Log-Periodic Monopole Array Spans 1.5 to 6.8 GHz

Communications for aircraft and ground vehicles require low-profile antennas capable of relatively broad bandwidths. To fill this need, researchers from various institutions in China developed a low-profile, log-periodic monopole array with end-fire radiation and vertical polarization. Using conductor-backed coplanar stripline feed and a broadband transition from coaxial cable to the stripline to facilitate measurements, the antenna array achieves a wide impedance bandwidth of 1.5 to 6.8 GHz for a VSWR of less than 2.30:1.

The antenna array also provides more than 4.5 dBi gain over that same wide bandwidth. In spite of the outstanding performance, the design has a low profile equal to only 0.047 λ_L (the free-space wavelength at the lowest operating frequency).

The antenna consists of 15 monopoles of different sizes/resonant frequencies printed on the top layer of a conductor-backed printed-circuit-board (PCB) substrate. The monopoles have the same height and are mounted in a straight line for a nearly symmetric radiation pattern in the end-fire direction. The monopole top hats feature an elliptical shape to avoid overlaps in radiation patterns of adjacent monopoles at lower frequencies.

In addition, the coplanar stripline is bent slightly to alternately feed the monopoles and achieve a phase difference of 180 deg. between adjacent monopoles across the slot in the stripline. A pair of lumped-element resistors are included to absorb the residual power at the end of the coplanar stripline. The antenna design was simulated using High Frequency Structure Simulator (HFSS) commercial electromagnetic (EM) simulation software from ANSYS.

A prototype antenna array was fabricated on a low-dielectric-constant (ε_r = 2.2) PCB substrate material and characterized with commercial test equipment. Only slight differences were found between measurements and simulations, which the authors attributed possibly to fabrication and assembly errors. The low profile, wide bandwidth, and planar structure of the antenna make it an attractive candidate for vehicular and airborne communications systems.

See “Low-Profile Log Periodic Monopole Array,” IEEE Transactions on Antennas & Propagation, Vol. 63, No. 12 December 2015, p. 5,484.

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