The coexistence of microwave and millimeter-wave technologies becomes the inexorable trend of future wireless communication systems. The corresponding components of the system are required to cover these two frequency bands simultaneously. However, it is difficult for the existing dual-band antenna configurations to achieve a frequency ratio larger than 3. In this paper, a novel topology is proposed to solve this problem. For the desired dual-band operation, a signal routing approach is proposed to guide the microwave/millimeter signals to the specific elements, respectively. The classical aperture coupling mechanism is deliberately utilized to block the millimeter-wave signal from feeding to the microwave antenna element and route the millimeter-wave signal into the millimeter-wave antenna element. Meanwhile, the substrate- integrated waveguide (SIW)-based millimeter-wave antenna element has the high-pass nature to reject the microwave signal. Therefore, the antenna elements for microwave and millimeter-wave bands can be designed separately with a high flexibility. First, a dual-band antenna was implemented to support 5.8 and 30 GHz simultaneously by integrating an annular-ring antenna element and an SIW slot antenna element. Furthermore, different radiation properties at the two bands can be easily realized owing to the high flexibility of the proposed approach. For validation, these two antennas were fabricated and measured.