Recent Advances in Theory and Applications of Substrate-Integrated Waveguides: A Review

Abhishek Sahu(1), Vijay K. Devabhaktuni(1), R. K. Mishra(2) and P. H. Aaen(3)

(1)Electrical Engineering. & Computer Science Department, The University of Toledo, OH 43606, USA.

(2)Electronic Science Department, Berhampur University, Ganjam, Odisha, India.

(3)Advanced Technology Institute, University of Surrey, Guildford, Surrey, UK.

E–mail: Abhishek.sahu@utoledo.edu

Abstract – The use of substrate integrated waveguides (SIW) for microwave and millimeter wave integrated components has increased dramatically over the last decade. They mimic the performance of conventional metallic waveguides and they are fabricated using printed circuit boards using the top and bottom metallisation with two rows of vias forming the side walls. This creates a low profile, compact, and light weight alternative to conventional metallic waveguides, and they allow a direct interconnection with printed circuit boards and active components. This paper reviews the fundamental theory, documents the research that has been performed over the past decade, and summarises progress up to the recent state-of-the-art including novel SIW structures for passive circuits and antennas as well as new applications for reconfigurable and printed circuits using SIW technology.

Keywords–Millimeter–waves, Substrate integrated circuits (SICs), Substrate integrated waveguide (SIW).

1. Introduction

The exploitation of millimeter waves for next generation mobile communications has

fostered significant advances in the development of new high-performance microwave

components to support applications ranging from mm-wave handsets to microwave back-haul [1-

2]. There has also been a considerable interest in a wide range of applications beyond mobile

communications that includes imaging sensors [3], automotive radars [4] and biomedical devices

[5]. All of these applications require microwave/mm-wave components to be integrated into a

multilayer circuit and existing manufacturing techniques, that use precision machining have

difficulties to economically achieve the required precision and they are also difficult to scale for

mass production [2]. To address this, innovative integrated waveguides, such as the substrate

integrated waveguide [6–8], post–wall waveguide [9], and laminated waveguide [10] have been

proposed. These waveguide technologies attempt to maintain the advantages of rectangular

waveguide but are significantly smaller in size, with lower profiles, and are straightforward to

fabricate using existing printed-circuit board (PCB) technology. Substrate integrated waveguide

(SIW) structures are fabricated by embedding two rows of connecting vias or slots in a dielectric

to connect the top and bottom metal planes as shown in Fig. 1. In this way, the non-planar

rectangular waveguide can be made in planar, and the SIWs are then compatible with existing

fabrication techniques

Fig. 1. Illustration of a substrate integrated waveguide (SIW) showing the via arrangement that

forms the side-wall of the waveguide.

such as printed circuit board (PCB) or low–temperature co–fired ceramic (LTCC).

Substrate-integrated waveguides exhibit the propagation and dispersion characteristics

similar to rectangular waveguides. As SIW structures provide a platform to integrate all the

components including active circuits, passive circuits, planar circuitry and even antennas, there

is a possibility of mounting one or more chip-sets on the same substrate. Moreover, there is no

need of transition between elements fabricated with different technologies, thus reducing the

losses and extending the concept of system-in-package (SiP) to system-on-substrate (SoS) [11].

Over the last decade SIWs have been used in design of many microwave components including

post and cavity filters [51–56], power dividers [77], phase shifters [78, 79], attenuators [82],

oscillators [84], various antennas (slot, array and conformal) [101–122], and reconfigurable

microwave resonators, antennas and filters [127–140]. Figure 2 shows the number of

publications using SIWs over the past decade showing the cumulative annual growth. This

paper summarizes the present state-of-the-art and the applications and uses of SIW technology.

2005 2006 2007 2008 2009 2010 2011 2012 2013 20140

500

1000

1500

2000

2500

0 160 330511

748972

12481583

1826

2258

Fig. 2. Cumulative plot of number of publications on SIW technology in last decade (source:

web of science: webofknowledge.com).

2. Recent Progress in Waveguide Structure and Analysis

The original SIW design was conceived in 1998, when it was known as the laminated

waveguide [10-11]. It is also noteworthy that integrated waveguide technique was proposed

much earlier through a patent [12] that was aimed at facilitating the processing of and improving

the productivity of waveguide based microwave components. The design used two rows of

plated through holes to connect the top and the bottom conductors. In each of the rows the

separation between two consecutive holes is smaller than the cut-off wavelength. But it was not

until 2000 when SIW technique was unified with substrate integrated circuits (SICs) [13], which

refers to the synthesis or conversion of any non-planar structures in planar form (such as

rectangular waveguide, coaxial line, non-radiating dielectric waveguide, and image waveguide),

and their full integration with other planar structures such as microstrips and coplanar

waveguides on single or multi-layered substrates. Thereafter, numerous attempts have been made

to formulate design equations for primitive SIW structures to simplify their design. In their

review work, Dr. Wu et. al. [11] provide an elaborate study on the operating principles and

development of different empirical relations to calculate the effective width for SIWs. In this

Section we will discuss recent approaches to design of different SIW structures and review their

operation principles.

A. Operation Principles and Loss Analysis:

Substrate-integrated waveguides have propagation characteristics similar to rectangular

waveguides when the metallic vias are closely spaced; as this minimizes radiation leakage. Their

TEno (n=1,2,…) modes coincide with a subset of the guided modes of the rectangular waveguide.

TM modes are not supported by SIW structures due to spacing between metalized vias [11]. The

metallic posts establish the boundary conditions for electromagnetic waves; as they conductively

connect the surface currents between the top and bottom planes. Various empirical relations

have been proposed relating geometrical dimension of SIW and rectangular waveguide. Two

such relations proposed in [14, 15] are

(1)

(2)

Where d is the diameter of the metal vias, w represents their transverse spacing and s represents

their longitudinal spacing. The effective width enables the SIWs to be analyzed as rectangular

waveguide, thus reducing the design complexity.

An improved closed form formula derived from an analytical method was proposed in [16].

(3)

Very recently, in [17] the authors showed that the accuracies of the above relations depend on

via diameter to via pitch (d/p) ratio. Hence, a new method to design the SIW width , in

terms of its equivalent dielectric waveguide width Wequi, based on the mode matching technique

was proposed in [17].

(4)

Where d and s are the diameter and pitch of the via respectively. The above relation covers all

practical SIW applications for which 0.5< <0.8.

Numerous attempts have been made for rigorous determination of the propagation and

dispersion characteristics of SIW structures based on numerical and modal analysis methods.

Analysis of the propagation characteristics of the EH1 mode of the HMSIW was proposed in

[18]; using method of auxiliary sources (MAS), which is an alternative numerical technique to

the widely used surface-integral formulation, that is suitable for solving elliptical boundary value

problems, which appear in electromagnetic-scattering analysis, antenna modeling and waveguide

structures. Addition theorems employing Bessel and Hankel functions were used to analyze the

full–wave behavior of the SIW devices [19]. In this approach, the equivalent circuit parameters

were extracted using the hybrid mode matching between guided and cylindrical modes.

Interested readers can refer to [17] and [19] to learn more about the mode matching technique.

An approximate analysis of a slotted substrate integrated waveguide with periodically loaded

elements using the transverse technique was proposed in [20]; for designing a travelling wave

attenuator where pin diodes are capacitively coupled to the waveguide slot.

The minimization of losses is a significant consideration in the design of SIWs. There are

three major mechanisms of loss: conductor losses, dielectric losses, and radiation losses [21].

SIWs conductor losses closely resemble those exhibited in rectangular waveguides and they are

primarily affected by the substrate thickness; as the conductor losses depend on the surface

integral of on the metal surface, more specifically on the top and bottom metal surfaces of

the SIW. Hence, increasing h results in a reduction of proportional to , further scaling the

conductor loss proportional to 1/h. Conversely, the dielectric losses depend on the volume

integral of . Since increasing h results in a reduction of proportional to , but linearly

increases the volume of the substrate, so overall there is no effect on the dielectric loss.

Analyzing the quality factor related to dielectric loss shows that it is independent of h and

depends on the substrate dielectric material, but not on the geometry of the SIW [21]. Earlier

research indicated that radiation losses can be reduced [21] if p/d<2.5, with p/d=2 as the

recommended value. Recently, a formula based on a semi–analytical model to calculate the

attenuation constant due to radiation leakage in SIW interconnects has been proposed [22] based

on the decomposition of fundamental mode of SIW into two plane waves. Lateral radiation

leakage can result in crosstalk between adjacent SIWs and in [23], an analytical formula has been

developed for the crosstalk. Considering a case of two adjacent SIW structures sharing common

row of metalized vias, as shown in Fig. 3, the authors formulated the forward crosstalk using

attenuation constant due to lateral leakage (αr), and the attenuation constants due to dielectric and

ohmic losses, αd and αc, respectively.

Fig. 3. Two adjacent substrate integrated waveguide layout for crosstalk determination [23].

The forward crosstalk C can be expressed as:

(5)

Where d, w and s are the diameter, transversal spacing and pitch of the via respectively. N is the

number of apertures and is the wavelength of operation. The number of apertures is directly

related to the SIW as (l= N×s).

The scalar parameters in (5) are extracted through a least square curve fitting and hence,

limits of validity have to be considered while using the same, in terms of operating frequency

and geometrical parameters. Equation (5) has been derived for validity in the single-mode band

of the fundamental SIW mode, according to the nominal band ranges from 1.25 f0 to 1.90 f0 (f0

being the cutoff frequency of the fundamental SIW mode). Finally, the geometrical limits are

directly described by the attenuation constants and SIW length, by the approximation (αr + αd +

αc) l <<1.

The insertion loss, which accounts for conductor, dielectric and radiation losses, can be

significantly affected by surface roughness. Although analytical models of losses due to surface

roughness have been developed for classical waveguides and incorporated in electromagnetic

simulators, no publications have been reported so far for insertion losses in SIW structures [11].

A detailed comparison of losses in SIW structures, microstrip lines and coplanar waveguides is

reported in [24], which shows that SIW structures losses are comparable or lower than those of

planar transmission lines. Furthermore, the recent advancements in waveguide technologies for

millimeter- and submillimeter- wave applications have led to development of new topologies

such as gap waveguides [25], hard-wall waveguides [26], microstrip-ridge gap waveguides [27].

Gap waveguides offer very low loss and manufacturing cost compared to SIWs, since there is no

need of dielectrics or any conductive joint between the top and bottom metal plates, which have

led to investigation of microwave circuits with improved performance over SIWs at frequencies

above 40 GHz. A comparison among the major features between SIW, gap waveguides,

rectangular waveguides and microstrip lines is presented qualitatively in Fig. 4.

Fig. 4. A spider chart showing SIW's performance qualitatively compared to gap waveguide,

ideal rectangular waveguide and microstrip line based on different criteria.

B. Recent configurations of SIWs:

The compactness of SIWs is one of the key attributes considered in designing efficient

commercial mm-wave components; different topologies have been proposed to improve the

compactness of SIW structures. Substrate integrated folded waveguide (SIFW) was proposed in

[28]; where a metal septum is introduced to permit folding of the waveguide, thus, reducing the

size by a factor of two at the cost of slightly larger losses, as illustrated in Fig 5(a). The half

mode substrate integrated waveguide was proposed in [29] based on the approximation of the

vertical cut of the waveguide as a virtual magnetic wall permitting reduction in size of almost

50% , as illustrated in Fig. 5(b).

(a) (b)

Fig. 5. (a) Substrate-integrated folded waveguide [28] (b) Half-mode SIW [29].

A quarter–mode substrate integrated waveguide is proposed in [30], based on the

approximation that centre symmetrical plane of HMSIW can also be regarded as magnetic wall

for several modes, allowing the cross section to be further bisected along the symmetrical plane,

as illustrated in Fig. 6(a); resulting in a size reduction of 75%. A miniaturization method is

proposed in [31], based on the concept of backward wave propagation; where a SIW when

loaded with SRRs shows uniaxial negative transversal permeability and the width becomes less

than half a wavelength at cut–off frequency (Fig. 6(b)). This miniaturized structure also provides

flexibility for tuning the cut–off frequency, provided the SRRs are designed accordingly.

(a) (b)

Fig. 6. (a) QMSIW topology [30] (b) An equivalent infinite parallel plate waveguide (PPWG) containing

the SRRs to analyze proposed SIW loaded with SRRs [31].

However, none of the above topologies focus on longitudinal dimension reduction. As

discussed before, the dielectric loss in a SIW depends on the substrate dielectric constant. This is

one of the reasons, that printed circuit board (PCB) technology is too often used for SIW

component fabrications. Alternatively, when SIW is realized using other fabrication technologies

such as low temperature co-fired ceramic (LTCC) and semiconductor IC’s, the relative high

dielectric constant often makes the width of SIW shrink to a size which is impossible for

practical use at mm-waves. Another important aspect to be accounted for the design of SIWs, is

the ease of integration of active components. Unlike traditional rectangular waveguides, SIW

avoids the requirement of complex mechanical structures, such as diode mount and RF

mechanical coupling between waveguide and active devices; consequently, the manufacturing

cost for these structures is reduced. To address these critical design issues, some SIW topologies

have been developed. The corrugated substrate integrated waveguide (CSIW) utilizing open–

circuit quarter–wavelength microstrip stubs, in place of vias, to form the electric sidewalls of

SIW (Fig. 7(a)) was proposed in [32]. This results in a structure that is completely dc isolated

from the ground plane and permits the shunt connection of active devices. The half–mode buried

corrugated substrate integrated waveguide (HMBCSIW) [33], is almost identical to HMSIW; but

offers a floating top conductor, thereby permitting ease in integration of active devices with an

additional advantage of a mono–mode bandwidth of 3:1. The empty substrate integrated

waveguide (ESIW) proposed in [34], removes the dielectric substrate, and a wideband transition

allows the excitation of the waveguide accessing the microstrip line so that waveguide can be

connected to traditional planar circuits (Fig. 7(b)). This approach enables very low loss circuits

with up to 4.5 times greater quality factor as compared to an equivalent design in SIW.

Extending the approach, a low-loss hollow substrate integrated waveguide (HSIW) was proposed

in [35], where the HSIW is realized by removing the inner dielectric of an SIW and therefore

incorporating a hollow cavity inside (Fig. 7(c)). As this structure incorporates a hollow cavity

inside, the losses again significantly reduce and the structure eases the integration with

rectangular waveguides of equal heights and design of antennas with better radiation

performance. A slow–wave substrate integrated waveguide (SW–SIW) was proposed in [36],

which requires a double layer substrate with a bottom layer including internal metalized via–

holes connected to the conducting ground plane (Fig. 7(d)).

(a) (b)

(c) (d)

Fig. 7. Topologies for (a) Half–mode corrugated SIW [33] (b) Empty SIW [34] (c) Hollow SIW

[35] (d) Slow–wave SIW [36].

Based on a double-layer substrate technology, these SW–SIWs exhibit a significant slow-

wave effect leading to a reduction in both transverse and longitudinal dimensions and thus, a

noteworthy surface miniaturization ratio.

3. Recent Progress in Passive Circuits:

SIW based filters and couplers are extensively documented in the literature. Various filter

topologies have been proposed which include filter with an inductive post, with irises, and cavity

filters with circular and rectangular cavities [7, 37, 38]. Subsequently, dual mode SIW filters

[39–41], wideband SIW filters [42], multi–band SIW filters [43–45], filters based on multi–

layered structures with cavities [46], and compact and super–wide band–pass filters using

electromagnetic band–gap structures [47] were reported.

With respect to passive components, the development of bandpass filters is an essential

component of a communication system. Several attempts have been made to design filters with

substrate integrated waveguides exhibiting a desired passband and high rejection level in the

stopband. Table 1 presents a qualitative comparison of recently published bandpass filters in

SIW technology. In addition to these bandpass filters, several other topologies including

introduction of complementary split–ring resonators (CSRRs) and composite right–left handed

(CLRH) transmission line [55, 56], a triple mode filter on quarter–mode SIW [57], balun filters

based on SIW technology [58, 59], and filters with controllable electric and magnetic coupling

[60, 61] were reported.

TABLE 1 QUALITATITVE COMPARISON OF RECENTLY DEVELOPED SIW BANDPASS FILTERS

Reference Size (mm2) Bandwidth Insertion loss (dB)

Center Frequency(GHz)

Technology

[48] 61 × 32.8 4% 1.8 5.05 Inclusion of planar resonators

63 × 33.13 4.2% 2.05 5.1

[49] 62.36×64.92 5.7% 2.4 5.25 Using frequency dependent coupling

[50] 12×19 10.1% 1.2 4.65 Using HMSIW and avoiding

the need of vias

[51] 30×15 42% <1.1 8.5 U shaped slot etched in SIW cavity to form a multiple mode resonator for wide band

[52] 28.13×28.13 10.9% 1.8 5.5 Hybrid structure of SIW and coplanar waveguide (CPW)

[53](Circular SIW)

1515.55 9.7% 0.29 2.06 Using a triple mode dielectric resonator implemented in SIW

[54](triple passband)

74.73×21.54 4%, 3.4%, 3.3%

0.33, 0.45, 0.3

9.72, 10.76, 11.76

Inclusion of six cascaded singlets in SIW

Fig. 8. The relationship between SIW filter and its substrate characteristics [64].

Very recently, a series of three papers summarizing the various filter topologies, design

considerations and practical aspects of SIW filters, have been published in [62–64]: basic design

rules and fundamental electrical characteristics to indicate the superior performance of SIW

structures are reported in [62]. Advanced design techniques such as cross–couplings realized by

physical and non–physical paths, SIW filters with dual–mode and multi–mode techniques,

miniaturization enabled techniques such as LTCC, wideband and multiband filters have been

presented in [63]. In [64] the authors have summarized a series of practical aspects and design

considerations of SIW filters. Critical aspects such as substrate selection, fabrication tolerance,

thermal stability, and power handling capability are discussed in depth with suitable examples.

The discussion showed that substrate characteristics play a crucial role on the electrical and

physical performance of the SIW filters, as shown in Fig. 8. The size, insertion loss, power

handling, temperature stability and fabrication cost of an SIW filter are all related to the

characteristics of its substrate.

Additionally, several coupler topologies have been developed in SIW. Among them, two

directional couplers were proposed. The first one uses two adjacent SIWs with a common wall

on which an aperture is utilized to realize the coupling between two SIWs working in TW10

mode. This topology was used to design and fabricate 3, 6, and 10-dB couplers [65]. The second

configuration presents a cruciform shape, and was adopted to design a super–compact 3-dB

directional coupler [66]. An efficient design technique for accurate design of wideband couplers

was proposed in [67]. In this design, the even mode propagation constant for tapered slot section

was first accurately extracted with the help of numerical thru–reflect–line calibration method.

Then, it was fitted into the model of a dielectric filled rectangular waveguide and thereafter

extrapolated to the operation range of odd-mode. Based on the extraction of the equivalent

circuit models of the waveguide bifurcation effects along with parametric values, a 90° 3–dB

coupler was developed to validate the approach. Design and fabrication of a 3–dB coupler based

on a hybrid HMSIW–slotline guided wave structure is proposed in [68]. With this approach, the

coupler achieved a relative bandwidth of 17%~22% with 0.5 dB amplitude imbalance. A variety

of ring coupler topologies have been proposed (Fig. 9). Among them, a SIW-ring coupler and

HMSIW ring coupler adopting closed side to closed side designing, for connections of HMSIW

[69] with 24.6% bandwidth for amplitude imbalance, a dual band ring coupler based on CLRH

folded substrate integrated waveguide (FSIW) [70] and a hybrid ring coupler based on compact

ridge substrate integrated waveguide (RSIW) [71] are noteworthy.

(a) (b)

Fig. 9. Layout for (a) Hybrid ring coupler based on HMSIW [69] (b) Lumped-element

equivalent of dual ring coupler unit cell based on CLRH folded SIW [71].

The design of SIW-based power dividers and phase shifters has also received significant

interest from the research community. Development of three configurations of SIW power

dividers were proposed in [77], including a compact radial cavity power divider developed in

[72], a frequency–selective power divider based on single–layer proposed in [73], and a multi–

layer four way out of phase power divider developed in [74]. Two topologies of ring power

divider were proposed in [75, 76]. Also, two configurations of phase shifters were proposed in

[78, 79]. One of these is based on several phase channels made by SIW resonators loaded with

additional metallic posts, to design a phase shifter with 10% fractional bandwidth and the other

design proposes a compact phase shifter using omega particles; thus, providing bandwidths of

around 55% for 90° and 45° versions of the phase shifter, with the accuracy of 3° and 60% with

the accuracy of 2.5°. A broadband magic T [80] and X–band SIW attenuator [81] were also

implemented and experimentally verified. The analysis of loaded SIW attenuators was presented

in [82].

4. Recent Progress in SIW Active Circuits

Complete system on substrate (SoS) integration is possible with integration of active devices

in SIW technology, which is yet to be a focus area of research in this field. The development of

SIW technologies have stimulated the possibility of design and optimization of components like

oscillators, mixers, and switches etc [11]. This section highlights several recent developments of

these components including new SIW-based switch topologies.

The first reported SIW oscillator used a rectangular SIW resonator appropriately placed in

the feedback path between the input and output nodes of an amplifier circuit designed using

Agilent ATF36077 pHEMT transistor [83]. The feedback transmission lines and the input and

output lines of the oscillator circuit were fabricated in microstrip technology and were connected

to the SIW cavity using appropriately designed transitions. This design demonstrated the

fabrication of low cost, high Q, oscillator circuits using SIW technology. Successively, two

oscillators based on Gunn diodes were proposed in [84, 85]. In [85], the authors achieved

frequency tuning for the oscillator by introducing a varactor diode along with a Gunn diode.

These reflection oscillators were designed based on linear simulation techniques in order to

estimate the resonance frequency of the oscillators. Linear simulation techniques are very

intuitive and provide a means for estimating the oscillation frequency using S-parameters

obtained from measurements of the passive resonator. However, as they rely on small-signal

operating conditions, they are unable to estimate the output power of the harmonic content of the

oscillator [11]. Addressing this problem, two X–band oscillators using rectangular cavity were

reported in [86]. In that work, the oscillators were designed by harmonic balance simulation,

where the S-parameters of the cavities were imported from an electromagnetic simulation. A new

technique based on self–compensation of the resonant frequency of SIW cavities was applied to

the oscillator design in [87], in which a nominal ratio of thermal expansion and permittivity

coefficients was defined to generate temperature compensation. In that work, a cavity of specific

substrate was designed, measured and compared to cavities realized on other substrates. Finally,

the cavity was integrated with an amplifier to build an oscillator which showed a stability of 2

ppm/°C in the temperature range of –40 to 80° C. Another Gunn oscillator design based on

HMSIW was reported in [88]. The circuit was composed of a Gunn diode, resonant cavity, direct

current power supply circuit and a transition of HMSIW to microstrip. The HMSIW cavity acts

both as a tuned resonant circuit and an energy coupling device of GUNN diode, coupling energy

from cavity to transmission line. A single–layer cavity backed antenna oscillator was

implemented in [89]. The active circuit and radiator were placed on the opposite side of the

substrate and within the cavity area allowing a compact configuration with 11.87–12.36 GHz

tuning range and phase noise better than –107 dBc/GHz. A push–push and push–pull oscillator

(Fig. 10 (a)) based on SIW and substrate integrated coaxial line (SICL) was presented in [90].

Design of a tunable oscillator based on mechanically tuned resonant cavity was reported in [91]

that allowed varying the oscillation frequency by 2%–3% by means of a flap (Fig. 10 (b)).

(a) (b)

Fig. 10. Schematic of (a) SIW push–push push–pull oscillator [90] (b) SIW oscillator with tuning

element [91].

While, considerable work on oscillators based on SIW technology have been reported in

literature, still there are several areas for future research. For example, phase noise can be

minimized by increasing the unloaded quality factor of the resonator, matching the noise

impedance or achieving a balanced characteristic of the nonlinearities. Similarly, recent

published results show a 2%-3% tuning range through the use of varactor diodes or

mechanically tuned resonant cavities, which can be potentially further improved by inserting

more diodes or introducing various resonant circuits as CSRRs or CLRH topologies. Further,

there is also scope for fabricating low cost frequency sources for high frequency range (100 GHz

or above).

As one basic building block of communication system, mm–wave mixers have widely been

studied, especially with regard to the aspects of port isolation and spurious rejection. Two

singly–balanced mixers, one: X–band single–balanced diode mixer and the other a part of 24

GHz automotive radar system–on–package front–end, were proposed [92, 93]. Further, a

broadband single–balanced mixer using a standard H–plane 3–dB coupler and a novel 90°

broadband SIW phase shifter, was proposed in [94]. The mixer had a conversion loss better than

10 dB across the frequency range 20–26 GHz and LO–IF RF–IF isolations as –55 dB and –45 dB

respectively. Additionally, a single balanced mixer based on a SIW Magic–T structure was

proposed in [95]. The structure showed conversion gain higher than –9.5 dB in the case of

applying –20 dBm RF signal for both high and low LO power configurations at 10.6 GHz, over

the entire RF frequency band of 10–10.55 GHz. Self–oscillating mixers are compact circuits

providing the functionality of both the oscillator and the mixer. They are designed by

appropriately biasing and loading oscillator circuits in order to optimize conversion gain [11].

Two prototypes of self–oscillating mixers were proposed: the first one, a self–oscillating mixer

based on a feedback oscillator that used a rectangular SIW cavity placed in the feedback path of

a field–effect transistor (FET)–based amplifier circuit [96], and the other, an X–band active

antenna self–oscillating down–converter SIW mixer [97], with IF frequency of 3.15 GHz for

conversion gain in at least 600 MHz bandwidth around the IF frequency. As in case of

oscillators, there are several areas where SIW technology can be explored for mixer design.

Some such areas include implementation of mixers in high frequencies (100 GHz and more);

integration of active devices in SIW circuits [11]; new topologies for self-oscillating mixers etc.

Besides oscillators and mixers, switches are essential components that are found in many

microwave applications and a switchable SIW was first proposed in [98]. This research

introduced a switch based on slotted–SIW (SSIW), whose mode of propagation can be switched

between two different modes controlled by integrated pin diodes (Fig. 11 (a)). The switch was

fabricated using microwave laminated SIW, and showed an isolation of approximately 50 dB

over the operating bandwidth with 3-dB insertion loss. In [99], a magnetically controlled switch

based on ferrite–loaded substrate integrated waveguide was proposed. Through an external

transverse magnetic field, the permeability of the ferrite slabs loaded inside the SIW cavity (Fig.

11 (b)) is tuned, thereby making the SIW mode cut–off frequency change, to enable the

switching function. This switch had a 1.1 GHz bandwidth centered at 1.1 GHz with an insertion

loss less than 1 dB in the on state and an isolation of 20 dB in the off state. The design and

implementation of a SIW SPDT switch for X–band applications was presented in [100].

Inductive posts with rectangular slots were embedded in the SIW to control the travelling EM

wave. By adding pin diodes on the top slots of inductive posts, the SPDT switch achieved

isolation (S31) greater than 10 dB, (S32) greater than 15 dB and an insertion loss less than 2.55 dB

in 8.24–10.36 GHz band.

Fig. 11. Layout for (a) Switchable SIW [98] (b) Geometry of a ferrite loaded SIW section [99].

5. SIW Antennas:

The development of millimeter wave technologies has led to a growing interest in SIW

antennas. Several early developed antenna topologies have been discussed in [11]. SIW based

antennas offer advantages similar to microstrip antennas as they can be fabricated with the same

planar printed technology. Furthermore, SIW as a waveguide structure, does not suffer from

unintentional radiation and surface wave loss, which reduces the limitation of thin substrates.

These merits make SIW a better candidate for antenna design. In this Section we will focus on

the recent advancements for several SIW antennas.

A. Slotted antennas:

In this technique, the radiation in SIW antennas is obtained by etching slots in the top metal

surface of the SIW structure (Fig. 12 (a)). The first slotted SIW antenna was based on a four–by–

four slotted SIW array operating at 10 GHz [101]. Recently, many topologies of SIW slot

antennas have been proposed for back lobe suppression. A SIW slot antenna array based on

comb–shaped chokes of an array of quarter–wavelength short–end parallel microstrip lines at the

bottom surface of array was reported for lowering the backward radiation by 10–dB [102]. A

SIW slot antenna with folded corrugated stubs was proposed in [103] for suppressing the back

lobes. Bandwidth broadening of SIW slot antenna was reported in [104]. The work proposed the

use of two unequal slots to increase the bandwidth up to twice of standard slot antennas. Two

design methods for SIW travelling wave slot antennas and planar slotted array antennas were

also reported in [105, 106]. Both design methods are based on the method of least squares and

Elliot’s design formulas using the concept of equivalent waveguide to formulate the error

function. Using the concept of hollow SIW, a slotted waveguide antenna array was designed in

[107]. Using classical log-periodic theory, SIW slot antennas with broadband performance were

presented in [108]. Based on the orientation of the slots a transverse SIW slot antenna and a

longitudinal SIW slot antenna were designed and results indicated a much wider impedance

bandwidth as compared to traditional slotted waveguide antennas.

B. Leaky–wave SIW Antennas:

In the SIW leaky–wave antennas, the radiation is obtained by increasing the longitudinal

spacing s of the side wall metal vias, as shown in Fig. 12(b).Two leaky wave antennas based on

CLRH SIW were presented in [116, 117]. In [116] a double periodic CLRH substrate integrated

waveguide is proposed and the equivalent circuit, the dispersion behavior and the expression for

cut–off frequencies were studied. Two SIW leaky wave antennas composed of periodic set of

transverse slots were proposed by Long et al. [118, 119]. Such structures were found to support

three kinds of modes (a leaky mode, proper waveguide mode, and a surface–wave–like mode).

Furthermore, a SIW leaky wave antenna with tapered transverse slots on top and bottom sides

was proposed by the same group for investigation of end–fire radiation with a narrow beam and

side lobe suppression [120]. The synthesis of one dimensional SIW–leaky wave antennas with

modulated geometry was presented in [121] using holographic concepts; its synthesis technique

demonstrated the capability of flexibly tailoring the radiated field’s pattern, both in near and far–

field regime.

C. Cavity backed Antennas:

Generally, there are two categories of SIW cavity backed antennas: SIW cavity backed

patch antenna (Fig. 12 (c)) and SIW cavity backed slot antenna. Numerous SIW cavity backed

antennas, with different configurations, different feeding structures, and different performances

have been developed by researchers around the globe. In 2013, Dr. X. H. Zhang et. al. reviewed

SIW cavity antennas [109]. Thereafter, some new topologies have also been introduced to this

area. A wideband SIW cavity backed antenna was proposed in [110]. In order to enhance the

bandwidth and radiation efficiency, the cavity was designed to resonate at its TE210 mode.

Another broadband SIW cavity backed antenna was introduced in [111], where the broadband

performance is achieved by introducing a bow–tie–shaped slot instead of a conventional

rectangular slot. A SIW–fed circularly polarized antenna array with a broad axial–ratio

bandwidth was proposed in [112]. The antenna array consisted of 16 sequentially rotated

elliptical cavities fed by slots on the SIW acting as radiating elements, four 1–to–4 SIW power

dividers, and a transition from a coaxial cable to SIW. Two dual band SIW cavity backed

antennas were introduced in [113, 114]: the first one, integrates cavity backed SIW and

triangular slot (TLS), where the dual mode of TLS is achieved by simultaneously exciting the

mode of the slot and the mode of the patch inside the slot; the other one uses a dumbbell shaped

slot along with thin substrate integrated waveguide cavity backing. The SIW–slot feeding

structure is introduced in a fully substrate–integrated thin Fabry–Perot cavity (FPC) antenna to

achieve low backward radiation levels [115].

D. Conformal Antennas:

A significant effort has been devoted very recently to the development of conformal SIW

antennas. There are a few concerns regarding analysis of the SIW conformal antennas and it

requires the synthesis of planar antenna to be carried over to conformal antennas. Unless a

systematic analysis method is developed to study the effect of conformal shape on the properties

of antenna, it is impossible to achieve satisfactory performance or special shaped beam

requirements with conformal antennas because the curved shape of the conformal antenna

determines its characteristics to a great extent [122]. However, some attempts have been made to

come up with accurate synthesis design for SIW conformal antennas. A 35 GHz slot array

antenna conforming to a prescribed curved surface with shaped–beam was investigated in [122],

which had low side lobe level in H–plane and a flat–topped fan–beam in E–plane (Fig. 12 (d)). A

conformal coplanar feed network was designed for the desired excitation. The structure had a –

27.4 dB side lobe level beam in H–plane, and a flat–topped fan beam with –38° ~ 37° 3–dB beam

width in E–plane, along with a cross–polarization lower than –41.7 dB at the beam direction.

Subsequently, a travelling wave slot array on a cylindrical substrate integrated waveguide

(CSIW) at K–band was presented in [123]. A 16 element longitudinal slot array on the broad

wall of CSIW was designed using Elliot’s procedure. For the structure, about 10° beam steering

is achieved when the frequency is swept from 24 to 26 GHz with an antenna gain of 14 dB. The

capability to conform a substrate integrated leaky wave antenna along an arbitrarily curved line

by suitably tapering the leaky mode along the antenna length was demonstrated in [124]. It was

shown that, by means of locally adjusting the pointing angle of the radiated wave, a coherent

plane waveform at the far–field region can be obtained. A conformal wideband SIW H–plane

horn antenna was presented in [125].

E. Active Antennas:

The term active antenna here refers to ‘circuit–antenna module’ and hence it includes active

integrated antennas, where an active device is integrated in the same substrate with the radiating

antenna structure. In addition, it includes antenna elements where an active device is used to

modify or reconfigure the properties of the antenna such as beam direction, polarization or

bandwidth [11]. Compact, single–substrate cavity–backed slot and patch oscillator antennas were

proposed in [126, 127], where a square cavity was used along with antenna and the feeding

networks, etched on the top and bottom layers respectively. Furthermore, a tunable oscillator was

presented in [128] by removing one via hole from the cavity wall and introducing a varactor

diode in its place.

Significant focus has been placed on design of frequency reconfigurable SIW antennas. A

frequency reconfigurable antenna based on HMSIW was presented in [129]. Its resonant

frequency was electronically controlled by a varactor–loaded interdigital capacitor on the top

plate of HMSIW and the bias network was designed on the bottom plate. A reconfigurable SIW

cavity–based antenna was presented in [130] by loading the SIW cavity with shorting posts and

manipulating the field distribution within the cavity. The frequency shift was controlled by the

number of posts and their location. Using this technique a frequency tuning ratio as high as an

octave (1.1–2.2 GHz) was achieved. A frequency reconfigurable SIW interdigital capacitor

antenna on a composite right/left handed (CRLH) resonator was presented in [131]. A varactor

diode was embedded on the interdigital slot of the short–ended SIW, whose zeroth–order

resonance varied from 4.13 GHz to 4.50 GHz by changing the bias voltage from 0 to 36 V. A

varactor loaded complementary split ring resonator (CSRR) was used to design a compact

eighth–mode substrate integrated waveguide antenna [132] operating in dual band. A broadband

frequency tuning was achieved in a ferrite loaded SIW antennas by simultaneously changing the

location of ferrite slabs in the antennas (mechanical tuning) and bias magnetic fields (magnetical

tuning) [133].

(a) (b)

(c) (d)

Fig. 12. Layout for (a) Slotted SIW (b) Leaky–wave SIW (c) Cavity Backed SIW (d) Conformal

SIW [122].

6. Re–configurability with SIW:

Reconfigurable components are essential for millimeter–wave multifunctional radio and

radar systems, such as smart and cognitive radio and radar techniques for better use of the

electromagnetic spectrum as they facilitate the simplification of multiband and wideband

wireless systems architecture [59]. These techniques can eliminate interference while preserving

good signal receiving condition. Through a dynamical reconfiguration of operational frequency

and bandwidth, tunable resonators efficiently cope with time and regional variation of traffic

demands. Hence, they are crucial building blocks for design and realization of tunable RF and

microwave components. One of the preliminary tuning solutions was proposed in [134] based on

the insertion of vertical capacitive posts integrated within SIW cavities. Further, a low–loss

tunable resonator based on combline–SIW cavity loaded with GaAs varactor was proposed in

[135]. A 2.6–3.1 GHz tunable band was obtained with a Qu between 180 and 70, a capacitance

variation between 0.25 and 1.25 pF. An ultra–wideband two–port resonator based on CSRR and

varactor based SIW was presented in [136]. The resonant frequency was varied between 0.83 and

1.58 GHz and has a wide tuning ratio of 90%. Subsequently, electrically tunable evanescent

mode HMSIW resonators were presented [137]; HMSIW loaded with CSRRs where a variable

capacitor connected to one of the conductors of CSRR changes its effective capacitance with

respect to ground, resulting in frequency tuning of the resonator. A novel approach for providing

SIW tunable resonators by means of placing an additional metalized via–hole on the waveguide

cavity was presented in [138]. The via hole contains an open loop slot on the top metallic wall.

The tuning range was defined by the dimension, position and orientation of the open–loop slot.

A novel tunable second–order filter was implemented on three–layer Rogers RT/Duroid

substrate using p–i–n diode switching elements [139]; the filter provided six states ranging from

1.55 to 2 GHz (25% tuning). A two pole filter has been demonstrated on a low–cost substrate

showing a tunable center frequency between 2.64 and 2.88 GHz with 1.27–3.63–dB insertion

loss across the tuning range [140]. Tuning a SIW cavity by embedding a frequency agile material

into an SIW cavity was presented in [141]. A specific switchable post constructed using plasma

(argon) was introduced in the SIW cavity. A compact tunable filter integrated in ferroelectric

ceramic substrate was reported [142] based on an evanescent–mode dielectric cavity loaded by a

pair of tunable–mushroom–type complementary split–ring resonators.

7. System on substrate (SoS):

The use of microstrip or coplanar waveguides is convenient for frequencies below 30 GHz,

but these techniques becomes impractical at high frequencies due to the incurred losses. The

previous sections highlighted the possibility of SIW technology in the development of various

active and passive components. These possibilities provide opportunities for design of system on

substrate (SoS), where all the components not included in chip-set are fabricated using SIW

technology. Two methodologies have been reported for the deployment and integration of SoS.

The complete front end of a 24 GHz frequency-modulation continuous-wave (FMCW) was

reported in [144]. All building blocks including up-converters, down-converters, power dividers,

one transmitting and one receiving 16-element slotted antenna array were integrated in the circuit

(Fig. 13 (a)). This approach showed that the surface hybrid integration scheme enables the

complete integration of planar and non-planar microwave circuits.

The other approach was based on deployment of multi-chip modules. Here, the active

portions of the circuit were implemented in the chipset whereas antennas, filters and other

passive components were developed off-chip on a multilayer substrate (Fig. 13 (b)). This

methodology offers a much simpler design, while exploiting the SIW technology for those

components which cannot be fabricated on-chip. A 60 GHz receiver based on this methodology

in GaAs technology was proposed in [145].

Fig. 13. Demonstration of SoS (a) 24-GHz FMCW radar front end [144] (b) 60-GHz multi-chip

module receiver [145].

8. Fabrication Technologies and Novel Materials

The fabrication is one of the key aspects in design of SIWs, especially for millimetre-wave

operation. PCB techniques is one of the most common technologies adopted for implementing

SIWs as they provide advantages in terms of low cost and good design flexibility. In the early

versions of SIW components, the holes made by a drilling machine and the metal vias were

implemented by using metal rivets. More recently, the metalized holes are usually obtained

either by micro–drilling or laser cutting, and their metallization is subsequently performed by

metal plating or by filling the holes by a conductive paste [146]. Apart from PCB, LTCC

technology has also been used in SIW implementation. The possibility of using several layers

and the tiny dimension of the via holes, provides a platform for implementation of extremely

compact SIW components.

The implementation of SIWs on novel materials have also been explored. The design of

SIW components and antennas based on polyethylene terephthalate was proposed in [145]. In the

work, initially SIW interconnects were designed and fabricated, which finally led to design of a

two cavity filter and a slotted waveguide SIW antenna (Fig. 14 (a)). The design and

implementation of an inkjet printed flexible broadband multilayer coupler on a polyimide film

(Kapton) in a SIW technology was demonstrated in [148] (Fig. 14 (b)). In [149], the flexible

liquid crystal polymer (LCP) was used to implement broadband bandpass filters, that features

7.5% size reduction compared to conventional SIW (Fig. 14 (c)).

(a) (b)

(c)

Fig. 14. Photograph of (a) SIW Filter based on PET substrate [147] (b) Inkjet-printed multilayer

broadband coupler on polyimide film [148] (c) Fabricated 3 pole filter on LCP [149].

Popovic and her research team are exploiting a new area of research on the micro-coaxial

array fed components at the University of Colorado, Boulder. Performance of broadband

Wilkinson dividers covering 2-22GHz (11:1 bandwidth) implemented in micro-coaxial lines

along with integration of active devices with micro-coaxial components for 20W transmitters

covering the 4-18GHz band was shown, and challenges associated with high power densities and

interconnect parasitic are addressed in [150]. Although, this technology requires high precision

and complex fabrication process, it could be one of the research area for SIW based components

operating in high frequencies. This technology provides advantages in terms of low loss, high

isolation, and large range of characteristic impedances.

9. Conclusions

The present state-of-the-art of the SIW technology is summarized in this paper, with a

focus on recent advancements. Design rules that are important for the development of

active and passive SIW circuits are discussed to highlight the attractiveness of this

emerging technology. The SIW technology has been compared with other recently

developed techniques from the view point of different critical aspects such as loss,

packaging and design complexity. In addition to the traditional components, the recently

explored areas in SIW technology such as switches, conformal antennas, have been

discussed. Furthermore, we have found that the attractive features of this technology make

it the ideal tuning platform for reconfigurable filters, antennas and impedance matching

networks. Finally, issues related to fabrication technologies and using novel materials for

SIW based components have been presented.

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