24 October 2013
Reconfigurable antennas have played an active role in the defence industry for more than a decade. We can really attribute the recent developments in reconfigurable antenna systems to these efforts, as well as the undertow of interest from federal agencies which have helped make many of the key advancements in this area possible. In the commercial domain these advances are being leveraged to implement low-cost reconfigurable antenna systems that are being rolled out in many new cell phones and laptop computers. It’s arguable that most mass-produced antennas are based on variants of the planar inverted F-antenna (PIFA), which has a number of ‘hooks’ in the design that engineers can use to facilitate reconfiguration, but I believe there some industrial-scale wireless platforms and a number of satellites which also exploit the capabilities of reconfigurable antennas. The economy of scale plays a large role in their adoption into mass production, so one of the main drawbacks has been the cost, losses and reliability of reconfiguration mechanisms. Integration is also a major issue which separates concept from practice; there are inherent physical challenges when integrating a terminally-connected reconfiguration mechanism (e.g. one that is electronically controlled through conductive biasing and control lines – RF MEMS, etc.) such that it does not negatively interact with the performance governing fields of the antenna.
Looking back, it’s very illuminating that, when I started working in this area in 2006, we put together a prototype model using coffee straws as a ‘fluidic system’ and samples of colloidal material that were donated remnants from a production run. This prototype was used to evaluate the analytical work we had prepared, and it worked very well to illustrate the concept, but microfluidics, microvascular systems and tunable fluidic materials have all continued their rapid march forward in leaps and bounds since the article appeared in 2011. There has also been a remarkable increase in the availability and accessibility of low-cost 3D printing technologies as well as more advanced 3D printing techniques. All of these areas have really demonstrated the special ability to create unique fluidic systems for use in reconfigurable antennas, and I think a review of the fluidic-enabled reconfigurable antenna systems being reported on now provides a real testament to the increased interest in this area. I’ve tried to avoid placing any expectation on the advancement of these systems, but I will say that I am very appreciative of the efforts that have gone into their improvement, and I’m always on the lookout for new developments in this area.
Eutectic Gallium Indium (EGaIn) is inherently ‘fun stuff’ for antenna engineers since it provides the freedom to physically displace conducting material around the antenna in a pressure-driven network. There’s been a growing interest in this area with regards to flexible, stretchable and reconfigurable antennas, and we’ve been tinkering with it for several years as well. This was a perfect opportunity to report on what we felt was a real advancement in the use of these material systems. From a pedagogical standpoint, there’s also a tangible ‘wow’ factor for students who’s ever seen ‘liquid metal’ in a sci-fi movie or other media, so this is a great recruitment tool for many future engineers since we really can demonstrate that some of these far-our futuristic concepts are within reach while still connecting them to some very fundamental antenna concepts. It also allows to you avoid biasing and control structures, which can be made transparent to the antenna in a pressure-driven system.
This is a two-state frequency-reconfigurable antenna that has taken many of the basic concepts from microfluidics and reconfigurable antennas and paired them down to the lowest common denominator. The antenna we present in this issue really highlights how a segmented (or piece-wise) fluidic system can be integrated onto the antenna to avoid all of the losses one might expect from commonly-used microfluidic materials.
Demonstrating the basic concept of segmenting the fluidic network and illustrating how materials not well-suited for antennas (e.g. PDMS) can still be used effectively are two major accomplishments. Indium is one of the rarest metals on earth, so our antenna won’t likely go into low-cost production soon, but there are definitely some application spaces out there that can benefit from the ability to provide displaceable conducting bodies without traditional biasing and control structures.
I would like to highlight that this work was a part of a research-based capstone senior design project in the TAMU ECE Dept. (Hannah McQuilken and Brittany Lawrence, our co-authors, worked very hard to complete the reconfigurable antenna and fabricate the fluidic systems), and mention that the effort was collaboratively mentored by Prof. Arum Han’s research group (microfluidics) and my own research group (reconfigurable antennas). I would also like to acknowledge the U.S. Office of Naval Research and U.S. National Science Foundation for their continued interest in superconfigurable antenna systems and biologically-inspired antennas, respectively.
Leaks and losses… One of the biggest challenges of this work was the fabrication of the fluidic system and its integration onto the antenna. The fluidic system was changed several times throughout the course of this project, and it seems simple now, but we eventually decided it made sense to try and be a little more clever in how we dealt with the fluids (e.g. strategically reducing the overall footprint they needed).
Reconfigurable antenna systems have a surprisingly long history, which I believe dates back to the rotating cam-and-gear mechanical system by Oliner and Rotman in the 1950’s that was integrated into trough waveguide antenna for beam-scanning. I bring this up only because it illustrates how many innovative reconfiguration mechanisms for antennas can sometimes find themselves out-of-phase with their ability to be implemented efficiently, and in the case of the troughguide the advent of low-cost electronic phase shifters provided a path of least resistance. This being the case, I believe there are two serious challenges related to bringing microfluidic antenna technologies into widespread use. First, we need to bring the applied electromagnetics community into the development of emerging microfluidic fabrication technologies so reconfiguration systems can be built around the basic need for low losses and controllable, high speed reconfiguration. Second, these systems must find their right place in the application design space, where it makes sense to have fluidic systems – this latter one seems challenging, but there are definitely some application spaces where these fluidic technologies can have a tremendous impact.
A large portion of my own group’s work in this vein of reconfigurable antennas involves the pressure-driven flow of nanoparticle dispersions and their electric field assisted control. We have some really exciting results coming from this work, but we are now integrating thermal control and structural morphing modalities into the reconfigurable antenna design space. There are a number of other biologically-inspired systems and other really exciting areas that are in the pipeline, and we (Prof. Jean-Francois Chamberland and I) are examining the interface between self-aware systems and reconfigurable antennas (fluidic and otherwise) through user interaction in smartphones, tablets and other devices… so please stay tuned!
This article is based on the Letter: ‘Frequency reconfigurable patch antenna using liquid metal as a switching mechanism’ (new window).
Huff Research Group: www.ghhuff.com (new window)
A PDF version (new window) of this feature article is also available.
Browse or search all papers in the latest or past issues of Electronics Letters on the IET Digital Library.