## 5 Conclusions  
A simple, small, via-less, impedance-transformer-less, side inserted, E-plane transverse, 90\({}^{\circ}\) hybrid, rectangular waveguide to fishtail-shaped microstrip line transition for Ka-band millimeter wave, wireless, satellite, and RADAR applications was optimized and investigated in the simplest manner possible. It provides a good transmission from the microstrip line end port to the waveguide port end and vice versa. An insertion loss of less than 0.50 dB and return loss lower than \(-15\) dB were achieved with an ultra-wideband fractional bandwidth of 66.15% of the chosen band of interest, with a lower than \(-10\) dB bandwidth. The tapered microstrip line probe inside the rectangular waveguide WR-28 adopts a fishtail-like structure and provides very good impedance matching, and thus the maximum electromagnetic energy flow between rectangular waveguide and microstrip transmission lines, with extremely low losses (3% reflections only). This design \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Ref.** & **Coupling Method** & **Type** & \begin{tabular}{c} **Need of** \\ **Impedance** \\ **Transformer** \\ \end{tabular} & \begin{tabular}{c} **IL** \\ **(\textgreater{}dB)** \\ \end{tabular} & \begin{tabular}{c} **RL** \\ **(\textless{}dB)** \\ \end{tabular} & \
\begin{tabular}{c} **FBW** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **RLC** \\ **Equivalent Circuit** \\ \end{tabular} \\ \hline
[3] & Narrow side wall (b) inserted, E-plane, side inserted magnetic coupling using semi-circular microstrip ring probe & Transverse &
\begin{tabular}{c} Yes \\ (multi-section step \\ transformer) \\ \end{tabular} & \(-0.95\) & \(-10\) & \(33\) & Not Given \\ \hline
[4] & Broadside wall (a) inserted, E- Plane (90\({}^{\circ}\)) & Transverse &
\begin{tabular}{c} Yes \\ (multi-section step \\ transformer) \\ \end{tabular} & \(-0.10\) & \(-10\) & \(40\) & Not Given \\ \hline
[5] & Broadside wall probe insertion through aperture (E-plane), via-less & Transverse &
\begin{tabular}{c} Yes (single-step \\ Impedance Transformer) \\ \end{tabular} & \(-0.30\) & \(-45\) & \(14.54\) & Not Given \\ \hline
[6] & Broadside wall probe insertion, E-plane 90\({}^{\circ}\) transition, Uses rectangular probe & Transverse &
\begin{tabular}{c} Yes (two-section step) \\ \end{tabular} & \(-0.30\) & \(-10\) & \(38\) & Not Given \\ \hline
[7] & Inline E-plane (90\({}^{\circ}\)), radial-shaped probe with notch cut and extended ground & Inline & No & \(-0.30\) & \(-10\) & \(69\) & Not Given \\ \hline
[12] & Q-band transition uses a section of the imaged waveguide and Chebyshev impedance transformer, simple and planar structure, increases the ease of fabrication and the compactness, good return loss, via-less, broad BW &
\begin{tabular}{c} Yes \\ (Chebyshev impedance transformer) \\ \end{tabular} & \(-0.26\) & \(-24\) & \(40\) & Not Given \\ \hline
[10] & Patch above a wedge-shaped cavity, low-cost, single-sided PCB, wideband compact structure and easy integration, high integration density & Inline & No & \(-0.40\) & \(-13\) & \(71.43\) & Not Given \\ \hline
[9] & Inline (90\({}^{\circ}\)) transition, 22 semi-circular ring probe & Inline &
\begin{tabular}{c} Yes \\ (single step) \\ \end{tabular} & \(-0.13\) & \(-10\) & \(48.3\) & Not Given \\ \hline
[11] &
\begin{tabular}{c} 90\({}^{\circ}\) WG-to-coaxial transition, field matching, and Impedance matching were achieved through a ridged waveguide, customized coaxial probe, and back-short distance \\ \end{tabular} & Transverse & No & \(-0.47\) & \(-23.4\) & \(38\) & Not Given \\ \hline
[8] & Broadside wall inserted, E-plane, 90\({}^{\circ}\), reverse approach of microstrip insertion & Transverse & Yes & \(-0.10\) & \(-10\) & \(72\) & Given \\ \hline This work & Broadside wall inserted, E-plane, 90\({}^{\circ}\)/fishtail shaped probe insertion, compact, easy fabrication of PCB & Transverse & No & \(-0.50\) & \(-10\) & \(66.15\) & Given \\ \hline \hline \end{tabular}
\end{table}