An Archiedean planar spiral antenna is numerically analyzed in the presence of a conducting plane reflector. The analysis shows that the spiral antenna backed by the plane reflector has two distinct regions in the current distribution, which explain the radiation of a circularly polarized wave for the outer circumference C ranging over shout 1.3 X < C < 1.5 A and C > 2.9 X, where A is a free-space wavelength. Further consideration is given to a truncated spiral antenna whose outer circumference is on the order of 1.4 A. The truncated spiral antenna maintains a decaying current distribution and radiates a circularly polarized wave over a k1.2 frequency bandwidth. It is also demonstrated that a power gain on the order of 8.5 dB is realized over the same frequency range.

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It is revealed that the combination of low pitch and a small number of turns realizes a low-profile helix as a radiating element of circular polarization. A two-turn helix of 4" pitch angle shows a bsndwidtb of 12% for a 3 dB axial-ratio criterion.

Abstract-An inverted FL antenna (InvFLA) is analyzed to obtain dual-frequency operation at 2.45 and 5.2 GHz (wireless LAN system frequencies). The InvFLA is composed of inverted FL elements, a parasitic element, and a ground plate, where these lie in the same plane, i.e., the structure is a card-type structure having a co-planar ground plate. The antenna height above the ground plate is very small: 5 5 mm = 0 045 wavelength at 2.45 GHz.The analysis shows that the InvFLA has a 4.1% bandwidth around 2.45 GHz and a 31.8% bandwidth around 5.2 GHz, both for a VSWR = 2 criterion. The gain is calculated to be 0.9 dBi at 2.45 GHz and 1.7 dBi at 5.2 GHz, with a small gain variation in each of the VSWR bands.

Abstract-A low-profile array antenna composed of 2-turn 4" pitch angle helices is designed for a frequency band of 11.7 GHz to 12.0 GHz. The feed wire of each helix is inserted into a radial waveguide through a small hole and excited by a traveling wave flowing in the transverse electromagnetic (TEM) mode between the two parallel plates of the waveguide. The measured aperture efficiency shows a maximum value of 77% for a beam radiated in the normal direction and 69% for a 30" beam tilt.