In this paper, two substrate-integrated waveguide (SIW)-based antennas are designed and optimized for automotive radar applications. In the initial stage, an antenna with a U-shaped slot in the ground plane is proposed. To enhance the bandwidth of this antenna, shorting pins are introduced between the resonant cavity and the microstrip feed line. In the next step, the original U-shaped slot is replaced with two symmetric L-shaped slots, which enable dual-frequency resonance. Subsequently, by optimizing the placement of the shorting pins, the dimensions of the slots, and the width of the transition, the surface current distribution on the top conductor is improved, leading to stronger current concentration along the edges. These modifications, accompanied by the simultaneous excitation of two different modes in the resonant cavity, result in improved radiation performance as well as the merging of resonant frequencies, thereby achieving a wide bandwidth of 3.56 GHz in the range of 25.47–29.02 GHz (13%) for the first antenna, and an operational range of 72–78.5 GHz for the second antenna. Simulation results indicate that the antenna gains at the resonant frequencies of 26.15 GHz and 28.5 GHz are 6.54 dB and 5.06 dB, respectively. Moreover, the antenna efficiency across the entire first frequency band is estimated to be approximately 86.6% on average, demonstrating optimized performance for automotive radar and fifth-generation (5G) communication technologies.