A K/Ka-Band Reconfigurable Substrate Integrated Coaxial Line to Waveguide Transition Technology

In this article, the designs of the K/Ka-Band reconfigurable Substrate Integrated Coaxial Line (SICL)-to-waveguide transitions are presented. Two types of reconfigurable SICL-to-waveguide transitions, with stepped impedance transformers waveguide transitions and ridge waveguide transitions, are designed for two operation frequency modes (K/Ka-band). Both simulated and experimental results are presented for the demonstration. Results indicate that the reconfigurable waveguide structure has the advantages of broad bandwidth and low return loss for both K/Ka-band compared with the conventional dual-band waveguide transitions. The presented waveguide transition designs with high function flexibility will be significant to satellite communication applications and massive MIMO beamforming networks with an active phased antenna array.

waveguide transitions at K/Ka-band (20 GHz/30 GHz). For this purpose, the two types of reconfigurable SICL-towaveguide vertical transitions with two operation frequency modes (K/Ka-band) were developed. They provide promising solutions to achieve better wideband performance for the K/Ka-band separated Rx and Tx, which traditionally have two different systems for Rx and Tx with different dimensions. These two types of reconfigurable SICL-to-waveguide transitions have the same waveguide dimension for the same operation mode. Furthermore, the two K/Ka-band mode waveguides share the same SICL PCB feeding network in beamforming applications. Only metallic waveguide parts need to be replaced to achieve K/Ka-band (Tx/Rx) operation modes switching, saving cost, simplifying installation and maintenance, and providing flexible functions.

II. DESIGN OF THE SICL-TO-WAVEGUIDE TRANSITION
In a beamforming system, the SICL-to-waveguide transition connects the feeding network in PCB and the waveguide antenna array. An example of a 2×2 beamforming antenna array is illustrated in Fig. 1 to show the beamforming system network. The phase and gain control section would be installed at the bottom of the multilayer PCB feeding network.

A. RECTANGULAR WAVEGUIDE TRANSITIONS WITH STEPPED TRANSFORMERS
The SICL structure, shown in Fig. 2, provides low loss and wideband performance for a high-frequency multilayer PCB feeding network in a beamforming system. Its two side rows of metallic vias reduce resonance interference in the beamforming feeding network in a compact structure [3]. The adaptable patches on top of the PCB could help match the impedance for the two modes and reduce the interference between them to achieve dual-mode operations at K/Ka-band, shown in Fig. 3 (a) and (b). Contrary to the conventional designs, the two modes for K/Ka-Band share the same PCB feeding network. The waveguide parts with two different dimensions for K/Ka-band connect to the same PCB. They can be installed and replaced easily (screws), as demonstrated in Fig. 1, to achieve different modes of operation at different frequencies. The stepped transformers are adopted in this waveguide transition design to increase the impedance from SICL to the waveguide and convert the propagation mode from quasi-TEM in a SICL to TE10 in a waveguide [15]. The side view and top view of the rectangular SICL-to-waveguide transition with stepped transformers are illustrated in Fig.4   To meet the requirements in a beamforming system to provide a better antenna elements distance for better performance at K/Ka-band, there is a dimension limitation of 10 mm for every element. Therefore, the broad wall width of the K-band and Ka-band modes waveguides in this design are 9 mm and 8.5 mm, respectively. When the broad wall is 9 mm, 2 VOLUME 4, 2016 This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2022.3184020 the cut-off frequency is around 16.7 GHz, and the waveguide still works in the required frequency range for K-band. The narrow wall width is around half of the broad wall width.
The SICL structure and the waveguide are connected by a via, whose equivalent circuit model is shown in Fig. 6 (a) [16]. The stepped transformers from via (Zvia) to waveguide (Zws) could be represented by the equivalent circuit model shown in Fig. 6 (b) [17]. The reactive energy of the fringing fields at each waveguide step is represented by susceptances B1, B2, B3 and B4. The Zd1, Zd2 and Zd3 represent the impedances of stepped transformers.

B. RIDGE WAVEGUIDE TRANSITIONS
The reconfigurable ridge waveguide transitions have a similar design to the waveguide transitions with stepped transformers to achieve K/Ka-band dual-mode operation by using the adaptable patches on top of the PCB. The identical waveguide dimensions and adaptable patches make these two types of SICL-to-waveguide transitions match the same PCB feeding network in a beamforming system for the same operation mode.
The ridge waveguide structure is adopted in this design to get a broader bandwidth performance [18] [19]. The loss of waveguide transitions with stepped transformers for K-band mode below 18 GHz increases due to its cut-off frequency being around 16.7 GHz. The ridge waveguide structure provides a lower TE10 cut-off frequency for better bandwidth performance for the same broad wall width to meet the requirements for a beamforming system.
The side view and top view of the ridge structure SICL-towaveguide transition are illustrated in Fig. 8 (a) and (b), and the E-field distributions are shown in Fig. 9 (a) and (b).
The ridge SICL-to-waveguide transitions simulation results are shown in Figs. 10 (a) and (b). They demonstrate wider bandwidth performance for both K/Ka-band operation modes with a bandwidth of 50.5% (16.3 GHz -27.3 GHz) VOLUME 4, 2016 and 43.7% (20.4 GHz -31.8 GHz) for return loss better than 10 dB, respectively, compared with the SICL-to-waveguide transition with stepped transformers. In a beamforming application, adding an 8.5 mm × 4.25 mm to 9 mm × 4.5 mm waveguide transition to the Kband mode waveguide port does not significantly influence the performance and helps the two modes designs have the same waveguide dimension on both K/Ka-band. Therefore, the two K/Ka-band waveguides could connect and feed the same wideband waveguide antenna array in a beamforming system with separated Rx and Tx, improving flexibility and cost-efficiency and simplifying installation. The cavity height L, via diameter Dv, ridge depth d1 and stepped transformer width W are critical to these two SICL-to-waveguide transition designs. The analysis in Figs. 11 (a), (b), (c), and (d) demonstrates the effects of these transition parameters on the K-band mode ridge SICL-towaveguide transition. The cavity height L is critical for the propagation mode transformation from a SICL to a waveguide. The via diameter Dv and width W of the ridge (or the stepped transformers) significantly influences the operation frequency range. The operation range moves to a higher frequency when the via diameter Dv increases or the ridge width W decreases. The ridge depth d1 helps the impedance matching to get better performance.

III. EXPERIMENT RESULTS
A grounded coplanar waveguide (GCPW) structure was deployed and extended from the SICL to the edge of the test ports on a K-connector to facilitate experimental testing of the two kinds of designed SICL-to-waveguide vertical transitions. The fabricated SICL-to-waveguide transition is shown in Fig. 12 (a). The back-to-back structure was used in the measurement, shown in Fig. 12  The SICL-to-waveguide transition with stepped transformers measured minimum insertion loss (per transition) is 1.2 dB for K/Ka-band. The measured back-to-back structure bandwidths (return loss below 10 dB) are around 35.8% (17.4 GHz-25.0 GHz) and 25.0% (24.5 GHz-31.5 GHz) for K/Kaband, respectively.
Both reconfigurable SICL-to-waveguide vertical transitions with stepped transformers and ridge SICL-towaveguide vertical transitions provided wideband performance at K/Ka-band. Table. 1 summarised and demonstrated the performance of recently published waveguide transition designs. Compared with the recent dual-band waveguide transition designs (with around 3% to 16% bandwidth), most waveguide transition designs with a single operational frequency band mode have better wideband performance and insertion loss. The comparison shows that the two presented designs have considerably better wideband performance than the recent dual-band waveguide transitions with around 25% to 45% measured bandwidth and could achieve the dualband operation at K/Ka-band simultaneously by using the reconfigurable structure. Furthermore, the sizes (estimated dimensions of the transition designs which exclude the 50ohm input and waveguide output) of all cited designs and these two proposed designs are listed in the table. The proposed designs have a compact footprint with 0.7λ and 0.9λ dimensions for K/Ka-band, respectively, applicable for an antenna array in MIMO beamforming applications at K/Kaband.

IV. CONCLUSION
A K/Ka-band reconfigurable SICL-to-waveguide transition technology has been presented in this article. Two SICL-to-  His research interests include the Multi-layer MIMO beamforming system based on the active phased array. VOLUME 4, 2016 7 This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/ACCESS.2022.3184020