Zain Shafiq, Maksim Kuznetcov, Victoria Gomez-Guillamon Buendia, Dimitris E. Anagnostou, Symon K. PodilchakInsitute of Sensors Signals Systems, Heriot Watt University, Edinburgh, United Kingdom

zs2, mk11, vg22, d.anagnostou, s.podilchak@hw.ac.uk

Abstract—We present a planar substrate integrated waveguide(SIW) horn antenna for efficient TM surface wave (SW) launch-ing at microwave and millimeter-wave frequencies. A designapproach is presented by selecting the correct substrate materialand operating frequency as well as the SIW horn aperture. Inaddition, simulations and measurements of a demonstrator pro-totype for operation at 24 GHz are reported. This type of planarsurface-wave launcher (SWL) can have multiple applications suchas feeding leaky-wave antennas and other SW-based structures.The proposed SIW horn can also be used as a standalone antennato radiate at end-fire.

Index Terms—antenna, propagation, measurement, surfacewaves.

I. INTRODUCTION

The substrate integrated waveguide (SIW) is a fairly newtype of transmission line [1] and SIW technology is a form ofwaveguide which can be integrated within a planar substrate.The idea of the SIW is very similar to a metallic rectangularwaveguide. In a rectangular waveguide the TE10 mode isdominant while in SIW, a TE-like mode propagates. The viasof an SIW structure can be thought of as the side walls of themetallic waveguide and the two top and bottom metallic platescan be considered as the upper and lower boundaries of thewaveguide. This can make the non-planar rectangular waveg-uide, compatible with existing fabrication techniques such asprinted circuit boards (PCBs), planar antennas, and integratedcircuits (ICs). SIW technology also provides benefits such ashaving a low profile, being light weight, and with reducedfabrication costs [2], [3].

On the other hand, guided surface waves (SWs) on aplanar grounded dielectric substrate (GDS) can be a meansof efficient power transfer achieving bound propagation overa dielectric slab and for planar antenna applications [4]–[6].There are other geometries where SWs can propagate suchas dielectric rods and corrugated conductors, and generallywithin these structures fields are mostly contained within thedielectric while propagation occurs at the relevant air-dielectricinterface [7]. Over the years there has also been great interestin other SW structures. Researchers have described multipletypes of structures (e.g. open-ended waveguides and antennas)and other methods of excitation. For example, a convenient

Fig. 1. Exploded view of the proposed SIW structure for planar SW launching.

method of exciting SWs along a grounded dielectric slab(GDS) is a truncated parallel plate structure [8].

Following these developments and the previous work inSIW horn antennas which have been shown to offer highquality factors, low losses, and high efficiency [9], [10], wepropose a surface-wave launcher (SWL) using SIW technol-ogy. A design approach is presented and simulations andmeasurements of a demonstrator prototype for operation at24 GHz are reported.

II. DESIGN PROCEDURE

As mentioned a convenient way of launching SWs isthrough the use of a truncated parallel plate structure [8],and SIW technology is a simple approach to achieve suchlaunching. This is because the dominant electric field of SIWcan be matched to that of the TM0 SW mode for a GDS.However, depending on the application, field confinement maybe needed to ensure efficient and directive SW launching. Thiscan be made possible by using an SIW horn antenna and ahigh dielectric constant material. However, SIW technologyis a practical structure and unwanted power losses can occurfrom the spacing between adjacent vias. If the pitch, andthe diameter of the vias are chosen according to the design

A Planar Horn Antenna for TM Surface Wave Launching using Substrate Integrated Waveguide

Technology

equations in [11], this unwanted leakage into the peripherysubstrate can be said to be negligible.

The dielectric material chosen for the SIW launcher isRogers 6010LM. This material has a high dielectric permit-tivity, εr of 10.2. When using a material which has such ahigh relative dielectric permittivity it is more efficient for SWexcitations [4]. Mainly, that the wave is better confined to thedielectric substrate. On the other hand, if the substrate thick-ness h is too thin, the majority of the power can radiate whichis undesired in our case. Equation 1 gives the desired substratethickness to achieve good efficiency for SW generation (whenεr >10) [12]:

h =λ0

4√εr

(1)

where λ0 is the relevant free-space wavelength.As the substrate thickness is increased the SW coupling ef-

ficiency can also increase. In our SIW structure, the employedsubstrate has a thickness of 1.27 mm and this has been shownto be adequate for efficient SW launching at about 20 GHzand above [4]. To ensure efficiency coupling from the SIWguide to the SW section it is also important to operate abovethe cut off frequency for the SIW TE1-like mode [5].

When designing the launcher, diameters and pitch of thevias were chosen to negate unwanted power leakage into thesubstrate periphery. It was noted that when using a structurewith a larger aperture, higher-order field configurations couldpropagate causing destructive interference and reduce SWexcitation efficiency. This is illustrated with the antenna withthe largest aperture (see Fig. 2 [top]). To avoid this unwantedfield configuration, other aperture sizes of the SIW hornwere explored (see Fig. 2). It was observed that, when usinga smaller angle for the SIW aperture, the S-parameters ofthe structure also improved. More importantly, the smallestaperture in Fig. 3 shows the SW propagating into the dielectricregion with a cylindrical-like field configuration which isdesired in our case.

The developed SIW-SWL has four parts to it: the microstripinput transmission line, the taper to SIW, the SIW horn aper-ture and the SW section. Also, all sections should have goodfield matching to avoid losses. The microstrip line operates bythe quasi-TEM mode, then there is a transition from microstripto the SIW which operates by the dominant TE10-like mode.The GDS supports SWs which are defined by the TM0 mode.

III. SIMULATIONS AND MEASUREMENTS

An SIW horn SWL operating at 24 GHz was designed andfabricated. The SWL was fed using a 50-Ω input microstriptransmission line. The annotated structure is shown in Fig. 4.The dimensions are as follows: W1 = 6 mm, W2 = 9.5 mm,L1 = 7.8mm, L2 = 18.2 mm, P = 2.6 mm and D = 2 mm.

Numerous simulations and optimizations were carried out toobserve the launchers operation and characteristics. A planar,single-ended launcher was first tested with an extended dielec-tric region. This region was extended to see the behaviour ofthe SWs along the air-dielectric interface. As shown in Fig. 3,

Fig. 2. An illustration of the E-fields for different apertures explored.

Fig. 3. E-field simulation of the SIW-SWL at 24 GHz.

it can be observed that the bound SW fields have a cylindricalpattern, typical to that of a horn antenna, as they travel awayfrom the launcher.

The simulated and measured reflection coefficient is shownin Fig. 5. It can be seen that at 24.5 GHz the measuredreflection coefficient is around -15 dB. Also, the bandwidthof both the measured and simulated reflection coefficient canbe said to be wide. More specifically, the measured |S11| forthe launcher is below -10 dB from around 24.1 GHz to 25.5GHz as shown in Fig. 5.

The difference between the measured and simulated resultscan be explained by fabrication tolerances due to the in-house

Fig. 4. Schematic of the proposed SIW horn antenna for SWs.

Fig. 5. Simulated and measured reflection coefficient versus frequency forthe measured prototype of finite size in Fig. 6. Results are also compared toa simulated structure considering an infinitely size substrate.

Fig. 6. Bottom (left) and top (right) of the fabricated structure.

fabrication process. Each via was created by a mechanicaldrilling machine which had a tolerance of about 0.1 mm to0.2 mm. This practically can explain the differences betweenthe measurements and simulations as can be observed in Fig.5. The top and bottom copper etching could also have beenmisaligned and this can further explain the minor disagreementbetween simulated and measured results as well. Images of thefabricated structure can be seen in Fig 6.

IV. CONCLUSION

A new type of SWL for the generation of SWs has beenproposed. This new approach uses a directive SIW planarhorn antenna to generate SWs in the end-fire direction. Thesimulation and measurement results are in agreement whichsuggests high SW launching efficiency. From the simulationand measurement results, it can be seen that the launcheroperates over a wide bandwidth as well. Applications includethe feeding of planar leaky-wave antennas and other SW-basedstructures.

ACKNOWLEDGMENT

This work was supported by Leonardo UK through theEngineering and Physical Science Research Council Centre

for Doctoral Training in Embedded Intelligence (grant no.EP/L014998/1). The project was also supported by the Euro-pean Horizon 2020 project CSA-EU. The authors would liketo indicate that the work is only the author’s views, and that theHorizon 2020 Agency is not responsible for any informationcontained in the paper.

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