H. Mimaki

A spiral antenna backed by a conducting plane reflector

An Archimedean 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 about 1.3 \lambda and C > 2.9 \lambda , where \lambda is a free-space wavelength. Further consideration is given to a truncated spiral antenna whose outer circumference is on the order of 1.4 \lambda . The truncated spiral antenna maintains a decaying current distribution and radiates a circularly polarized wave over a 1:1.2 frequency bandwidth. It is also demonstrated that a power gain on the order of 8.5 dB is realized over the same frequency range.

A curl antenna

A radiation element, designated as a curl antenna, is proposed for a circularly polarized antenna. The radiation characteristics of the curl are numerically analyzed. The gain is approximately 8.4 dB, and the 3-dB axial ratio criterion is 6.7%. Two aspects of curl array antennas are also presented: a decoupling factor between two curls and a circular array antenna consisting of 168 curls. Calculations show how the decoupling factor depends on the relative rotation angle of the two curls. The 168-curl array antenna shows a high aperture efficiency of 95%. >

Realization of dual-frequency and wide-band VSWR performances using normal-model helical and inverted-F antennas

The effects of parasitic elements on the voltage standing wave ratios (VSWRs) of two antennas are investigated. First, a parasitic monopole is used for a normal-mode helical antenna. The VSWR investigation shows that dual-frequency operation is obtained by the parasitic element effects. The dual-frequency operation is revealed as a function of monopole position above a ground plane (monopole height). As the monopole height decreases, the separation of a higher resonance frequency f/sub H/ from a lower resonance frequency f/sub L/ increases. For a monopole length of L/sub MP//spl ap/0.4/spl lambda//sub HX/, where /spl lambda//sub HX/ is the resonance wavelength of the helix, the frequency bandwidth for a VSWR=2 criterion is 12.5% in the lower frequency f/sub L/ region and 5.2% in the higher frequency f/sub H/ region, with a frequency separation ratio of f/sub H//f/sub L/=2.14. Secondly, L-figured parasitic elements are used for an inverted-F antenna (IFA). The parasitic elements improve the VSWR performance. The frequency bandwidth for a VSWR=2 criterion is approximately two times as wide as that of the single IFA. It is also found that bending the horizontal sections of the IFA and parasitic elements contributes to reducing the antenna size while not significantly deteriorating the VSWR bandwidth. The radiation patterns are also presented and discussed.

Low-profile helical array antenna fed from a radial waveguide

A low-profile array antenna composed of two-turn 4 degrees 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 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 degrees beam tilt. >

Extremely low-profile helix radiating a circularly polarized wave

It is shown 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 degrees pitch angle shows a bandwidth of 12% for a 3 dB axial-ratio criterion. The numerical results presented lead to new aspects of a low-pitch helix as an effective radiating element. >

An inverted FL antenna for dual-frequency operation

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.

Effects on the radiation characteristics of using a corrugated reflector with a helical antenna and an electromagnetic band-gap reflector with a spiral antenna

An axial-mode helical antenna backed by a perfect electric conductor (PEC reflector) is optimized to radiate a circularly polarized (CP) wave, using the finite-difference time-domain method (FDTDM). After the optimization, the PEC reflector is replaced with a corrugated reflector. The effects of the corrugated reflector on the current distribution along the helical arm and the radiation pattern are investigated. A reduction in the backward radiation is attributed to the reduction in the current flowing over the rear surface of the corrugated reflector. A spiral antenna backed by a PEC reflector of finite extent is also analyzed using the FDTDM. As the antenna height decreases, the reverse current toward the feed point increases, resulting in deterioration of the axial ratio. To overcome this deterioration, the PEC reflector is replaced with an electromagnetic band-gap (EBG) reflector composed of mushroom-like elements. Analysis reveals that the spiral radiates a CP wave even when the spiral is located close to the reflector (0.06 wavelength above the EBG surface). The input impedance for the EBG reflector is more stable over a wide frequency band than that for the PEC reflector.

Center-fed grid array antennas

Single- and double-layer grid array antennas are numerically investigated. A radiation beam normal to the antenna plane is realized by using appropriate configuration parameters. The difference of the gains between the upper-fed and lower-fed grid arrays is very small (0.5 dB over a frequency range of 12.5 to 12.75 GHz).

Backfire radiation from a monofilar helix with a small ground plane

Numerical analysis of a helical antenna is made in the presence of a finite-grid ground plane. It is found that the transition from forward-fire radiation to backfire radiation occurs as the ground-plane diameter is reduced to the order of the helix diameter. The investigation of the backfire helix with a small ground plane shows that a circularly polarized wave is radiated over a frequency range of more than 1 to 1.3. An application of the backfire helix to primary feed for a paraboloidal reflector is also described, in which the far-field pattern is evaluated by the physical-optics approximation. >

A monofilar spiral antenna excited through a helical wire

The radiation characteristics of a monofilar spiral antenna excited through a helical wire are investigated using the method of moments. The spiral has a circumference of more than two wavelengths (2.3/spl lambda//sub 6/, where /spl lambda//sub 6/ is the wavelength at a test frequency of 6 GHz) to obtain a tilted beam. An input impedance of approximately 50 /spl Omega/ is realized by appropriately selecting the diameter and pitch angle of the helical wire. The antenna shows a gain of approximately 8 dB relative to an isotropic source in the maximum beam direction (18/spl deg/ from the direction normal to the antenna plane) and a VSWR frequency bandwidth of approximately 12%.