Compact Planar Loop–Dipole Composite Antenna With Director for Bandwidth Enhancement and Back Radiation Suppression

A wideband and unidirectional loop-dipole composite antenna with a planar structure is proposed. The antenna includes a half-wavelength dipole and a one-wavelength loop that are printed on different sides of a substrate. To control the radiation characteristics of each element separately, the dipole is fed using a coaxial cable, whereas the loop is fed using offset broad-side coupling. To overcome the low front-to-back ratio (FBR) at lower and upper frequencies of the band, where the loop-dipole mode is not dominant, while maintaining a compact size, a parasitic strip is located in the vicinity of the loop. The strip acts as a director for the dipole at the low frequencies and for the loop at the high frequencies and thus significantly improves the FBR at those frequencies. The antenna achieves a wide fractional operating bandwidth of 49% at 0.7-1.15 GHz, a size reduction of more than 50% compared with recent similar designs, stable gain with a peak value of 4.8 dBi, and a minimum FBR of 11 dB across the operating bandwidth. Owing to its compact and planar structure, lightweight, simple fabrication, and stable radiation characteristic, it can be utilized in various wireless applications operating at the ultrahigh-frequency band.