Jiangfeng Wu

Microwave Backscatter From Arctic Lake Ice and Polarimetric Implications

Polarimetric synthetic aperture radar satellite and ground-based Ku- and X-band scatterometer measurements are used to explore the scattering mechanism for ice in shallow Arctic lakes, wherein strong radiometric responses are seen for floating ice, and low returns are evident where the ice has grounded. Scatterometer measurements confirm that high backscatter is from the ice/water interface, whereas polarimetric decomposition suggests that the dominant scattering mechanism from that interface is single bounce. Using Fresnel equations, a simple model for surface bounce from the ice/water interface is proposed, and its predictions are supported by experimental parameters such as co-pol phase difference, co-pol ratio, and the results of rigorous numerical modeling. Despite early research suggesting double-bounce scattering from columnar air bubbles and the ice/water interface as the dominant scattering mechanism in shallow lakes, this paper strongly supports a single-bounce model.

Dual Polarized Wideband Directional Coupled Sectorial Loop Antennas for Radar and Mobile Base-Station Applications

This paper presents a new dual polarized antenna structure with matching and radiation characteristics suitable for radar and base-station applications. Two slightly different designs based on a dual polarized radiating element are presented. For radar applications, the radiating element is used to excite the two degenerate modes of a cavity with square cross-section. The resulting overall antenna size is 0.5λ_m × 0.5λ_m × 0.2λ_m, where λ_m is the free space wavelength at the minimum operating frequency. The measured results showed isolation better than 30 dB and voltage standing wave ratio (VSWR) less than 2.5 over 1-1.95 GHz as well as good radiation pattern performance. For base-station applications, the exciting element was placed in front of a metallic ground plane to increase the beamwidth. This results in a smaller antenna size (0.38λ_m × 0.38λ_m × 0.25λ_m) at the expense of reduced directivity. The antenna is designed to work over 1710- 2170 MHz which covers three common frequency bands of mobile radio application. The measured results showed return loss better than 14 dB over the operating band and more than 30 dB polarization isolation.

Compact Omnidirectional Circularly Polarized Antenna

In certain radar and wireless communication applications, omnidirectional circularly polarized (CP) antennas are desired but existing solutions have complex 3-D structures and are large in size. In this paper, a compact and low profile CP antenna having omnidirectional pattern on azimuthal plane is demonstrated. The CP waveform is synthesized by combining two linearly polarized and omnidirectional antennas with 90° phase difference on their terminals. The most challenging part of the design is the realization of the horizontally polarized antenna with omnidirectional radiation pattern. This antenna is designed using a novel two-faced slot antenna structure capable of emitting in-phase radiation of equal magnitude on both sides of the antenna. The two faces of the low-profile slot antenna are connected by a thin meander parallel plate waveguide to provide antiparallel field distributions over the two faces of the slot radiators. This special design removes radiation nulls on E-plane that exists on conventional slot antennas, the mechanism of which has been carefully discussed in this paper. The other antenna is a compatible pair of planar inverted-F antennas constructed on the sides of the finite-ground plane two-faced slot antenna for the provision of vertically polarized omnidirectional radiation. A feed network with power splitter and phase shifter is also accommodated on the compact CP antenna. The antenna is designed and fabricated, and its performance is measured against numerical simulations. It is shown that the fabricated antenna at 2.4 GHz can provide measured axial ratio of lower than 1.8 and gain variations of less than ±1.5 dB over all azimuth angles.

Reactive Impedance Surface TM Mode Slow Wave for Patch Antenna Miniaturization [AMTA Corner]

This paper presents a two-layer mushroom-like reactive impedance surface (RIS) and its application to patch-antenna miniaturization. A reactive impedance surface, known as a meta-substrate, has been shown to present the ability to miniaturize printed antennas when serving as the substrate for the antenna. However, the area of conventional reactiveimpedance- surface substrates is usually much larger than that of the miniaturized antenna. Here, a reactive impedance surface with very small unit-cell dimensions (a cell area reduction of 95.6%, compared to a traditional reactive impedance surface) is proposed, and utilized to design a miniaturized antenna over a reactive-impedance-surface substrate with the same size as the antenna itself. The two-layer mushroom-like reactive impedance surface is analyzed theoretically and numerically. The reflection phase diagram of the reactive-impedance-surface substrate and the reactive impedance surface/PEC parallel plate waveguide are studied to verify the analytic model. It is shown to have a high propagation constant near the resonant frequency of the reactive impedance surface. The effect of vias in the mushroom-like reactive impedance surface is also discussed. Applying the two-layer reactive-impedance-surface substrate and an optimized miniaturized patch antenna topology, several UHF-band patch antennas, working around 400 MHz, were designed and fabricated. Using this approach, a miniaturized antenna with dimensions λ0/11.4×λ0/11.4×λ0/74, including the reactive-impedance-surface substrate, was developed.

Scattering phenomenology of arctic lake ice

This paper presents the radar scattering phenomenology of arctic shallow lake ice. This is an important feature for monitoring the arctic climate for this purpose, a full-wave simulation model to analyze lake ice scattering that includes columnar air bubbles and rough interface is presented. The proposed model can handle complex interactions between high density air bubbles and rough ice surfaces. Based on this model, the scattering contributions from the rough ice/water interface and columnar air bubbles in the ice at C band are calculated. It is concluded that the roughness at the interface between ice and water is the dominate contributor to the observed backscatter.

Anisotropic magneto-dielectrics for wideband planar antenna miniaturization applications

Summary form given only: In this paper, the application of magneto-dielectric materials and their anisotropic electromagnetic properties for designing wideband planar antennas with small form factors is studied. At the lower UHF band (~300 MHz), the sizes of antennas are rather large and oftentimes antennas with wide bandwidth have 3-D structures. For many modern military applications planar structures with low profile is of great interest. In the area of planar antenna miniaturization, application of high permittivity materials as substrates has been studied by many researchers. However, high permittivity materials tend to reduce the antenna bandwidth and lower the radiation efficiency. Impedance matching of the antenna also becomes difficult due to the large capacitive nature of high permittivity materials. On the other hand magnetic materials at VHF and UHF bands tend to be lossy due to the proximity of ferrite magnetic resonances to the desired operational frequencies. In this presentation, composite magneto-dielectric materials are proposed as a way to control the material loss while maintain a high index of refraction with almost equal permittivity and permeability. Magneto-dielectrics can be made up of dielectric and magnetic materials that are arranged in different periodic configurations. Different periodicity arrangements, orientation, and volume fraction ratios provide much flexibility in designing anisotropic medium for a desired antenna. When used as substrates for planar antennas, the magneto-dielectric can also miniaturize the antenna using moderate values of effective permittivity and permeability simultaneously. With the lower effective loss factor, the antenna radiation efficiency can be improved. Moreover, since the characteristic impedance of the magneto-dielectric is close to that of a free space, the antenna impedance matching in a wide frequency range will become less difficult. Different antenna configurations with different magneto-dielectric composite materials will be considered and the resulting antenna performance will be compared with dielectric only materials have the same index of refraction.

Small planar omni-directional circularly polarized antenna

Circularly polarized (CP) antennas are commonly needed for satellite communication, WLAN, RFID, Radar and many other systems. Circularly polarized waves can avoid the negative effects of polarization rotation in propagation channels, such as ionosphere. It can also reduce the effect of multipath fading for co-polarized transceiver systems as the odd bounce reflections change the handedness of the wave polarization. For many communication and radar problems with omni-directional antenna elements are needed. For the communication the reason is obvious as the direction of arrival or the orientation of the antenna is arbitrary. For the radar, distributed imaging network of transmitters and receivers are envisioned to create all direction imaging from a number of moving receivers. To reduce the direct signal between the transmitter and receivers CP antennas with orthogonal polarization can be used. Existing omni-directional CP antennas have complex 3D structures and are large in size. In this paper, a planar CP antenna having omni-directional pattern within a ring area is designed. The CP polarization is synthesized by combining two linearly polarized and omni-directional antennas and feeding them with signals having the same amplitude but 90 degree phase difference. The first antenna is a slot antenna on a finite and small ground plane which is made to have an omni-directional pattern on its E-plane. To achieve that, the radiation null on the E-plane that exists on conventional slot antennas is removed by constructing a parallel plate waveguide delay line between the two sides of the antenna and forming an antiparallel field distribution on the aperture of each side. As a result, the far field radiation from both apertures adds up in phase and becomes uniform. The other antenna consists of two PIFA elements which radiate omni-directionally on their H-plane. The feeding network splits the input power equally to excite both antennas and introduces an extra 90 degree phase delay for one of the antennas. The feeding network is realized by microstrip lines and is optimized to have an axial ratio lower than 2 within a bandwidth of 2–3%. The simulation results demonstrate that the antenna has a figure-eight pattern in elevation with a beamwidth of about 100 degree. The gain of this small antenna is 2dB with small fluctuation as a function of angle in the horizontal plane.

MF band compact helical antenna design and imaging techniques for hydraulic fractures detection

In this paper we present medium frequency (MF) band imaging systems to estimate the sub-surface hydraulic fractures. We propose a helical dipole antenna for the borehole imaging application. The proposed antenna is loaded with a ferrite-bundle and is optimized to achieve an ultra-compact size to fit within a volume of λ0/1400 × λ0/1400 × λ0/35 while maintaining a relatively high efficiency of about 10%. The feeding network is designed to match the antenna input impedance with the frequency tunability. Based on the proposed antenna, radio-wave techniques for detecting fractured regions in subsurface rock layers are developed. Compared with conventional high frequency mapping system, the penetration distance has increased to kilometers range due to the low propagation loss at MF band. Multi-hole and single-hole setups with different imaging algorithms are studied in the paper. Many methods such as beam-forming, adaptive null-generating and time-gating are performed to minimize the direct-link. Post-processing maps the conductivity contrast in the target area with a two-dimensional resolution.

Super-miniaturized borehole antenna design and radio-wave estimation of sub-surface hydraulic fractures at MF band

In this paper we present a medium frequency (MF) band imaging system to estimate the sub-surface hydraulic fractures. We propose a helical dipole antenna for the borehole imaging application. The proposed antenna is loaded with a ferrite-bundle and is optimized to achieve an ultra-compact size to fit within a volume of λ0/1400 × λ0/1400 × λ0/35 while maintaining a relatively high efficiency of about 20%. The feeding network is designed to match the antenna input impedance with the frequency tunability. Based on the proposed antenna, a radio-wave technique for detecting fractured regions in subsurface rock layers is developed. Compared with conventional high frequency mapping system, the penetrating distance has increased to kilometers range due to the low propagation loss at MF band. The method is based on single-hole setup with separate transmitter and receiver antennas. The time-gating is performed to minimize the direct-link signals. Post-processing maps the conductivity contrast in the target area with a good resolution.

Design and evaluation of a class of miniaturized metal-backed antenna

This paper presents a two-layer mushroom-like reactive impedance surface (RIS) and its application to patch antenna miniaturization. RIS, known as meta-substrate, have shown the ability to miniaturize printed antennas with omni-directional radiation pattern, when served as the substrate for the antenna (H. Mosallaei and K. Sarabandi, IEEE Trans. Antennas Propag., 52, 9, 2403-14, 2004). However, the area of conventional RIS substrate usually has to much larger than that of miniaturized antenna, since the cells dimension is comparable with the antenna, even using a high dielectric constant of 10.2 or 25. The proposed compact two-layer RIS can achieve 76% lower frequency than conventional one-layer RIS with the same size, whose cell dimensions are limited within λ0/45 in length and λ0/100 in height while the dielectric constant is kept below 3.5. This makes it possible to design a miniaturized antenna over an RIS substrate with the same size as the antenna itself.