Y. Iitsuka

Low-Profile Equiangular Spiral Antenna Backed by an EBG Reflector

The bi-directional beam from an equiangular spiral antenna (EAS) is changed to a unidirectional beam using an electromagnetic band gap (EBG) reflector. The antenna height, measured from the upper surface of the EBG reflector to the spiral arms, is chosen to be extremely small to realize a low-profile antenna: 0.07 wavelength at the lowest analysis frequency of 3 GHz. The analysis shows that the EAS backed by the EBG reflector does not reproduce the inherent wideband axial ratio characteristic observed when the EAS is isolated in free space. The deterioration in the axial ratio is examined by decomposing the total radiation field into two field components: one component from the equiangular spiral and the other from the EBG reflector. The examination reveals that the amplitudes and phases of these two field components do not satisfy the constructive relationship necessary for circularly polarized radiation. Based on this finding, next, the EBG reflector is modified by gradually removing the patch elements from the center region of the reflector, thereby satisfying the required constructive relationship between the two field components. This equiangular spiral with a modified EBG reflector shows wideband characteristics with respect to the axial ratio, input impedance and gain within the design frequency band (4-9 GHz). Note that, for comparison, the antenna characteristics for an EAS isolated in free space and an EAS backed by a perfect electric conductor are also presented.

Unbalanced-Mode Spiral Antenna Backed by an Extremely Shallow Cavity

This paper describes a two-arm Archimedean spiral antenna backed by a conducting cavity, where only one arm is directly excited, with the other arm being parasitically excited; in other words, the spiral arms are excited in an unbalanced mode. A balun circuit required for a conventional two-arm spiral is not used for this unbalanced-mode spiral. The design of the unbalanced-mode spiral is performed over a frequency range of fLd = 3 GHz to fHd = 9 GHz (1:3 bandwidth), where the antenna height is selected to be extremely small (7 mm = 0.07 wavelength at fLd) to realize a low-profile antenna. For reference, a corresponding spiral antenna excited in balanced mode is also analyzed. It is found that the unbalanced-mode spiral shows an acceptably small VSWR over the design frequency range of fLd to fHd. The radiation is circularly polarized around the antenna axis normal to the spiral plane. The gain shows behavior similar to that of the balanced-mode spiral. Results for other antenna heights (5 mm, 10.5 mm, and 14 mm) are also presented and briefly discussed. It can be said that the unbalanced-mode spiral is a circularly polarized wideband antenna with a simple feed system.

Equiangular Spiral Antenna Backed by a Shallow Cavity With Absorbing Strips

When a conducting shallow cavity is placed behind an equiangular spiral to obtain a unidirectional beam, the inherent wideband characteristics of the spiral deteriorate. To restore the wideband characteristics, a ring-shaped absorbing strip (R-ABS) is placed under the spiral arms. Analysis of the equiangular spiral with the R-ABS is performed using the finite-difference time-domain method. It is found that the R-ABS successfully restores the wideband characteristics. Subsequently, the R-ABS is divided into two absorbing strips, each specified by an arc-angle . Analysis reveals that the radiation characteristics obtained using the R-ABS are reproduced when the arc-angle is greater than . Throughout this paper an extremely small cavity depth is selected for the analysis: 0.07 wavelength at the lower operating design frequency of 3 GHz.

Loop-Based Circularly Polarized Grid Array Antenna With Edge Excitation

A loop-based grid array antenna (GAA) is proposed. The GAA is excited at one edge of the array and terminated to the ground plane at the other edge through a resistor. It is found that the proposed GAA, designated as the prototype GAA, has a frequency scanning function, with two circularly polarized (CP) frequency regions and a non-CP broadside-radiation frequency region. Subsequently, an advanced loop-based GAA is investigated to change the non-CP broadside radiation to CP radiation. For this, perturbation elements are added to the loops. It is revealed that the perturbation elements make the current flow along the loops in a traveling wave fashion. The advanced GAA has a seamless CP frequency scanning beam. The gain bandwidth is 23% (from 5.4 to 6.8 GHz), where the axial ratio is less than 3 dB, the radiation beam scans 51° (from -22° to +29°), and the VSWR exhibits an acceptable value (less than 2.5) for practical applications.

Rhombic Grid Array Antenna

A rhombic grid array antenna (RGAA) is proposed and its frequency-scanning performance is discussed. The RGAA has numerous bent radiation elements, forming rhombic cells and loops. As the frequency is increased, the radiation beam from the RGAA becomes a forward-fire beam. It is found that, as the bend angle of the radiation elements is decreased, the useful antenna characteristics shift to a lower frequency region. It is also found that the RGAA provides a more stable gain within a frequency band bounded by the 3- and 4-guided-wavelength resonances of the loops, compared with a conventional grid array antenna having non-bent radiation elements. Further investigation reveals that the VSWR is small within a frequency band bounded by the 3- and 4-guided wavelength resonances, as desired, and the bend radiation elements of the RGAA contribute to increasing the radiation efficiency.

Grid array antenna composed of V-shaped and rhombic elements for beam scanning

A grid array antenna composed of V-shaped and rhombic elements is proposed. It is found that the beam direction of the proposed antenna with 2α = 60° scans the range of θ = 3° to 31° as a function of frequency. The gain for this scanned beam is almost constant, at approximately 21 dBi.

Extremely low-profile spiral antenna with PEC and EBG reflectors

An extremely low-profile spiral antenna above a PEC reflector does not show wideband antenna characteristics. However, placing ring-shaped absorbing material between the outermost spiral arms and the PEC reflector mitigates deterioration in the antenna characteristics. Further investigation shows that removing absorbing material and replacing the PEC reflector by an EBG reflector contribute to realizing a low-profile antenna with high radiation efficiency.

Circularly polarized grid array antenna composed of open-loop elements for beam scanning

An edge-fed Kraus-type grid array antenna (GAA) radiates a linearly polarized (LP) beam, whose direction varies with frequency [1]. The long and short side lines of each grid cell for this GAA act as the transmission-line and radiation elements, respectively.

Consideration on radiation from an unbalanced-mode spiral antenna

This paper describes the radiation from an unbalanced-mode two-arm spiral antenna, where the antenna arms are defined by the Archimedean spiral function. The input impedance, current distribution, radiation pattern, axial ratio, and gain are investigated. The investigation reveals that the unbalanced-mode spiral radiates a circularly polarized wave over a wide frequency range as the balanced-mode spiral.

Circularly polarized array antenna composed of four open-loop elements

A circularly polarized array antenna composed of four open-loop elements is proposed for a CP beam scanning antenna. The array has a low-profile structure; the antenna height above the ground plane is small, 0.1 wavelength at 6 GHz. Analysis reveals that a scan angle range of 40° is obtained. The 3-dB reduction gain bandwidth is 27% (from 5.35 GHz to 7.0 GHz). Within this bandwidth, the axial ratio is less than 2 dB and the VSWR is less than 2.