In this article, a 2-18-GHz double-ridged horn antenna is designed, and the effects of the E-plane sidewalls on antenna performance are examined. The modifications implemented in the shape of the ridges cause a better voltage standing wave ratio over the whole frequency band. The double-ridged horn antenna is designed to have fewer parts that reduce the possibility of performance reduction due to the production process. The antenna is modeled using commercial 3D electromagnetic simulation software, CST Microwave Studio (Computer Simulation Technology, Germany). After the simulations, a prototype of the designed model is manufactured and tested. The measurement results are in good agreement with the simulation results. The results show that the antenna has low side-lobe levels with a voltage standing wave ratio less than 1.85 (typically 1.5) over the entire frequency band. The method outlined in this article can be used to design a double-ridged horn antenna as a test antenna in electromagnetic compatibility (EMC) applications.