Study On Low Profile,Wideband,and Unidirectional Antenna In VHF/UHF Bands

Light weight,low profile antennas with good radiation characteristics are drawing more and more attention in modern wireless communication and radar applications. Because they can be easily built conformal to instal ation platforms such as vehicles, vessels and aircraft. The low profile design usual y refers to an antenna structure whose overall height or width is less than one-tenth of the operating wavelength.Low profile antennas are of special importance within the VHF/UHF bands where they are used as communications antennas on airborne platforms, they can reduce platform visibility, decrease antenna weight and lower wind drag. In addition to lowprofiles, these antennas also require wide bandwidths. Meeting both requirements at the long wavelengths involved poses special difficulties.The gain of most present airborne low profile antennas is low, this paper proposes two kinds of high gain, low profile, unidirectional and wideband antenna in the VHF/UHF band.The first antenna is a resonating cavity-based low profile antenna loaded with anisotropic metamaterials, the operating band is from 200 MHz to 500 MHz, in which the VSWR is below 1.5 and the realized gain is from-1.8~4.5d Bi, and the total height of the antenna is 0.056?max. The antenna is composed of a rectangular probe which is placed inside a half open resonant cavity to stimulate electromagnetic fields and achieve high gain and unidirectional radiation patterns. To reduce the profile of the antenna, high index materials including isotropic and anisotropic materials are loaded inside the cavity, the influence of those material loading is also studied in this thesis. And a novel antenna structure is proposed to suppress higher order resonances.The second prototype is a low profile wideband antenna based on a bowtie-shapedplanar printed dipole, the upper structure is a bowtie-shaped planar antenna printed on the two sides of a FR4 substrate, a certain distance from the upper structure is another FR4 substrate, which a metal ground plane is printed on the top surface, so as to produce a unidirectional pattern. Most importantly we introduce a short-circuited structure with two slots, which can reduce the height of the antenna and broaden the bandwidth. Simulation results show the the VSWR is below 2 within the band of 200~500MHz, the gain is above 3.5 d Bi and the total height is 0.06?max.