Conference PaperPDF Available

Microstrip to Ridge Gap Waveguide Transition for 28 GHz Steerable Slot Array Antennas

Microstrip to Ridge Gap Waveguide Transition for
28 GHz Steerable Slot Array Antennas
Alireza Bagheri∗†, Hanna Karlsson, Carlo Bencivenni, Abolfazl Haddadi,
Thomas Emanuelsson∗‡, Andr´
es Alay´
on Glazunov†‡
Gapwaves AB, Gothenburg, Sweden
University of Twente, Enschede, Netherlands
Chalmers University of Technology, Gothenburg, Sweden
Abstract—In this paper three types of contactless vertical
transitions from microstrip to double ridge waveguide are
presented. The designs are compact in size and robust, with
improved isolation by employing a pin structure, making them
ideal for 5G mmWave phased arrays. All transitions cover the
26.529.5GHz band, their dimensions are less than half a
wavelength in pitch and have insertion losses less than 0.6dB.
The three designs apply different matching strategies and offer
a trade-off between bandwidth and PCB areas. Finally, the
behavior within an array configuration is analyzed.
It is well-known that the gap waveguide is an advantageous
wave guiding solution for mmWave frequency bands. Indeed,
it provides attractive characteristics such as low-loss, wide
bandwidth and ease of manufacture [1]. This technology is
based on the concept of two parallel PEC and PMC plates,
which when placed less than a quarter of wavelength apart,
prevents wave propagation in undesired directions while al-
lowing wave propagation along a desired path. A guiding
structure is then obtained when the appropriate geometry is
inserted, e.g., in the form of the groove, the microstrip and the
inverted microstrip gap waveguides [2]. Similarly to classical
waveguides, a ridged gap waveguide is obtained by placing
a metal strip in a hollow cavity. This configuration supports
a quasi-TEM propagation mode in a compact and low-loss
fashion. In order to implement the PMC boundary, a bed of
periodically arranged pins is typically used as an artificial
magnetic conductor (AMC). In practice, the contactless nature
of the gap waveguides helps for mass-producible antenna
solutions, especially at microwave and mmWave frequencies
However, all microwave technologies must offer good inte-
gration with PCB technology as the carrier of most, if not all,
active components. It is thus important that such transitions
offer good performance, robustness and simple integration.
This is especially true in phased arrays for 5G applications,
where the large number of densely packed antenna elements
create issues of spacing and isolation [6], [7]. The three types
of transitions presented in this paper target specifically this
scenario and has thus been tested in an array configuration
similar to previously published arrays [8], [9].
In the past years, a large number of microstrip to gap
waveguide transitions have been designed. A horizontal con-
tactless transition is presented in [10], but the one wavelength
width makes it unsuitable for phased arrays. This issue is
solved in [11] which is a horizontal, compact and contactless
transition. However, verticals transition are the only practical
solution for phased arrays as they leave the space below the
antenna available for PCB routing and active components.
Ultra-wideband transitions have also been proposed in [12],
[13], but the need for galvanic contact between waveguide
and microstrip make them sensitive to assembly [14]. Double-
and single-ridge gap waveguides allow to design narrow
waveguide-based radiating elements to be used in phased
arrays with wide steering ranges. The proposed transitions
can be used for gap waveguide-based arrays which has been
investigated to be a promising solutions for upcoming 5G
applications [5].
In this paper, three types of contactless vertical transitions
with application in antenna arrays based on gap waveguide
technology are proposed. Section II describes the design
of transitions, Section III considers the integration with an
antenna array with center-fed elements. Section IV provides
the conclusions.
The structure of the transitions is shown in Fig. 1. All three
transitions consist of a PCB with a rectangular patch coupled
to a double-ridge waveguide surrounded by a pin structure.
The double-ridge waveguide has dimensions 4.25 ×2.1mm
with a ridge height of 1.4125 mm and a ridge width of
1.4mm. The double-ridge is extended towards the PCB
on each side of the waveguide opening to ensure sufficient
coupling between the transition patch and the waveguide. The
pin structure is designed to have a stopband in the desired
frequency range, as to increase isolation between adjacent
antenna columns, decrease leakage from microstrip patch, and
consequently improving signal transmission level to double-
ridge waveguide. The role of pin structure is very essential and
the transitions do not work without them. The pin dimensions
are 2.7×1.58 mm with a pin period of 2.8mm. A 10 mils
Rogers RO4350B-substrate with a relative dielectric constant
of εr= 3.66 is used for the PCB, with a copper thickness
of 0.035 mm and a microstrip width of 0.5mm. The specific
transition type configuration is explained below:
Type 1 transition has a matching stub in series with the
transition patch as shown in Fig. 1a. Both the size of the
Port 6
Port 5
Port 4
Double ridge
Transition patch
Matching stub
Port 1
Port 2
Port 3
(a) Type 1
Transition patch
with inset feed
(b) Type 2
Transition patch
λ/4 transformer
(c) Type 3
Fig. 1. The structure of three microstrip to double ridge waveguide transitions. In these figures, substrate and ground layer of microstrip lines are not shown.
24 26 28 30 32
(a) Type 1
24 26 28 30 32
(b) Type 2
24 26 28 30 32
(c) Type 3
Fig. 2. Scattering parameters of each transition.
Transition Bandwidth [GHz] Max. insertion loss [dB]
in 26.529.5GHz Advantage Disadvantage
|S22|<15 dB |S22 |<10 dB
Type 1 6 (21.3%) 6.8 (24%) 0.62 Wideband Not Compact
Type 2 3.7 (13.3%) 5.4 (19.2%) 0.6Compact on PCB Narrowband
Type 3 4.6 (16.8%) 5.8 (21.2%) 0.49 Relatively wideband -
stub and the distance from the patch is tuned to achieve
optimal performance. The patch and stub dimensions are
2.42 ×2.6and 2×2.73 mm2, respectively and have
1.7mm distance.
Type 2 transition is shown in Fig. 1b. It uses an inset of
the feed into the patch. This gives a more compact design
compared to the type 1 transition. Patch dimensions are
2.54 ×2.73 mm2and inset gap and length are 0.42 and
0.24 mm, respectively.
Fig. 1c shows type 3 transition. It uses a quarter-wave
transformer to transform the impedance of the microstrip
to that of the transition patch. The patch and transformer
dimensions are 2.6×2.68 and 0.43 ×2.44 mm2.
The transition patch is optimized individually for each tran-
sition type regarding its length, width and center point offset
relative the center point of the waveguide.
Fig. 3. Integration of transition type 1 with a 5G antenna array based on gap
waveguide technology [9]. A part of slot layer is cut for viewing purposes.
The number of port for each microstrip line is shown in left side picture.
25 26 27 28 29 30 31
(a) Type 1
25 26 27 28 29 30 31
(b) Type 2
25 26 27 28 29 30 31
(c) Type 3
Fig. 4. Reflection coefficient and coupling coefficients with neighbor ports in the array, when the transitions are used to excite the antenna.
The software Computer Simulation Technology (CST) was
used for simulating and optimizing the three transition types.
The scattering parameters of each transition are shown in
Fig. 2a- 2c, where the port numbering corresponds to Fig. 1.
For transition type 1, the scattering parameters in Fig. 2a
shows a wideband performance with a 15 dB return loss
bandwidth of 21.3% while having an insertion loss of max-
imum 0.62 dB within the band of interest. The coupling
to the adjacent transition columns is kept below 15 dB
throughout the whole simulation range and is less than 20 dB
for 26.529.5GHz. The scattering parameters for transition
type 2 is shown in Fig. 2b displaying a 15 dB return loss
bandwidth of 13.3% and an isolation towards neighboring
columns higher than 17 dB between 26.529.5GHz. The
maximum insertion loss within the frequency band is 0.6dB
for transition type 2. For type 3, the results are displayed in
Fig. 2c and shows a 15 dB return loss bandwidth of 16.8%
with a maximum insertion loss of 0.49 dB.
A summary of the performance of all three transition types
is given in Table I, where advantages and disadvantages are
also listed. Comparing the results shows that transition type
1 gives a wideband performance, while being less compact to
other solutions. Type 2 is the most compact of the transitions
but comes with the disadvantage of being less wideband
compared to type 1 and 3.
To further study the performance of the proposed transitions,
we consider the integration with a previously designed phased
array based on gap waveguide technology [9]. Note that neither
the antenna nor the transition has been tuned to each other,
thus sub-optimal performance is expected. The front and back
view of the complete antenna and type 1 transition assembly
are shown in the left and right side of Fig. 3, respectively.
The array is composed by 8center-fed subarrays of 8 longitu-
dinal and horizontally polarized slots. The horizontal spacing
between columns is 5.6mm (0.52λ0at 29.5GHz).
Fig. 4 shows the embedded scattering parameters of the
array ports, when the three types of transitions are used. Due
to symmetry in the structure, simulation results for ports 5
to 8are omitted. The antenna array integrated with every
types of transitions has scattering parameters below 10 dB,
without any retuning for the new transitions. The coupling
coefficients for adjacent ports are better than 13 dB, with
the non-contiguous being below 20 dB.
Three types of microstrip-to-ridge-gap-waveguide are pre-
sented in this paper. Their characteristics, including return
loss, insertion loss and isolation are discussed. Transitions
are vertical, compact and contactless. They are designed for
26.529.5GHz, which all types show insertion loss less
than 0.6dB, return loss less than 15 dB and isolation
better than 20 dB. These features make them ideal for 5G
mmWave phased arrays based on gap waveguide technology.
All transitions are designed for 28 GHz as the center frequency
and their performance is evaluated when integrated with a
previously designed antenna array.
This project has received funding from the European Union
Horizon 2020 research and innovation program under the
Marie Skłodowska-Curie grant agreement No. 766231 WAVE-
COMBE H2020-MSCA-ITN-2017.
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... In order to maximize the power transfer to the radiating element from the GaN-based RFFE, a through-substrate probe transition from microstrip to ridge GW has been used. The vertical, compact and contactless transition is a practical solution for phased arrays as it leaves space below the antenna for PCB routing and active components [24]. The designed transition simulation and measurement results with a two-layer 10 mil Rogers RO4350 substrate are reported in [14]. ...
p>A high equivalent isotropic radiated power (EIRP) active phased array antenna system has been designed and experimentally verified at the 28 GHz band. The phased array employs Gallium Nitride (GaN) based radio frequency front-ends with 31 dBm output power in transmit mode and 3.5 dB noise figure in receive mode. A fully metallic gapwaveguide technology has been employed in order to achieve an efficient heat dissipation per aperture area of the array as well as low-loss array antenna elements that are easily manufactured. The phased array is realized by sub-arraying an 8×8 slot array antenna with horizontal polarization. The presented antenna system is capable of analog beamforming in the range of ±60 degrees in E-plane. The presented high-bandwidth phased array antenna system is a potential candidate for high power and compact size 5G base station antennas for wireless communications requiring high temperature stability at the millimeterwave bands.</p
This article discusses three integration and packaging techniques for gap waveguides and monolithic microwave integrated circuits (MMICs) suitable for multilayer waveguide applications. Two vertical transitions between microstrips and ridge gap waveguides (RGWs) are presented. The first vertical transition connects RGW to a microstrip line from the top where a rectangular patch has been used. Measured results of the transition in a back-to-back structure show that the reflection coefficient is better than −10 dB from 75 to 83 GHz, and the insertion loss for a single transition over the frequency band is 0.65–0.85 dB. The second vertical transition connects RGW to a microstrip line from the back by a slot in the ground plane. Measured results of the transition in a back-to-back structure show that the reflection coefficient is better than −10 dB from 69 to 86 GHz, and the insertion loss for a single transition is 0.65–1 dB over the frequency band. Commercially available E-band MMIC amplifiers are integrated with RGWs using the two proposed transitions. Moreover, for the very first time, the integration of MMIC with inverted microstrip gap waveguide (IMGW) is realized by a compact packaging structure that utilizes bond wires and capacitive pads. All the three active integrations are consistent with the passive measurements in terms of operational bandwidth, losses, amplifier gain flatness, and unwanted resonance suppression.
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In this letter design of a novel, wideband and low loss microstrip line transition to ridge gap waveguide is described. The transition is made up of two sections. The first section is a microstrip line that feeds a slot on the ground plane. The microstrip line is placed inside a metal box. The second part is the slot to ridge gap waveguide matching section. The proposed design has a broadband coupling to ridge gap waveguide and covers the whole stop-band of the periodic gap structure. The design optimization is performed for the microstrip line, slot size and ridge gap waveguide matching ridges. A prototype in Ku-band is manufactured and measured that shows a return loss less than 14 dB and a back-to-back insertion loss smaller than 0.5 dB for the given bandwidth. The microstrip transition enables integration of monolithic microwave integrated circuits (MMICs) in ridge gap waveguide technology.
This paper presents a gap waveguide based compact monopulse array antenna, which is formed with four unconnected layers, for millimeter-wave tracking applications at W-band (85105 GHz). Recently developed gap waveguide technology removes the need for galvanic contact among metallic layers of waveguide structures, and thereby, makes the proposed antenna suitable for easy and low-cost manufacturing. In this context, a low-loss planar Magic-Tee is designed to be used in a monopulse comparator network consisting of two vertically stacked layers. The gap waveguide planar monopulse comparator network is integrated with a high-efficiency 1616 corporate-fed slot array antenna. The measured results of the comparator network show the amplitude and phase imbalance values to be less than 0.5 dB and 2°, respectively, over the frequency band of interest. The fabricated monopulse array antenna shows relative impedance bandwidths of 21% with input reflection coefficients better than 10 dB for the sum and difference ports. The null in the difference radiation pattern is measured to be 38 dB below the peak of the sum radiation pattern at 94 GHz. The measured gain is about 30 dBi for the same frequency. The low-loss performance of the comparator network and the feed-network of the proposed array, together with the simple and easy manufacturing and mechanical assembly, makes it an excellent candidate for W-band compact direction-finding systems.
A 42 element microstrip parasitic patch antenna is developed in the millimeter-wave band for 5th Generation (5G) mobile communication base stations. A metalized elliptical stripline to embedded microstrip transition with adaptive via hole arrangement as well as a 20dB Chebyshev tapered 6 way power divider is proposed to have an insertion loss of 0.045 dB. To confirm the feasibility of the antenna, the antenna has been measured to provide a 6.2 % impedance bandwidth from 26.83 GHz to 28.56 GHz at VSWR of less than 1.96 dB. The array antenna gains of more than 21.4 dBi has been realized with side lobe levels (SLLs) of less than -19.4 dB operating within 27.5 GHz to 28.5 GHz in both the azimuth and elevation directions.
This paper describes the design and realization of a transition from a microstrip line to a ridge gap waveguide operating between 95 and 115 GHz. The study includes simulations, measurements, and a Monte Carlo analysis of the assembly tolerances. The purpose of this tolerance study is to identify the most critical misalignments that affect the circuit performance and to provide guidelines about the assembly tolerance requirements for the proposed transition design.