Using advances in the areas of multi-mode, closely coupled miniaturized radiators and artificially engineered materials, ultra-broadband, highly efficient, electrically small antennas were developed for operation in military communications systems. A low-profile UWB antenna was developed that is composed of two electrically small loops coupled together in their near fields. Each loop has a three-dimensional surface with a bent diamond-arm shape. Half of each loop placed on top of an infinite conducting ground plane is used in the design. Each loop is loaded with a top hat to reduce the lowest frequency of operation of the antenna.
The antenna radiates like a vertically polarized monopole with omnidirectional, vertically polarized radiation patterns in the azimuth plane. The antenna demonstrates consistent radiation characteristics over a 4:1 frequency band. At its lowest frequency of operation, the antenna has an extremely low height of 0.033 λmin, where λmin is the free-space wavelength at the lowest frequency of operation of the antenna. Moreover, the antenna has lateral dimensions of 0.22 λmin × 0.22 λmin at the lowest frequency of operation.
While the antenna shown in Figure 1 is capable of delivering impressive performance levels, its bandwidth is limited to two octaves. To extend the bandwidth of this antenna, we examined the factors that limit its bandwidth. The bandwidth of this antenna is limited by the fact that its radiation patterns are deteriorated as frequency increases. This is due to the fact that as frequency increases, the radiation emanating from the different locations of the antenna has a larger phase difference between them. This way, the radiation emanating from the antenna adds constructively at some angles and destructively at others, resulting in deterioration of the radiation pattern of the structure from the desired omnidirectional patterns at higher frequencies.
To overcome this limitation and extend the bandwidth of the antenna, we proposed a new design of a wideband antenna that takes advantage of the previous antenna in a dual-antenna system. This significantly enhances the bandwidth over which the antenna can maintain its omnidirectionality. The topology and the photograph of a fabricated prototype of this modified antenna is shown in Figure 2. We demonstrated experimentally that this antenna can cover up to a decade of bandwidth with consistent, vertically polarized, omnidirectional patterns across the entire band. At its lowest frequency of operation, this antenna has electrical dimensions of 0.026 λmin × 0.026 λmin × 0.046 λmin, where min is the wavelength at the lowest frequency of operation.
Small UWB Antenna Occupying a Cubic Volume
We also investigated the development of small, ultra-wideband antennas that efficiently occupy a cubic volume. The antenna developed is an electrically small, low-profile, ultra-wideband antenna with monopole-like radiation characteristics. Figure 3 shows the topology and the photograph of a fabricated prototype of this antenna. The antenna is composed of a monopole bowtie antenna reactively loaded with a cascaded system of top hats, two shorting arms, and a ring slot cut into the ground plane. The reactive loads are used to introduce two additional resonances close to each other and below the lowest resonant frequency of the bowtie. This results in a very compact, ultra-wideband antenna that utilizes the available volume inside the Chu’s sphere rather efficiently. At the lowest frequency of operation, the proposed antenna has electrical dimensions of 0.085 λmin × 0.19 λmin × 0.19 λmin, where min is the free-space wavelength. The antenna demonstrates a VSWR of 2.2:1, and consistent monopole-like, omnidirectional radiation patterns over a 5.5:1 bandwidth.
Compact, Ultra-Wideband, Circularly Polarized Spiral Antenna
We developed a new technique for designing low-profile, compact spiral antennas with broadband circularly polarized (CP) responses. The antenna is backed by a ground plane and has unidirectional radiation patterns over its entire frequency band of operation. Figure 4 shows the topology and the photograph of the fabricated antenna. This antenna is a multilayer structure composed of a center-fed modified Archimedean spiral that exploits a novel loading structure, a ring-shaped absorber, and a feed network, which includes a 180° power splitter. The loading structure possesses both inductive and capacitive characteristics, which increase the equivalent electrical length of the antenna while maintaining its maximum dimensions. The Archimedean spiral is integrated into the multilayer dielectric structure along with its differential feed network. An optimized ring-shaped absorber is used on the periphery of the antenna to reduce the ground effects on the antenna performance.
The antenna developed in this part of the project occupies a volume that is 89% smaller than that occupied by a conventional ground-plane-backed Archimedean spiral antenna. At its lowest frequency of operation, the antenna has electrical dimensions of 0.21 λmin × 0.21 λmin × 0.09 λmin, where λmin is the free-space wavelength at the lowest frequency of operation (0.5 GHz). Over the frequency range from 0.5 to 1.4 GHz (2.8:1), the antenna has a VSWR of 2.4:1, and it has a CP radiation pattern with an axial ratio better than 1.2 dB. Within this frequency range, the antenna has minimum and maximum realized gain values of−5.0 dBiC and 3.1 dBiC, respectively.
RF Signature Reduction of Linearly and Circularly Polarized Antennas
We examined the design of low-observable antennas where the radar cross-section of the antenna is reduced by using a miniaturized-element frequency selective surface (MEFSS) and integrating it with a low-profile, wideband, circularly polarized or linearly polarized antenna.
We introduced a new technique for designing miniaturized-element frequency selective surfaces having bandpass responses and no spurious transmission windows over extremely large bandwidths. The proposed harmonic-suppressed MEFSSs consist of multiple metallic and dielectric layers. Each metallic layer is in the form of a two-dimensional arrangement of capacitive patches or an inductive wire grid with extremely sub-wavelength periods.
Harmonic-free operation in these structures is achieved by using multiple, closely spaced capacitive layers with overlapping unit cells to synthesize a single, effective capacitive layer with a larger capacitance value. This allows for reducing the unit cell size of a conventional MEFSS considerably and moving the natural resonant frequencies of its constituting elements to considerably higher frequencies. Consequently, the spurious transmission windows of such MEFSSs, which are caused by these higher-order harmonics, can be shifted to very high frequencies and an extremely broad frequency band free of any spurious transmission windows can be obtained. Using this technique, a number of MEFSSs with second-order bandpass responses were designed.
Integration of the Antennas with Harmonic Suppressed FSSs
The MEFSSs were also integrated with low-profile, linearly and circularly polarized antennas. Figure 5 shows a photograph of the linearly polarized antenna integrated with the MEFSS. The antenna radiates similar to a vertically polarized monopole, but is completely flush-mounted. The antenna also has omnidirectional radiation patterns in the azimuth plane. The antenna is cavity-backed and the frequency selective surface is completely integrated with the antenna within the cavity. The FSS is designed to be transparent in the frequency range where the antenna is expected to radiate. Outside of this frequency range, it presents a metallic ground plane where the FSS shields the antenna, at frequencies where the radar cross-section of the antenna may be high.
The radiation properties of the antenna were measured, and along the azimuth plane, the antenna showed a completely omnidirectional radiation pattern. The antenna has radiation patterns similar to those of a monopole antenna on top of a finite ground plane.
Wideband Antennas with Directional Radiation in the Azimuth Plane
A low-profile, compact, and wideband vertically polarized antenna was developed that demonstrated directional radiation characteristics in both the azimuth and the elevation planes of radiation. The antenna is composed of four bent-diamond-shaped half loops placed on a ground plane. A photograph of the fabricated prototype of this antenna is shown in Figure 6. The half loops are fed at their centers and short-circuited to the ground at their ends. Two of the half loops are fed in phase while the other two are fed with 180° phase difference generating omnidirectional and figure-eight-shaped radiation patterns, respectively. Coherent combination of these radiation patterns generates a cardioid-shaped directional pattern. A prototype of the antenna was fabricated and characterized. The antenna has electrical dimensions of 0.54 λmin × 0.4 λmin × 0.116 λmin at its lowest frequency of operation, and operates over a 2:1 bandwidth.
Enhancement of the Bandwidth of HF Antennas
Many antennas working at the high-frequency (HF) band have significantly smaller dimensions than the operating wavelength, and thus suffer from narrow bandwidths. In many military applications, such HF antennas are mounted on relatively large metallic platforms. We studied how a platform-mounted antenna can be used to excite the natural resonant modes of the platform to increase the overall bandwidth of the system. This way, the platform acts as the main radiator, and the mounted antennas act primarily as the coupling mechanism between the antenna and the external circuit. We used the theory of characteristic modes to identify the appropriate platform modes and determine the efficient means of exciting them. This allows for significantly increasing the bandwidth of the antenna system compared to what is achievable using the mounted antennas in isolation.
This approach was employed to successfully enhance the bandwidth of a horizontally polarized HF antenna system by as much as 10 times compared to a standalone antenna operating in free space. Scaled models of the proposed antennas were fabricated and experimentally characterized. Measurement results were observed to be in good agreement with the theoretically predicted results, and demonstrated the feasibility of using the proposed approach in designing bandwidth- enhanced platform-mounted HF antennas.