Recent advances in millimeter-wave (mmWave) and terahertz (THz) technology for high-speed wireless communication and high-resolution radars have increased the popularity of lens antennas due to their large gain and multi-beam capability. Wideband flat lenses offer a practical alternative to bulky curved dielectric lenses at millimeter-wave (mmWave) frequencies, thanks to their low profile and compatibility with planar fabrication.

A key challenge in implementing quasi-optical systems with wideband flat lens antennas is the design of a suitable feed. The feed should provide efficient illumination of the lens in the large bandwidth of operation to maximize aperture efficiency. Furthermore, the feed should be suitable for integration with the electronics at millimeter waves. Existing wideband lens of reflector feeds are based on conical horn with corrugated or optimized profiles, small elliptical lenses, and near-field focusing arrays. However, horns and small lenses do not reach high aperture efficiency over a band exceeding an octave, while near-field focusing arrays are characterized by high losses due to the feeding network.

In this work, different compact feeding solutions for wideband flat lenses are proposed and investigated. A comparative analysis between a connected array and a continuous transverse stub array implementation shows that the continuous transverse stub offers the best performance.

The design of a dedicated feeding network for the continuous transverse stub array is then presented. This consists of two feeding microstrips, a tapered power combiner, and a transition to a coaxial connector. The fabrication of the assembly components for the continuous transverse stub array is discussed, along with the printed circuit board (PCB) implementation of the feeding network. Through-Reflect-Line (TRL) calibration structures are used to de-embed the coaxial-to-microstrip transition.

Finally, the feed is measured in combination with a previously developed flat lens prototype. The experimental results confirm the wideband properties of the feed, which can operate across the entire bandwidth from 30 to 60 GHz.