A Broadband SIW-Fed Rhombic Loop Antenna With Endfire Radiation for Millimeter-Wave Applications

A broadband endfire antenna with three rhombic loops is designed for millimeter-wave (MMW) applications in this letter. The broadband response is attributed to traveling waves along rhombic loops which are constructed by two sine-like strips in an 180° rotation symmetry printed on two opposites of a laminate, respectively. Then using U-notches and quasi-trapezoid grounds improves the gain on lower and higher bands, respectively. Also, a pair of quasi-half elliptical matching stubs is employed for impedance bandwidth (IBW) near the feed end of loops. Finally, the antenna is excited by a substrate integrated waveguide (SIW) realizing a planar and low profile of 0.16 λ
₀
(λ
₀
is the free-space wavelength at 33 GHz). By these methods, the antenna prototype shows an IBW of 73.7% (21–45.5 GHz), a peak gain of 10.3 dBi, a 3 dB gain bandwidth of 61.2% (25–45.2 GHz) and radiation efficiencies of higher than 90% within the operating bandwidth.