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We propose a wideband multibeam circular array antenna for directional networking applications at VHF/UHF frequencies. We introduce a directional antenna element by combining the radiation patterns of a monopole and a loop element. Combining the figure-eight-shaped and omnidirectional radiation patterns of the loop and the monopole antenna results in a directional antenna element. We demonstrate that a significantly larger bandwidth can be achieved by moderately relaxing the size constraints placed on the antenna element. A circular array consisting of eight of these radiating elements was developed. This array has a low profile, fitting in a 4 $\times $ 4 $\times $ 1 ft3 volume. The antenna shows a well-matched frequency response with S11 below −10 dB over an octave frequency bandwidth of 225–450 MHz. The good impedance matching of the array is maintained if the array elements are individually excited (e.g., to provide eight beams) or if multiple adjacent elements are excited together to provide fewer, more directional beams. We validated our simulation results by fabricating and experimentally characterizing a 3:1 scaled model of the proposed antenna. The proposed array can achieve gain values of 6–8 dBi within its operational bandwidth if each radiating element is excited individually. This gain increases to 10–12 dBi by exciting three adjacent elements. [ABSTRACT FROM AUTHOR]

This communication presents a low-profile and efficient low-very high-frequency (VHF) antenna designed for compact unmanned ground vehicles (UGVs). An effective miniaturization strategy for a top-loaded multielement monopole antenna is proposed to improve its gain while maintaining a low profile. The approach introduces multiple closely spaced short vertical radiating elements and a structurally constructed 180° phase shifter connecting the elements on a small ground plane. The phase shifter enables electric currents flowing through the vertical elements to be in phase, resulting in enhanced radiation without increasing the antenna profile. In order to improve the antenna bandwidth, a monopole configuration with a sleeve feed and capacitive top loading is adapted. The proposed antenna optimized for a compact UGV via parametric analysis has a height of $0.028\lambda $ at 42 MHz (the corresponding electrical size $ka=0.24$). The measured performance of an antenna prototype shows the 3 dB bandwidth of 0.67 MHz and the peak gain of −2.5 dBi. The proposed design can achieve more than 16% reduction in lateral dimensions and 1 dB improvement in gain compared to the state-of-the-art design with similar topology. The performance of the antenna integrated on an actual UGV system is also evaluated by experiments. [ABSTRACT FROM AUTHOR]

The radiation pattern of an antenna within an array environment is modified by mutual coupling to neighboring elements, which can create unknown phase and amplitude variation on the order of 10° and 5 dB. This uncertainty in the element patterns results in beamforming errors, which can be particularly problematic for applications requiring precise pattern control. A new approach is reported to compensate for the mutual coupling of single-mode radiators using only the measured S-parameters and received signal from several far-field sources. The technique relies on the fact that dominant mode scattering from each antenna can be removed by terminating it with the proper impedance. Optimized beamforming weights are derived that compensate for mutual coupling. It is shown that implementing these optimized weights is equivalent to terminating all elements with a “virtual” impedance that cancels scattering from neighboring antenna elements (i.e., mutual coupling). Simulations of perfectly uniform and nonuniform linear dipole arrays show that compensating for mutual coupling using the proposed technique reduces the variation of the embedded element patterns by an order of magnitude compared to more conventional techniques. The proposed approach could be useful for a wide variety of antenna arrays and may even lend itself to self-calibration in the field using signals of opportunity. [ABSTRACT FROM AUTHOR]

Active matching techniques with non-Foster reactive elements have been introduced to overcome fundamental performance limits on electrically small passive antennas. While several experimental studies have demonstrated successful bandwidth enhancement with active matching applied to small unmatched passive antennas, their radiation performance against similar-sized, efficient, resonant antennas alone (and/or applied to active matching) has not yet been confirmed to assess their effectiveness for practical wireless communications applications. Beginning with a highly miniaturized efficient resonant antenna (${\lambda }_{0}$ /50 in height), we design an active matching network derived from an antenna impedance model. We conduct an extensive simulation analysis to address challenging issues associated with the design arising from a tradeoff between degree of bandwidth enhancement and stability of the matching network. Based on experimental performance assessment, the effective 3 dB bandwidth of the proposed actively matched antenna is improved by more than three times compared with the passive version and improved in efficiency by more than 10 dB relative to cases that apply active matching techniques to similar-sized, unmatched passive antennas. [ABSTRACT FROM AUTHOR]

We present a design strategy for platform-based antennas operating on electrically small platforms such as unmanned ground vehicles (UGVs). Specifically, we report the design of a platform-based antenna operating at the lower end of the very-high-frequency (VHF) band for a small UGV with physical dimensions of 99 cm × 67 cm × 39 cm. The antenna is mounted on the bottom of the platform resulting in an extremely low-profile design and generating vertically polarized, monopole-like radiation patterns. Using characteristic-mode theory (CMT), performance metrics of the antenna including its maximum potential bandwidth and radiation patterns are investigated. The performance of the antenna when operating in a realistic operational environment is analyzed using full-wave electromagnetic simulations. A full-scale prototype of the antenna mounted on the bottom of the vehicle was fabricated and experimentally characterized. Finally, the design of this low-profile antenna is extended to a two-element antenna array mounted on the same platform and a prototype of this two-element array was fabricated. Basic experiments were conducted to study impedance matching and mutual coupling effects in this electrically small antenna array. The performance of this array as part of a two-element direction-finding system was investigated through computer simulations. [ABSTRACT FROM AUTHOR]

The lower VHF band has potential for low-power, short-range communications, as well as for geolocation applications, in both indoor and urban environments. Most prior work at low VHF focuses on longer range path loss modeling, often with one node elevated. In this paper, we study indoor/outdoor near-ground scenarios through experiments and electromagnetic wave propagation simulations. These include the effects of indoor penetration through walls and obstacles, as well as indoor/outdoor cases, for both line of sight (LoS) and nonLoS (NLoS), at ranges up to 200 m. Mounting our receiver (Rx) on a robotic platform enabled the collection of thousands of measurements over an extended indoor/outdoor test area. We measure the channel transfer function, employing bandpass waveform sampling, with pulse and tone probe signals. Based on statistical tests, we show that the measured channels have a nearly ideal scalar attenuation and delay transfer function, with minimal phase distortion, and little to no evidence of multipath propagation. Compared with higher VHF and above, the measured short-range VHF channels do not exhibit small-scale fading, which simplifies communications Rx signal processing, and enables phase-based geolocation techniques. [ABSTRACT FROM PUBLISHER]

This paper investigates the advantages offered by radiation pattern diversity using a new physics-based analysis that takes into account the complex radiation patterns of the transmit(Tx) and receive(Rx) diversity antennas in conjunction with an accurate deterministic, coherent, and polarization preserving propagation model for a complex indoor scenario. Unlike techniques that utilize spatial diversity, radiation pattern diversity offers a unique opportunity to achieve compact diversity antenna systems especially with the advent of enabling antenna miniaturization techniques. In this work, a co-located antenna radiation pattern diversity system is proposed and its performance is analyzed using an accurate physics-based diversity analysis technique. The proposed analysis technique utilizes an efficient deterministic propagation model modified by incorporating the complex gain of the Tx and the complex effective heights of the Rx antennas into the rays launched and received by the antennas in a coherent manner. The proposed system is realized and tested in complex indoor scenarios based on which complex correlation coefficients between various channels and the effective diversity gain are computed which are then utilized as a figure of merit for improved channel reliability. For the scenarios considered, it is shown that the apparent diversity gain is at least 9.4 dB based on measured results. [ABSTRACT FROM PUBLISHER]

This paper presents a novel antenna architecture to achieve an extremely small form factor for HF band applications. The approach is based on manipulating the topology of a short monopole antenna without utilizing a high index material. A new architecture incorporating two radiating elements is configured, which allows significant gain enhancement. It is shown that such architecture can render a miniaturized HF antenna on air substrate having lateral and height dimensions as small as 0.0115\lambda0 \times 0.0115\lambda0 \times 0.0038\lambda0 (150 mm\,\times\,150 mm\,\times\,50 mm for operation at 22.9 MHz). It is found that the measured gain of such architecture can be as high as -18.1 dBi, which is 16.7 dB higher than a reference inverted-F antenna realized on a high index material (\varepsilonR =10.2) having exactly the same dimensions. The proposed antenna architecture is composed of two in-phase radiating vertical elements connected to two inductors between which a capacitive top load is connected to achieve the desired resonant condition. The two vertical elements act effectively as a monopole having increased height. It is also shown that the gain of the antenna can be increased monotonically by increasing the quality factor (Q) of the phase shifter. High Q air-core inductors that can be accommodated in electrically small monopole antenna are designed and incorporated in the phase shifter to achieve a gain value of -17.9 dBi. Details about the proposed design approach, simulation, and measurement results are discussed. [ABSTRACT FROM PUBLISHER]

A sub-wavelength source tracking system utilizing highly miniaturized antennas in the HF range for applications in GPS-denied environments including indoor and urban scenarios is proposed. A technique that combines a high resolution direction finding and radio triangulation utilizing a compact transmit (Tx) and receive (Rx) antenna system is pursued. Numerical models are used to investigate wave propagation and scattering in complex indoor scenarios as a function of frequency. We choose HF band to minimize attenuation through walls and multipath in indoor environments. In order to achieve a compact system, a low-profile and highly miniaturized antenna (\lambda /300 height and \lambda /100 lateral dimensions at 20 MHz) having omnidirectional, vertically polarized field is designed. At such low frequencies, accurate measurement of the phase difference between the signals at the Rx antennas having very small baseline is challenging. To address this issue, a biomimetic circuit that mimics the hearing mechanism of a fly (Ormia Ochracea) is utilized. With this circuit, very small phase differences are amplified to measurable values. The numerical simulations are used to analyze direction of arrival retrieval and source localization in highly cluttered environments. A compact system prototype is also realized and source tracking in complex indoor scenarios is successfully demonstrated. [ABSTRACT FROM AUTHOR]