## 1 Introduction  
Owing to the development of integrated circuits (ICs), the millimeter wave spectrum has been receiving more and more attention. Many applications in the millimeter-wave band are under development, such as 5G communication [1], 77/79 GHz automotive radars [2; 3], wireless backhaul [4], and 94 GHz radar/imaging systems [5; 6]. Benefiting from the low loss and high power capacity, the waveguide components are widely used in millimeter-wave systems, such as high-\(Q\) filters, horn antennas, power dividers, etc. However, waveguide components cannot be directly connected to the monolithic microwave integrated circuits (MMIC). The microstrip line is an essential component for interconnecting with the MMIC. Thus, a microstrip-to-waveguide transition is required to integrate the waveguide with the MMIC.  
Various technologies have been explored for realizing microstrip to waveguide transition. In [7; 8; 9; 10], these transitions using a multi-stage impedance transformer can achieve wide working bandwidth. For example, a microstrip-to-waveguide transition is presented in [8] by coupling energy from the microstrip line to the stepped ridge and subsequently to the rectangular waveguide. The transition achieves working bandwidth of 38% from 50 to 72 GHz and insertion loss below 0.4 dB. However, the disadvantage of this type of structure is that they have a large volume due to the multi-section waveguide or ridge for the impedance transformer. The transitions employing antipodal fin-line [11; 12; 13; 14] are another type of broadband structure. However, they require a relatively long length to realize wide bandwidth. A microstrip-to-waveguide transition based on an H-plane split waveguide is proposed in [15]. The conversion from microstrip mode to TE\({}_{10}\) mode of the waveguide is realized by a resonant cavity. The transition realizes a working bandwidth of 35%. In [16],the transition using quasi-Yagi antenna is designed and it achieves a bandwidth of 35%. However, the quasi-Yagi antenna requires a high dielectric constant substrate to achieve a shorter arm length for mounting to the waveguide. The E-plane probe-type transition has the characteristic of low insertion loss and wide impedance bandwidth. Several in-line transitions based on an E-plane probe or dipole are proposed in [17; 18; 19; 20]. They achieve wide working bandwidth covering full-band and low insertion loss. Nevertheless, an extra bend waveguide will be required if the input and output ports need to be arranged vertically, which will cause additional loss and large volume. An E-plane probe vertical transition from the microstrip line to the waveguide is reported in [21]. The probe deviates from the center of the waveguide to obtain a good impedance matching, which will increase the difficulty of manufacturing and assembly.  
In this work, an E-plane probe transition with a perpendicular arrangement is proposed in W-band. The probe is embedded along the center of the waveguide broad wall to excite the TE10 mode. To improve impedance matching, a parasitic patch is introduced to the E-plane probe. The introduction of the parasitic patch generates a new resonance mode. Therefore, both bandwidth and impedance matching can be improved. The proposed transition can achieve wide impedance bandwidth covering the full W-band with a reflection coefficient below \(-\)20 dB. Moreover, a back-to-back is designed and fabricated for verification. The experimental results prove a good performance of the transition.