We present the reconfiguration of a circular feed horn operating in bandwidth from 1.4 GHz to 1.8 GHz for radio astronomical applications, adapting it to the 21 cm band (1.42 GHz), and performing observations of the hydrogen line. The proposed model was implemented using cylindrical structures. The design, simulation, measured performance, and reconfiguration are detailed. This feed horn is used at the prime focus of a parabolic reflector antenna. A prototype was designed and characterized, and the simulated and measured results were in good agreement. The reflection coefficients (S 11) of the feed horn was below-15 dB, and the voltage standing wave ratio (VSWR) remained below 1.43. The gains obtained from simulation and measurement were 12 dBi and 12.3 dBi, respectively. To determine the suitability of the proposed design in a practical environment, spectral measurements were performed using an FPGA-based spectrometer. The results showed that the intensity of the signal reached 3.5 dB more than that of the original feed horn. The overall performance of the proposed feed horn suggests that it can be applied to the optics of the 4.9 m reflector antenna.