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It is well known that the beamwidths of horn antennas are inversely proportional to frequency. Previous studies point to minimum ±12% beamwidth stability within a maximum of 2.5:1 bandwidth (BW) ratio. To extend this BW, a design methodology for broadband double‐ridged horn antenna (DRHA) is proposed and applied to 4.5–18 GHz frequency band. Firstly, a conventional DRHA is designed and studied to compare the beamwidth variation of wideband horn antennas. By explicitly addressing the shortcomings of conventional DRHA, DRHA is redesigned and near‐constant beamwidth DRHA is proposed using the modifications implemented on the sidewalls. The antenna utilises ridges to extend the frequency band, unlike other wideband constant beamwidth horns. The authors show that ridged horns, whose dimensions are appropriately designed and modified, can have stable beamwidths. Then, properly positioned curved pinwalls are designed to provide considerable improvement in H‐plane beamwidth constancy. Broadband double‐ridged waveguide‐to‐coaxial adapter is also designed for 50 Ω reference. The prototype is manufactured, and the measured antenna exhibits wideband characteristics with 30.9° ± 2.7° half power beamwidth in H‐plane over 4:1 BW ratio. This variation corresponds to ±8.7% beamwidth stability along with the target frequency band.

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