Design of passive components (couplers, filters and antennas) and MMIC packaging validation intended for RF front ends for microwave links at 38 GHz using gap waveguide technology have been shown. The use of this technology allows all-in-one integration of receiver, transmitter, and diplexer including the antenna into one mechanical unit. These initial designs and studies have been made at the microwave band for validation and measurement purposes, and for comparison with existing technology. They represent the first step towards the application of gap waveguides to millimeter-wave systems, where gap waveguides could have a large potential

Alfonso, Esperanza

Kildal, Per-Simon

Pucci, Elena

Zaman, Ashraf Uz

Chalmers Publication Library

Chalmers Publication Library
Gap waveguide components for millimeter-wave systems. Couplers, filters, antennas,
MMIC packaging
This document has been downloaded from Chalmers Publication Library (CPL). It is the author´s
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International Symposium on Antennas and Propagation, ISAP 2012, Oct 29-Nov. 2 2012,
Nagoya, Japan (ISSN: 1522-3965)
Citation for the published paper:
Alfonso, E. ; Zaman, A. ; Pucci, E. (2012) "Gap waveguide components for millimeter-wave
systems. Couplers, filters, antennas, MMIC packaging". International Symposium on
Antennas and Propagation, ISAP 2012, Oct 29-Nov. 2 2012, Nagoya, Japan pp. 243-246.
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Gap Waveguide Components for Millimetre-Wave 
Systems: Couplers, Filters, Antennas, MMIC 
Packaging.  
 
#Esperanza Alfonso, Ashraf Uz Zaman, Elena Pucci, Per-Simon Kildal 
1 Department of Signals and Systems, Chalmers University of Technology, 
SE-412 96 Göteborg (Sweden), esalal1@gmail.com 
 
 
 
1. Introduction 
 
 Gap waveguides were first presented in [1] as an alternative guiding technology especially 
attractive for frequencies over 30 GHz up to THz. At those frequencies, the current technologies 
show some deficiencies regarding to the performance, integration ability, or product cost. Planar 
technologies, such as microstrip and coplanar, are often chosen due to their good integration ability 
and manufacture simplicity, but they suffer from higher losses with increasing frequency as well as 
from the presence of cavity resonances when encapsulated. Hence, hollow waveguides are usually 
resorted for low-loss applications, in spite of their difficulty for integration with active components 
and a high manufacturing cost. The need of new transmission line technologies for mm- and sub 
mm-wave systems is leading to the apparition of alternative technologies. Substrate Integrated 
Waveguide (SIW) technology has been widely used for high-frequency applications [2], but it 
exhibits significant losses at increasing frequencies due to wave propagation in substrate. Gap 
waveguides, on the contrary, support waves in the air gap between two metal plates. One of the 
plates is provided with a texture, in the form of a bed of nails, to create a high impedance condition 
at the surface, which in turn forces a cut-off for the parallel-plate modes [3]. On the same plate, 
there are metal ridges in between the nails providing a path to the waves so that fields are confined 
to the air gap between the ridges and the metal plate on top. This propagation path can alternatively 
be provided by a microstrip line lying on the bed of nails, or by a groove in between the nails. An 
interesting application using similar technology can be found in [4] where a multi-layered phased 
array antenna developed in Japan was presented. 
 On the other hand, RF front ends of cellular radio base stations for point-to-point 
microwave links often make use of microstrip lines to interconnect different components. But for 
low-loss passive components the use of waveguides is usually resorted. For instance, for full-duplex 
systems, the diplexer is a critical component, since it separates the TX and RX channels and 
connects them to a common antenna port. The diplexer filters are normally constructed with iris 
filters in waveguides to fulfil the stringent requirements regarding to the low-loss and high roll-off, 
and they represent a significant product cost. Moreover, these filters contribute to increase the size 
and complexity of the system, as they must be connected to the electronic modules, which contain 
active components and MMICs, mounted on a PCB and interconnected with microstrip lines. An 
additional problem comes out at high frequencies when those circuits are packaged, since metal 
walls and absorbers are the techniques commonly used. Therefore, new solutions have to be 
investigated for radio links at mm- and sub mm-wave frequencies. In this paper, the use of gap 
waveguide technology is proposed as a possible solution. On one hand, gap waveguides have been 
used to package microstrip circuits without creating cavity resonances [5]. On the other hand, they 
can be used as transmission lines to realize passive components like filters and couplers [6]. In this 
way, by making use of these two functionalities, gap waveguides can provide complete integration 
of all parts of the system. Passive and active circuits can be integrated in the same module; even the 
antenna can be included. This is actually one of the main advantages of gap waveguide technology, 
to provide system integration between two parallel-metal plates, which do not require any 
conducting contact between them. As a result, the system becomes more compact and the 
manufacturing difficulty and cost are reduced notably.  
243
  
 This paper presents the progress made so far on the design of passive components and 
MMIC packaging demonstrated for RF front ends for microwave links at 38 GHz using gap 
waveguide technology. Initial designs are made at the microwave band for validation and 
measurement purposes, and for comparison with existing technology. But it is for millimeter- and 
sub millimeter-wave applications where gap waveguides represent a promising solution. 
 
2. Couplers 
 
 A short-slot hybrid coupler at 38 GHz in groove gap waveguide with the specifications 
shown in Table 1 was designed on the basis of the techniques used for waveguide couplers. The 
Riblet short-slot hybrid coupler was constructed by placing two groove gap waveguides side by side 
and removing a section from the center pins separating both waveguides. The length of this section 
determines the coupling. Indentations were provided by adding extra pins to the sidewalls of the 
coupling section in order to match the phase of higher order modes. A pin in the centre of the 
coupling section with reduced height was in addition used with same purpose. Pucks, pins of 
reduced height and increased size, were introduced before and after the indentations for a good 
match at all ports. H-plane 90  bends were used in order to get the required distance to mount 
standard flanges (WR-28) at the coupler ports. Fig. 1 shows the prototype and measurement results. 
4% bandwidth for ± 0.25 dB amplitude imbalance between the two output ports can be seen. A 
phase difference of ± 2.5 , and return loss and isolation better than 20 dB over the entire band were 
obtained. 
 
Table 1: Specifications for the 3 dB hybrid coupler 
Frequency band 37-40 GHz 
Coupling 3 dB 
Isolation 20 dB 
Return loss 20 dB 
 
 
(a) 
 
(b) 
Figure 1: (a) Coupler prototype; (b) Measured S parameters.  
 
3. Filters 
 
 A narrow-band band-pass diplex filter for radio links at 38 GHz in gap waveguide 
technology with the specifications shown in Table 2 was designed. The stringent specifications of 
these filters regarding to the low loss and high selectivity require a number of resonators with high 
value of the quality factor (Q). This high Q is provided by waveguides. Groove gap waveguide 
resonators have been shown to provide values of Q comparable to those provided by rectangular 
waveguides [7]. Hence, groove gap waveguide resonators were chosen for this design. A general 
technique for designing coupled-resonator filters was used, which is based on coupling coefficient 
(K) of intercoupled resonators and the external Q of the input and output resonators. The coupling 
  
between adjacent resonators and the coupling from the input/output resonators to the external 
network (WR-28) is done through ridges, whose dimensions determine the values of the filter 
parameters (K and Q). The optimized design and manufactured prototype can be seen in Fig. 2 and 
Fig. 3a. The band-pass filter is fabricated between two metal plates leaving an air gap between 
them, allowing airing and cooling. Measurements show a minimum in-band insertion loss of 1 dB 
and agree quite well with simulations (Fig. 3b). 
 
Table 2: Specifications for the 38 GHz diplex filter 
Passband 37.058-37.618 GHz 
Stopband 38.318-38.878 GHz 
Insertion loss 1.5 dB 
Attenuation 70 dB 
Return loss 17 dB 
 
 
Figure 2: Sketch of the optimized 7th order diplex filter using groove gap waveguide resonators. 
 
 
(a1) 
 
(a2) (a3) 
 
(b) 
Figure 3: (a) Photos of the silver-plated prototype: (a1) Top and bottom plates (with holes to 
assemble both parts), (a2) Front view, (a3) Side view (with no sidewalls); (b) Filter responses: 
measured (solid line) and simulated (dotted line). Specifications are also shown for reference.  
 
4. Antennas and MMIC packaging 
 
 New antenna designs in gap waveguide technology are now being developed in order to get 
complete system integration. High gain and directive antennas are required for radio links. Hence, 
we are looking into slot arrays and cylindrical reflectors. Gap waveguide distribution networks are 
used to transmit the power to the radiating elements or to generate linear phase fronts (see Fig. 4a). 
 Gap waveguide technology was used to package the active microwave circuits of the RF 
front end for 38 GHz radio link. Validation was done by measuring the isolation between the TX 
and RX amplifier chains, and the maximum stable gain of a single amplifier chain (see Fig. 4b). 
Stable gain higher than 60 dB in 6.5 × 7.3 cm2 was obtained for a chain of 4 amplifiers with 
maximum gain of around 18 dB each. A TX-RX isolation higher than 80 dB is needed in practice in 
order to avoid performance degradation due to undesired crosstalk and feedback loops leading to 
oscillations and system instability. This is fulfilled by the separation of eight rows of pins between 
TX and RX chains. 
36 36.5 37 37.5 38 38.5 39
-80
-70
-60
-50
-40
-30
-20
-10
0
Frequency (GHz)
(dB
)
 
 
37 37.2 37.4 37.6-5
-4
-3
-2
-1
0
 
  
 
(a) 
 
(b) 
Figure 4: (a) Distribution network in ridge gap waveguide; (b) MMIC packaging using conventional 
method, metal walls and absorbers (left lid) and using gap waveguides (right lid). 
 
6. Conclusions 
 
 Design of passive components (couplers, filters and antennas) and MMIC packaging 
validation intended for RF front ends for microwave links at 38 GHz using gap waveguide 
technology have been shown. The use of this technology allows all-in-one integration of receiver, 
transmitter, and diplexer including the antenna into one mechanical unit. These initial designs and 
studies have been made at the microwave band for validation and measurement purposes, and for 
comparison with existing technology. They represent the first step towards the application of gap 
waveguides to millimeter-wave systems, where gap waveguides could have a large potential.   
 
Acknowledgments 
 
This work was supported by The Swedish Governmental Agency for Innovation Systems 
(VINNOVA) within the VINN Excellence Center Chase and the Swedish Research Council VR. 
 
References 
 
[1] P.-S. Kildal, E. Alfonso, A. Valero-Nogueira, and E. Rajo-Iglesias, “Local metamaterial-based 
waveguides in gaps between parallel metal plates,” IEEE Antennas Wireless Propagat. Letters, 
vol. 8, pp. 84-87, 2009. 
[2] Y. Cheng, P. Chen, W. Hong, T. Djerafi, and K. Wu, “Substrate-Integrated-Waveguide 
beamforming networks and multibeam antenna arrays for low-cost satellite and mobile 
systems,” IEEE Antennas and Propagation Magazine, vol.53, no.6, pp.18-30, Dec. 2011. 
[3] P.-S. Kildal, A. Zaman, E. Rajo-Iglesias, E. Alfonso, and A. Valero-Nogueira, “Design and 
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[4] H. Kirino, K. Ogawa, “A 76 GHz multi-layered phased array antenna using a non-metal contact 
metamaterial waveguide,” IEEE Trans. Antennas Prop., vol.60, no.2, pp.840-853, Feb. 2012. 
[5] E. Rajo-Iglesias, A. Zaman, and P.-S. Kildal, “Parallel plate cavity mode suppression in 
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[6] E. Alfonso, M. Baquero, P.-S. Kildal, A. Valero-Nogueira, E. Rajo-Iglesias, and J.I. Herranz, 
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[7] E. Pucci, A. Uz Zaman, E. Rajo-Iglesias, P-S Kildal, and A. Kishk, “Study of Q-Factors of 
ridge and groove gap waveguide resonators,” IEEE Trans. Components, Packaging and 
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