3.4/4.0 GHz Tunable Resonant Cavity in SIW Technology Using Metal Post and PIN Diode on a Low-Cost Biasing Network for 5G Applications

This paper presents a dual-band resonant cavity in Substrate Integrated Waveguide (SIW) technology to operate in the range from 3.3 GHz to 4.2 GHz, spectrum considered for the Fifth Generation (5G) network. The cavity was designed to operate at 3.4 GHz and 4.0 GHz. To achieve different states, a jumper and a PIN diode switch are considered as switching elements, connecting and disconnecting the bottom and the upper walls of the SIW resonant cavity through a metal post inserted inside its internal volume. A fitting equation is proposed to predict the maximum resonance frequency caused by the insertion of a single metal post inside the internal volume of the SIW resonant cavity and a new low-cost biasing network is designed on a thin dielectric substrate allocated on the upper wall of the SIW resonant cavity, using transmissions lines and a single capacitor, reducing the final cost and the insertion losses. Good agreement was observed among the fitting equation, computational electromagnetic simulations, and experimental results, validating the proposed methods. Index Terms — Biasing Network, SIW technology, Tunable circuits, 5G frequencies.


I. INTRODUCTION
Industry 4.0 is a new concept being widely explored and considered in many companies and institutions of the world due to the impact on the manufacturing and automation technologies. This new term is also referred to as the fourth industrial revolution and considers the insertion of smart systems in the manufacturing and automation processes aiming to increase productivity and to reduce losses, thus improving capitalization. Smart manufacturing is completely changing the production cycle of industries specialized in different kinds of products and goods. In this new concept, machines are equipped with communication systems, sensors, and actuators; all these components are connected to each other, generating a large amount of collected data to be processed. The emerging concept of Intelligence, and computational simulations [1]- [4].
The IoT is a recent technological diffusion that aims to connect billions of smart objects on the internet. These objects are expected to have network connectivity through an Internet Protocol, allowing them to transmit and receive data via internet networks. Several challenges need to be faced 3.4/4.0 GHz Tunable Resonant Cavity in SIW Technology Using Metal Post and PIN Diode on a Low-Cost Biasing Network for 5G Applications for the IoT, such as data reliability and communication performance due to a huge quantity of objects connected [5]- [6].
To improve the performance of the smart communication systems used in Industry 4.0, IoT and to accommodate the explosive growth of data traffic, the Fifth Generation (5G) network is being widely developed and explored, expected to be launched around 2020. The 5G technology is expected to offer data rate above 1 Gbps in a local area network and 500 Mbps in a wide area network, uploading by the insertion of a single metal post inside the internal volume of the resonant cavity and a new lowcost biasing network is also proposed, using transmissions lines and a single capacitor, leading to a low fabrication cost and low insertion losses in comparison with works presented in the literature.
In the remainder of this paper, Section 2 presents the design considerations, proposing a fitting equation and presenting the design of a tunable resonant cavity and including its biasing network.
Section 3 presents the fabricated tunable resonant cavity and the experimental results. Section 4 presents the conclusions. Section 5 shows the acknowledgments and Section 6 shows the references.

II. DESIGN OF A TUNABLE RESONANT CAVITY IN SIW TECHNOLOGY
This section presents the design of a SIW resonant cavity operating in TE101 mode, perturbed by the insertion of a metal post through its internal volume, as shown in Fig. 1. The metal post is permanently connected at the bottom wall of the resonant cavity, being connected and disconnected at the upper wall, resulting in two operating states, with two different resonance frequencies.
As shown in [18] and [20], when the metal post is disconnected from the cavity upper wall, the resonance frequency of this resonant cavity is close to the resonance frequency of the unperturbed cavity. The maximum resonance frequency is achieved when the metal post is connected at the upper wall of the cavity and located at the cavity center, where the electrical field of the TE101 mode has its maximum value. The resonance frequency of the cavity can be tuned between these two frequency limits locating the metal post out of the resonant cavity center. It is important to know the maximum resonance frequency of a perturbed resonant cavity because it imposes limits on the design of reconfigurable SIW filters. With this motivation, an analytical equation is proposed to estimate the maximum resonance frequency of a SIW resonant cavity when perturbed by a single metal via through its internal volume, connecting its upper and bottom metal layers.
A jumper and a PIN diode switch are used as switches to connect and to disconnect the upper and bottom walls of the cavity, as used in [18]. A new biasing network employing transmission lines and lumped components is used to bias the PIN diode, resulting in an easy-to-assemble and low-cost solution. where ℎ is the dielectric substrate thickness, and is the metal post position and is the effective width and is the effective length of the SIW cavity [11]- [16], given by The resonance frequency of the unperturbed resonant cavity ( ) is given by (3) A fitting equation (4) was proposed to predict the resonance frequency of a square resonant cavity perturbed by a single metal post inserted through its internal volume and located at the center of the cavity ( = /2 and = /2), connecting bottom and upper walls, therefore causing its maximum fields perturbation and, consequently, the maximum frequency variation [18].
The is the maximum resonance frequency achieved by a square resonant cavity operating at TE101 mode when perturbed by a single metal post located at its center. To obtain (4), three square SIW resonant cavities using feeding lines and taper lines were designed employing as dielectric substrates RO4003C™, RT/duroid 6006™ and RT/duroid 6010™, with a thickness of 0.635 mm, from Rogers Corporation. Cavities with sides ranging from 20 mm to 40 mm were simulated in the High Frequency Structure Simulator (HFSS), demonstrating resonance frequencies ranging from 2 GHz to 7 GHz. The metal post considered in all computational simulations has a diameter of 1 mm and was located at the center of the cavity.
The several perturbed resonance frequencies obtained by computational simulations were tabulated, whereupon a factor and hyperbole of first degree were considered to obtain an appropriating fitting, as given by The resulting resonance frequencies are presented in Fig. 3. The results presented in Fig. 3 show good agreement between simulated frequencies and the ones predicted by (4), considering dielectric substrates with dielectric constants of 3.35, 6.15 and 10.2. The ratio between and for a square resonant cavity using (3) and (4) is indicating that the maximum resonance frequency increases 18% in comparison with the unperturbed resonant cavity when a 1 mm diameter metal post is inserted at its center, connecting the upper and bottom walls. This frequency variation is due to the changes in the stored electric energy and magnetic energy caused by the metal post inserted in the cavity. According to [17], [20] and [22], it is mathematically described by where ∆ and ∆ are the changes in the stored magnetic energy and electric energy, consecutively, after a shape modification and + is the total energy stored in the cavity. The metal post reduces the cavity internal volume, modifying the electric and magnetic field distributions, therefore causing an increase of the frequency variation. If the metal post is moved from the center of the resonant cavity toward the sidewalls, the resonance frequency decreases, thus reducing the frequency variation.

B. Design of the tunable resonant cavity
A square SIW resonant cavity was designed to resonate at 3.4 GHz and TE101 mode. Inserting a metal post with a 1 mm diameter at the center of this cavity, a resonance frequency of 4.0 GHz is predicted by (5). The designed tunable resonant cavity is fed by a 50 Ω microstrip line and the impedance matching between the feeding line and the resonant cavity is performed by a tapered line.
One end of the metal post is connected to the bottom wall of the cavity, and its other end can be left open or can be connected to the upper wall of the cavity using a switch. The present work uses a PIN diode and a jumper as switching elements.
According to [20] and [22], a metal post connected only at the bottom or upper wall does not significantly change the field distributions inside the resonant cavity; the resonance frequency of the cavity must thus be close to 3.4 GHz in this condition. Therefore, considering (3), (4) and (5), the tunable resonant cavity was designed on a Rogers RO4003C™ and its physical dimensions are presented in Fig. 1 and Table I.

C. Biasing Network
Generally, to use PIN diodes as switching elements, a three-layer PCB implementation is necessary to separate the switching device biasing network from the microwave planar filter. In this paper, the biasing network was designed on a thin RT/duroid 5870 with a thickness of 254 µm, where this dielectric substrate is assembled on the upper wall of the resonant cavity. Figure 4 shows a description of the proposed tunable resonant cavity, where the dotted line denotes the cross-section for a better understanding.  Two paths are considered to connect the metal post to the upper wall of the cavity. The first path uses the metal pin RF in 1 and a PIN diode switch to allow the microwave signal to flow from the upper wall to the metal post. The second path is established using a jumper and the metal pin RF in 2.
The jumper is considered to verify the performance of the biasing network used to bias the PIN diode.
The connection using the jumper path is close to the ideal case when the metal post is directly  In the series configuration, the switch is ON when the diode is forward-biased and OFF when it is reverse-biased. Bias voltages of 0.85V and -15V were used for forward and reverse bias, respectively.
The jumper ON state is close to a short circuit, while its OFF state is considered an open circuit.  Regarding the fabrication process, the SIW walls, metal post, RF in 1 and RF in 2 were implemented using metallic pins of low-cost pinhead connectors. Fig. 6 shows in detail the geometry of the hole in the resonant cavity which metal post goes. The metal post and RF in 1 are spaced by a distance of 1.27 mm, as well the metal post and RF in 2.    Figure 8 shows the S11 parameters of the unperturbed resonant cavity and the electric field plotted on the ground plane at 3.40 GHz. In this case, the metal post was not inserted inside its internal volume.   Table II. According to Table II, state 1 is achieved when the PIN diode is OFF and the jumper is OFF. The S Parameters related to state 1 and the electric field plotted on the ground plane are presented in Fig. 9.   [20] and [22], close to the TE101 unperturbed mode.
According to Table II, state 2 is achieved when the PIN diode is ON and the jumper is OFF. The S Parameters related to state 2 and the electric field plotted on the ground plane are presented in Fig. 10.  According to Table II, state 3 is achieved when the PIN diode is OFF and the jumper is ON. The S Parameters related to state 3 and the electric field plotted on the ground plane are presented in Fig. 11.  This situation is closer to the ideal case when the metal post is directly connected at the upper and bottom walls. Fig. 12 shows all the S11 measured parameters. Figure 12 shows that the resonance frequency of the cavity can be tuned by 19.5% using the jumper to act as an ideal switch. A tuning range of 14.9% was obtained using the PIN switch, having been reduced by the no ideal behavior of the PIN diode at the reverse and forward bias conditions. These tuning ranges are compatible with the 18% range predicted by (5). All the states show good agreement for S11 and S21, validating the proposed fitting equation and the biasing network employing transmissions lines and a single capacitor assembled on the upper wall of the resonant cavity in SIW technology. Some comparisons between this and previous works are summarized in Table III. predict the maximum resonance frequency of a square SIW resonant cavity perturbed by a single metal post inserted inside its internal volume. An error of 3.50% in comparison with the measured value was obtained for the resonance frequency using the proposed fitting equation. Aiming to reduce the fabrication cost and the insertion losses of the dual-band resonant cavity, a new low-cost biasing network was designed on a thin dielectric substrate using transmission lines and a single capacitor.
Experimental and electromagnetic simulation results showed an error of less than 1.3 % for states 1 and 2 and 2.8 % for state 3. The proposed dual-band resonant cavity can be applied as a narrowband filter to frequencies below 6 GHz in receivers and transmitters to be used in 5G networks. The proposed biasing network can be applied to other types of filters based on resonant cavities and waveguides in SIW technology, resulting in a compact and low-cost structure, besides reducing the insertion losses.