitas Negeri Jakarta (2019) described an H-slot SIW structure designed specifically to reduce surface-wave loss in microstrip antennas at 28 GHz, demonstrating improved gain and impedance matching through better surface-wave confinement.

A quantitative demonstration of via-wall confinement at Ka-band was provided by Chung-Ang University (2018). Using tungsten-coated TGSV structures fabricated by deep reactive ion etching (DRIE), the researchers achieved an average measured insertion loss of only 0.69 ± 0.18 dB/cm at Ka-band. The TGSV fabrication approach also eliminates the time-consuming metal-filling process of conventional through-glass via substrates, further improving structural fidelity and reducing ohmic loss variability.

Substrate Selection and Air-Equivalent Structures: The Material Dimension of Loss Reduction

Substrate material choice is decisive for SIW insertion loss because dielectric loss — proportional to the loss tangent of the substrate — scales with frequency and becomes a primary contributor at millimeter-wave bands. Rogers RT5880, with a loss tangent of 0.0009, is the reference material across multiple 5G SIW studies. A direct comparison at 28 GHz (Universiti Teknologi Malaysia, 2020) confirmed that SIW implemented on Rogers RT5880 outperforms microstrip patches fabricated on the same substrate in both return loss and radiation pattern, isolating the structural advantage of SIW over and above the material advantage both structures share.

The logical endpoint of substrate loss reduction is removing the dielectric entirely. The 3D-printed honeycomb SIW study from Chung-Ang University (2019) demonstrated this principle by replacing solid PLA substrate material with a honeycomb structure that approximates an air substrate. The measured average insertion loss dropped from 3.15 dB to 1.38 dB over the 3.4–5.5 GHz range. While demonstrated at sub-mmWave frequencies, the principle extends directly to millimeter-wave designs: the honeycomb structure reduces dielectric loss tangent contributions by creating an air-equivalent medium, leaving only conductor and radiation losses.

“Replacing solid dielectric with a 3D-printed honeycomb structure reduced measured average insertion loss from 3.15 dB to 1.38 dB — a reduction of more than 56% — by approximating an air substrate and eliminating dielectric loss tangent contributions.”

The structural extreme of this approach is the Air-Substrate Integrated Waveguide (ASIW), developed by the University of Alabama (2021). The ASIW removes the dielectric substrate from within the SIW entirely and replaces the via fence with metallic sidewalls, yielding 18% higher efficiency than conventional SIW slot arrays operating in the n257 5G band. According to IEEE standards for antenna efficiency measurement, an 18-percentage-point efficiency gain at millimeter-wave frequencies represents a substantial reduction in effective insertion loss from feed to radiated field. The trend across these material innovations is consistent: reducing or eliminating the dielectric within the SIW channel progressively lowers the dominant loss mechanism at millimeter-wave frequencies.

Cavity-Backed Slot Architectures: Directing Energy Upward, Not Sideways

SIW cavity-backed slot antennas (SIW CBSA) reduce effective insertion loss — defined as signal loss from source to radiated field — by leveraging the enclosed cavity to direct energy upward through a precisely engineered slot rather than allowing it to dissipate as surface currents or backward radiation. The review from NIT Warangal (2022) identifies SIW CBSAs as emerging candidates for planar IC technology in millimeter-wave bands precisely because the cavity provides high gain, high front-to-back ratio, and low cross-polarization — all indicators of efficient power transfer from feed to radiation and minimal loss into ground or surface modes.

Slot geometry directly governs bandwidth and impedance matching quality, both of which control in-band insertion loss. The dumbbell-shaped radiating slot patent (MR. Dokuparthi Jagadeesh, 2020) introduced a dumbbell slot in an SIW cavity with metallic vias at the cavity centroid, achieving wide impedance bandwidth, high gain, and unidirectional pattern — outcomes consistent with efficient power transfer and reduced mismatch-driven insertion loss. The I-shaped slot patent from Anil Kumar Katta (2023) perturbed the TE210 mode to split it into odd and even sub-modes, yielding measured impedance bandwidths of up to 14.4%, directly reducing the in-band mismatch losses that contribute to insertion loss.

For large-scale millimeter-wave array deployments, the stacked modular SIW architecture from Universidad de Granada (2022) achieved directivity scaling from single to eight radiating layers while maintaining in-phase, equal-power distribution through efficiently implemented H-plane and E-plane corporate feeding networks embedded in the SIW layer stack. The modular design minimises lossy inter-layer transitions by using well-characterised SIW aperture coupling rather than coaxial probes or microstrip stubs — a principle consistent with guidance from ITU on minimising passive intermodulation in multi-layer antenna structures.

Integrated Feeding Networks and Transition Engineering: Closing the Loss Budget

The feeding network is often the largest single contributor to insertion loss in a millimeter-wave antenna array system, and SIW technology enables feeding networks — power dividers, couplers, and phase shifters — to be fully integrated on the same substrate layer as the radiating elements, eliminating the connector transitions and impedance mismatches that introduce loss in hybrid waveguide-microstrip systems. The patent from IIT Indore (2023, active) explicitly targets 5G millimeter-wave bands with an H-shaped SIW power divider designed for low-loss power distribution across the 3–300 GHz range, without relying on microstrip transmission lines that would exhibit prohibitive radiation loss at higher millimeter-wave frequencies.

Beamforming network integration compounds these benefits. The 5G Butler matrix study from Multimedia University (2019) described a system at 24 GHz using SIW directional couplers, achieving 16.85% bandwidth with beam steering between −29° and +29°. The use of SIW for the entire beamforming network — not just the radiating elements — means all signal paths share the same low-loss dielectric confinement regime. The University of Perugia review (2019) confirmed that SIW couplers and magic tees for beam-forming networks fully exploit the low-profile, low-insertion-loss, and low-interference characteristics of SIW technology. As documented by ITU and 3GPP specifications for 5G NR millimeter-wave base stations, feed network insertion loss directly limits the effective isotropic radiated power (EIRP) achievable at the array aperture.

Transition design is a critical insertion loss control point. The Hankyong National University study (2022) investigated microstrip-to-SIW transitions using balanced and single-slot techniques optimised by genetic algorithm, demonstrating that transition design directly controls the operating band and insertion loss of the entire SIW interconnect. The D-band waveguide-SIW transition from Chungang University (2022) achieved −10 dB impedance bandwidth of 26.5 GHz (135–161.5 GHz), with the transition optimised to shift an in-band null away from the operating band — directly reducing in-band insertion loss. At sub-terahertz frequencies, the MIT triplexer (2020) operating at 220–330 GHz demonstrated that replacing standard via fences with continuous trench vias in an Intel organic substrate directly reduces ohmic loss of ridged SIW waveguides compared to conventional via fences, yielding 3–7 dB insertion loss across a 40% fractional bandwidth — a principle with direct implications for upper-millimeter-wave 5G and future 6G deployments. Standards bodies including IEEE have documented ohmic loss scaling laws for metallic waveguide structures that underpin these experimental findings.

Patent Landscape and Key Innovation Players: Who Is Solving the SIW Loss Problem

The SIW insertion loss patent landscape is geographically diverse but institutionally concentrated, with distinct technical specialisations emerging across industry and academia. The dataset of over 50 sources reveals four dominant assignee clusters, each approaching loss reduction from a different angle.

Telefonaktiebolaget LM Ericsson

Ericsson holds multiple active and inactive patents directed at reducing mutual coupling between neighbouring SIW antennas in arrays. By adding electrically conducting vias along the substrate extending from the antenna aperture, Ericsson’s designs reduce both mutual coupling and back radiation — two distinct sources of effective insertion loss in phased arrays operating for 5G base stations and handsets. Key patents include an improved antenna (US, 2018), an SIW antenna arrangement (EP, active, 2019), and a surface integrated waveguide antenna transceiver array (US, 2019).

Universidad de Granada

Among the most prolific academic producers in the dataset, Universidad de Granada contributes studies on double-slot cavity arrays, modular stacked SIW arrays, and Ka-band aperture arrays. Their focus on equal-power, in-phase corporate feeding networks within SIW layer stacks positions them as a leading group in low-loss array-level integration, with contributions spanning 2020 and 2022.

IIT Indore and IIT BHU

IIT Indore holds an active patent (2025) on a compact wideband millimeter-wave power divider using SIW, and IIT BHU has published on wideband cavity-backed slot antennas for 5G. Their emphasis is on SIW-compatible power divider designs that eliminate microstrip radiation losses in the feeding network across the 3–300 GHz range.

Sony Mobile Communications

Sony holds an active EP patent (2018) incorporating top wave traps integrated into the SIW first metal layer to prevent surface-wave radiation from degrading efficiency — directly addressing one of the major loss mechanisms at millimeter-wave frequencies for mobile device integration. This approach is consistent with ETSI requirements for mmWave antenna efficiency in mobile handset designs.

The trend across the dataset is clearly toward architectures that: (1) eliminate all microstrip transitions within the array feed network; (2) employ low-loss substrate materials such as Rogers RT5880 and Duroid 5880, or air-equivalent structures; and (3) use cavity-backed geometries to maximise radiation efficiency, particularly in the 28 GHz, 38 GHz, and 60 GHz 5G bands.