Kazuhide Hirose

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).

A low profile Archimedean spiral antenna

An Archimedean spiral antenna, two arms of which are located above a conducting plane reflector at a small height, is numerically analyzed. Resistive loads are used to reduce standing wave currents on the arms. At the lowest frequency (the antenna height corresponding to one-twentieth the wavelength), the spiral shows a gain of approximately 3 dB with an axial ratio of approximately 4 dB.<<ETX>>

An integral equation and its application to spiral antennas on semi-infinite dielectric materials

This paper presents an integral equation that can handle wire antennas on a semi-infinite dielectric material. The integral equation is reduced to a set of linear equations by the method of moments. For efficiency, the impedance matrix element Z/sub m,n/ is divided into two parts on the basis of weighted Greens function extractions. The far-zone radiation field, which is formulated using the stationary phase method, is also described. After the validity of the presented numerical techniques is checked using a bow-tie antenna, a spiral antenna is analyzed. The current distribution, radiation pattern, axial ratio, power gain, and input impedance are discussed. It is found that the radiation field inside a dielectric material is circularly polarized. As the relative permittivity of the dielectric material increases, the angle coverage over which the axial ratio is less than 3 dB becomes narrower.

A spiral antenna backed by an electromagnetic band-gap material

A spiral antenna backed by an electromagnetic band-gap material, referred to as the EBG spiral antenna, has been analyzed using the finite-difference time domain method. The analysis shows that, even though the antenna height is less than one-quarter wavelength, the EBG spiral can radiate a CP wave. The spiral, which supports first mode radiation at 6 GHz, is found to have a frequency bandwidth of 11% for a 3-dB axial ratio criterion. Within this axial-ratio bandwidth, the gain variation is 4.1 dB.

Analysis and measurements for improved crank-line antennas

This paper describes the radiation characteristics of crank-line antennas radiating a circularly polarized wave. First, the radiation efficiency versus substrate permittivity is evaluated. Second, a 12-cell crank-line antenna of substrate permittivity /spl epsi//sub r/=1 and antenna height B=/spl lambda//sub 11.85/ is investigated as a reference antenna, where /spl lambda//sub 11.85/ is the wavelength at a frequency of 11.85 GHz. It is found that the main beam direction of the reference antenna varies 7/spl deg/ over a frequency range of approximately 6%, with an axial ratio of less than 3 dB and a gain of approximately 21 dB. Third, attention is paid to the gain behavior versus the antenna height. A way to increase the gain by modifying the antenna height is proposed. An increase of 1.5 dB from the gain of the reference antenna is demonstrated. Finally, the axial ratio, gain, and decoupling factor for crank-line antenna arrays are presented and discussed.

An array antenna composed of outer-fed curl elements

An array antenna consisting of four outer-fed curl elements is analyzed using the method of moments. It it found that, when the starting winding angle of each curl element is appropriately chosen, the array antenna can radiate a circularly polarized wave with an axial ratio of 0.1 dB. The calculation also shows that the bandwidth for a 3-dB axial ratio criterion is 5%, where a gain of approximately 13 dB is obtained.

Effects of arm bend and asymmetric feeding on dipole antennae

Abstract This paper describes the characteristics of modified dipole antennae whose arms are bent at their middle points. On the basis of an integral equation method, we first analyse a centre-fed half-wave dipole, paying special attention to the bending angle of the antenna arm. It is found that the bent dipole with a bending angle of 90° can be made to resonate with nearly a pure resistance of 50 Ω. Secondly, we analyse an asymmetric-fed full-wave bent dipole for the purpose of improving the directivity.. Calculations show how a bidirectional beam of increased directivity can be formed as the bending angle of the arm is increased.

Fan beam formation using spiral array antennas

Two types of spiral array antennas that form circularly polarized fan beams are numerically analyzed. One is composed of two-arm elliptical spiral elements, and the other is composed of single-arm round spiral elements. It is found that the elliptical spiral array antenna can radiate a wider fan beam than a conventional round spiral array antenna. It is also found that the single-arm spiral array antenna can radiate a fan beam off-normal to the antenna plane.

Numerical analysis of asymmetrical spiral antenna

The radiation characteristics of a two-wire spiral antenna in which the one arm is truncated are evaluated on the basis of the analyzed current distribution. It is numerically demonstrated that either sense of circular polarization is generated by changing the phase relation of feed points. The variation in the axial ratio is examined as a function of truncation amount of the one arm.

Low-profile circularly polarized radiation elements —loops with balanced and unbalanced feeds

In this paper, the two types of reflector-backed loop antennas have been numerically analyzed. The analyses reveal that a loop with a balanced feed shows a CP wave bandwidth of 15%, which is eight times as wide as that for the other loop with an unbalanced feed, when each loop height above the reflector is 0.25lambda0. As the loop height decreases to 0.10lambda0, the difference between the bandwidths is found to disappear. These results (for the loop resonant antennas) are compared to those for previous studies on reflector-backed spiral and curl (non-resonant) antennas. It is found that the difference between the CP wave bandwidths for the resonant and non-resonant antennas also disappears, as their antenna heights decrease to 0.10lambda0.