Patent Grant
8872725
Planar RF/microwave/mm-wave antennas, whether mounted to an airframe, vehicle, helmet or radio housing, are often backed by a conducting layer. Often times the antenna assembly also requires that the antenna be conformal to the conducting surface to which it is to be mounted. In the case of biomedical sensing, and in particular for radiometric sensing, it is necessary to have a flexible, low-profile antenna that is adjustable to the environmental loading effects that arise when the antenna comes into close proximity to an object or material (such as the human body). Antenna assemblies for use in biomedical radiometric sensing applications, such core body temperature measurement and pressure ulcers progress monitoring, require a flexible, low-profile antenna capable of adjusting to the environmental loading effects experienced by the antenna.
In the case of a radiometric antenna having a conductive layer backing, the presence of the conducting layer greatly limits not only the type of antenna element that can be used, but also the extent to which the profile of the antenna assembly can be reduced. In an antenna assembly employing an antenna element and a conducting layer, the conducting layer must be separated from the antenna element by an effectively large distance due to the natural tendency of ground currents to inhibit efficient radiation of the antenna element, thereby increasing the profile of the antenna assembly.
The microstrip patch antenna is commonly known in the art for the design of planar radiating elements above a conducting layer. The microstrip patch antenna is typically narrow-band, and bandwidth enhancement requires a large antenna-to-ground separation. In a low-profile antenna, the large antenna-to-ground separation is undesirable because it increases the profile of the antenna assembly. Additionally, the designs known in the art for microstrip patch antennas of this type do not allow for end-fire radiation.
Commonly, there is a need to severely limit background radiation for highly sensitive sensing applications. The need to severely limit background radiation in these applications requires backing the printed antenna element with ground plane shielding. However, it is often also required that these antenna assemblies are low profile antenna assemblies and as such, the ground plane must be placed in close proximity to the antenna element to reduce the profile of the assembly which also results in poor radiation characteristics of the antenna due to cancellation from image currents. Moreover, in the case where multiple antennas share the same ground plane, the surface currents add unwanted mutual coupling.
It is known in the art to reduce the ground interference of the low-profile antenna assembly by introducing a textured periodic surface above the ground plane that alters electromagnetic characteristics of the ground place. This textured periodic surface is known in the art as a high impedance surface, frequency-selective surface (FSS) or electromagnetic band gap (EBG) structure and prevents the propagation of radio frequency surface currents within the band-gap structure. The limiting effect of ground plane interference has been addressed by individuals in the art through electromagnetic band-gap (EBG) technology. However, work in the art has not addressed the need for tuning of the antenna to adjust to environmental loading effects experienced by the antenna when it is placed in close proximity to an object or material.
Accordingly, what is needed in the art is a low-profile, tunable electronic-band gap antenna assembly that is also flexible and therefore suitable for conformal mounting.
The present invention provides a low profile antenna that utilizes a flexible substrate with embedded elements to provide frequency tuning. A particular embodiment of the invention consists of a printed dipole that is loaded with two parallel sleeves, and has parasitic capacitive loading at the ends of the dipole arms. The loading elements in this embodiment offer design miniaturization. This design is attractive due to its high radiation efficiency and inherently broad bandwidth.
In a particular embodiment, the present invention provides a low profile microwave antenna assembly including, a planar antenna fabricated on a first flexible polymer substrate, a ground plane and a segmented textured periodic surface. Each of the segments of the segmented textured periodic surface are fabricated on a hard substrate and then integrated into a flexible substrate so that the overall textured periodic structure is flexible. The segmented textured periodic surface and the embedded reactance devices within the second flexible polymer substrate are positioned between the first flexible polymer substrate and the ground plane.
In a specific embodiment, the flexible polymer substrate is a liquid crystal polymer substrate and the planar antenna is an end-loaded planar open sleeve dipole (ELPOSD) antenna.
The segmented textured periodic surface in accordance with the present invention may be a high impedance surface, a frequency-selective surface or an electromagnetic band gap (EBG) surface. In a particular embodiment, the textured periodic surface is a Jerusalem Cross structure comprising a plurality of conductive patch elements electromagnetically-coupled to the ground plane to form a continuous textured metal structure. In a specific embodiment, the textured periodic surface is fabricated on a magnesium oxide substrate and the embedded reactance devices of the textured periodic surface are ferroelectric devices.
The present invention may further include one or more microwave monolithic integrated circuit (MMIC) integrated onto the first polymer substrate.
In an additional embodiment, low-density, low-loss material layers are positioned between the first flexible polymer substrate and the second flexible polymer substrate and between the second flexible polymer substrate and the ground plane.
The present invention enables antenna elements to be in close proximity to conducting layers without severely diminishing their performance, while also providing frequency tuning to enhance operational bandwidth. The flexible, low profile antenna in accordance with the present invention has the capability to electronically adjust to the environmental loading effects that arise when the antenna comes into close proximity to an object or material. The added feature of flexibility will increase the range of platforms into which such a technology can be integrated.
For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:
With reference to
In a particular embodiment of the present invention, the high impedance surface 40 comprises a plurality of segments, each of the segments begin created on a hard substrate such as MgO. The segments are integrated into a low loss, polymer substrate stack 70 to form the segmented high impedance surface that supports the dipole antenna 30 and ground plane 50 as shown in
With reference to
The need to severely limit background radiation requires that the ELPOSD be backed by ground plane shielding. Unfortunately, for low-profile antennas, having the ground plane in close proximity to the antenna results in poor radiation characteristics due to cancellation from image currents. Moreover, in the situation where multiple antennas share the same ground plane, the surface currents add unwanted mutual coupling. The ground interference issue can be resolved by introducing a textured periodic surface above the ground that alters its electromagnetic characteristics. This structure is known as a high impedance surface, frequency-selective surface (FSS) or electromagnetic band gap (EBG) structure, and operates in a similar fashion as two-dimensional photonic crystals to prevent the propagation of RF surface currents within the band-gap.
The embodiment illustrated in
A major challenge in the art that is addressed by the present invention is the integration of high performance tunability in flexible antenna systems. The choices that are available for achieving tunability can be broadly categorized as either semiconductor-based, field-tunable oxides or micro electro mechanical systems. In virtually all microwave applications, performance and cost are the most critical factors that drive technology-related decisions and high-quality field-tunable oxides are generally regarded as the best compromise among the three categories. The quality of the films and the performance of the devices, measured in terms of dissipation loss and percent tunability, are optimum when high process temperatures, vacuum deposition and micron- or sub-micron scale lithography can be used. To meet these optimum objectives, the present invention suggests a hybrid method in which ferroelectric devices used for frequency tuning are fabricated on a hard substrate using sputtering and semiconductor processing techniques, and subsequently packaged within the flexible substrate in a multi-chip-module (MCM) approach.
In addition to improvements in high-performance ferroelectric device technology, the present invention will advance the field of reconfigurable planar antenna design and therefore broadly impact many areas of wireless sensing and communications.
It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Now that the invention has been described,
This application claims priority to U.S. Provisional Patent Application 61/250,968, entitled, “Flexible Low Profile Microwave Antenna,” filed Oct. 13, 2009, the contents of which are herein incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 61250968 | Oct 2009 | US |
