# Design of a Broadband Transition from a Coaxial Cable to a Reduced-Height Rectangular Waveguide
## 2 Review of Existing Work  
Previously, [PERSON] and coworkers proposed a broadband CWT with a corner transition structure implemented between a coaxial line with an inner/outer radius of 1.52/3.50 mm and a rectangular waveguide with the narrow-to-wide wall ratio or the waveguide height(\(b\))-to-width(\(a\)) ratio \(b/a\) of 5/35 [5]. The transition structure consisted of a rectangular waveguide with an input guide size of \(a\)\(\times\)\(b\) = 35 mm \(\times\) 8 mm and an output guide size of \(a\)\(\times\)\(b\) = 35 mm \(\times\) 5 mm, and a coaxial line terminated by a disk probe. The disk was directly connected to the inner conductor of a coaxial line that served as a probe. A rounded step was implemented in the rectangular waveguide to ensure a smooth transition between the input and the output waveguides and to reduce the possibility of electrical breakdown. This CWT achieved a reflection coefficient of less than \(-\)20.8 dB at 5.0-7.5 GHz. Another work by [PERSON] and coworkers [6] implemented a similar structure for a transition with an input waveguide of \(a\)\(\times\)\(b\) = 28.5 mm \(\times\) 7.61 mm and an output waveguide of \(a\)\(\times\)\(b\) = 28.5 mm \(\times\) 5.5 mm. These dimensions of the reduced-height CWT were obtained by numerical optimization for a return loss of greater than 30 dB at 6.1-10.6 GHz.  
A conically-coupled CWT was introduced by [PERSON] and coworkers [7] for a reduced-height waveguide with \(a\)\(\times\)\(b\) = 12.94 mm \(\times\) 1.40 mm covering 14.5-18.5 GHz. In this work, the CWT was implemented using a conical probe, which was fed by a coaxial line with a microstrip line. The conical probe served as a stepped-transformer stub, ensuring a good impedance matching between a coaxial line and a reduced-height waveguide. Tuning the radius of the conical probe yielded optimum performance with a return loss of >20 dB at 14.5-18.5 GHz.  
CWT transitions can vary depending on the structure of the waveguide. For example, the transition from a coaxial line to dielectric-filled waveguides would be more challenging. An airgap CWT configuration was implemented by [PERSON] and coworkers [8] for a dielectric-filled waveguide with a small value for \(b/a\). The coaxial probe's diameter was increased gradually, followed by a tapered profile to reduce the input inductance. The airgap served as a capacitor to further reduce the input inductance. A return loss of >20 dB across 1.2-2.4 GHz was recorded using a conical probe in an 83 mm \(\times\) 10 mm waveguide filled with a dielectric material of \(\varepsilon_{r}\) = 2.53.  
In [9], [PERSON] and coworkers proposed a CWT consisting of a rectangular-to-ridge waveguide transition and a ridge waveguide-to-coaxial transformer. The ridge width and height and the waveguide width and height were chosen to allow a uniform distribution of the electric field at the interface where a ridge waveguide comes into contact with a coaxial line. In the case of the coaxial-to-waveguide transformer, the inner conductor of the coaxial line was shorted to the wall of the waveguide through a taper. This work reported a return loss of greater than 10 dB at 0.70-2.50 GHz with a 250 mm \(\times\) 25.4 mm waveguide.  
[PERSON] and [PERSON] [10] employed a tapered section of a double-ridge waveguide to achieve a right-angle coaxial-to-rectangular waveguide transition with \(a\)\(\times\)\(b\) = 150 mm \(\times\) 14 mm operating at 1.18-2.91 GHz with a return loss of 20 dB and a higher-order mode level of \(-\)24 dB. [PERSON] and coworkers [11] proposed a right-angle CWT with \(b/a\) = 0.5 operating at 1.47-2.56 GHz with a 15 dB return loss. They employed a printed coaxial probe and a shaped reflecting plate placed between the back short and the probe in their design.  
In this work, we introduce a new broadband transition between a coaxial line and a reduced-height rectangular waveguide. We seek to further increase the operational bandwidth of the transition in comparison to previously reported works.