Laboratory Research Twists Antenna Technology
Scientists bend, not break, the laws of physics.
Faced with limitations imposed by physics, laboratory researchers are generating antenna innovations by tweaking constructs to change the rules of the antenna game. Their efforts do not seek to violate long-held mathematical theorems or laws of physics. Instead, they are working to find lawful ways of working around limitations that long have inhibited the development of antennas that would suit user needs with fewer tradeoffs.
Currently, many types of antennas can be made small enough to fit in a tight area. Yet, they suffer performance drawbacks or are extremely limited in their application. Conversely, the type of antenna suitable for high-bandwidth links may prove detrimental to a use that requires low observability.
Laboratories in industry and academia are pursuing different approaches for future antenna technology breakthroughs. These efforts involve materials, architectures and network topologies. If successful, this research could lead to unobtrusive panels that replace large antennas as well as new capabilities for antenna-bearing platforms.
Howard Stuart, technical staff member at LGS Innovations, explains that the art of building smaller antennas comes up against the laws of physics. The issue is not one of miniaturization but of signal performance when antennas are built below a certain size.
“You can’t keep making antennas smaller and smaller,” Stuart points out. “There are fundamental physical limitations, and beyond that, [the antenna] is just not going to work anymore. Or, you’re going to have to give up something, such as gain.”
Stuart continues that developing small antennas involves addressing these fundamental questions, especially with regard to size versus bandwidth limitations. The relationship between size and bandwidth was formulated in 1948 by L.J. Chu of the Massachusetts Institute of Technology. As antennas are built to smaller sizes, their bandwidth shrinks and their efficiency is reduced. The Chu limit established a formula for determining the smallest size an antenna could be formed based on its bandwidth.
At the heart of this formula is an antenna’s Q factor, which defines quality. To develop as low a Q factor as possible for the widest possible bandwidth, engineers should build a spherical antenna, Stuart offers. He continues that researchers have been working on building spherical antennas so they could determine how close they could move toward Chu limits. Ultimately, they have determined that some types of structures allow building antennas that approach 1.5 times the lower limits, which is a benchmark for performance.
One development based on this methodology represents a new approach to spherical antenna technology. Stuart relates that company scientists have built a spherical antenna that does not involve twisted wires, but instead is defined by printed structures on circuit boards. The circuit boards are arrayed in a pattern so that the conductors resonate as a spherical antenna without running electrical connections from one board to another. The connector and the feed attach to only one of the boards, with the wires in the other boards electromagnetically coupled to the structure so that it resonates as a single entity. “Two-D [two-dimensional] building blocks give you a three-dimensional radiator,” he explains.
The result is an antenna with a Q factor close to that 1.5 Chu limit. It also has a double resonance in its impedance response, which is desirable for increased versatility. These types of antennas could be useful in applications in which antenna space is at a premium.
Stuart reports that other groups, particularly in academia, are trying to determine Q limits for arbitrarily shaped volumes, not just spheres. Some organizations have reported advances in that realm.
Another research area involves possible alternatives to patch antennas. The goal of this research, Stuart offers, is to determine if the patch construct is the bandwidth-optimized solution for that type of radiator—as opposed to a small dipole, such as in the spherical antennas. Patch antennas are made as thin as possible for many of their applications, but that thinness narrows its bandwidth.
One design that already has been built offers four times wider bandwidth than conventional patch antennas of the same size. Stuart explains that the new antenna’s Q factor is twice as low as a regular patch antenna because its directivity is lower. He describes this new patch antenna as a periodic surface antenna, because it has a periodic array of gaps across its surface. This construct also offers greater versatility in antenna type, as simple tweaks can change the antenna to suit different functions.
In a more theoretical realm, researchers are exploring the use of new materials to revamp antenna capabilities. This effort entails building antennas with magnetic materials. Stuart explains that, while these materials have high magnetic permeability, they traditionally also have high loss. This prevents them from being used for high frequencies.
He continues that some researchers are examining how to build magnetic antennas that could operate at hundreds of megahertz. Success in this endeavor could open the door to better electric dipole antennas. That advance is not yet commercially viable, he adds.
Potentially, the technology could produce radars that go below the 1.5 Chu limit all the way down to the limit itself. Being able to use magnetic materials would overcome the problem of stored energy that inhibits approaching the Chu limit.
Ultimately, improved magnetic material antennas could lead to vertically polarized electric dipole antennas that have no height. This capability would be especially useful for military platforms ranging from land-based vehicles to large ships. Instead of having numerous antennas bristling skyward—and thus increasing the platform’s visibility to the enemy—engineers could employ magnetic materials to build flat antennas. Similarly, aircraft could be equipped with conformal antennas that do not overly inhibit their aerodynamic characteristics.
More exotic work is aiming at developing non-Foster matching antennas. Stuart elaborates that among the fundamental limitations on antennas are those imposed on passive devices. In particular, Foster’s Reactance Theorem states that negative capacitors and negative inductors cannot exist in passive circuits.
In non-Foster-matching antennas, the components include active components such as transistors to make elements behave as if they were negative capacitors or negative inductors. While these two negative elements cannot exist in nature, an active device can generate their effects.
Stuart continues that a negative capacitor could be placed in front of a short dipole antenna, which has a high positive capacitive reactance. This would cancel out that reactance over a broad bandwidth, which would enable a small antenna to operate over a wide band.
This is a fairly active area of research, he allows, but the effort still has a long way to go before useful advances are realized. Stuart notes that one major issue that remains to be resolved involves circuit stability. Addressing that challenge will require “being really, really good at network theory,” he says. Having that skill will help determine the right circuit topology for building a non-Foster-matching circuit that can enhance an antenna’s bandwidth capacity.
“If that problem can be solved—and I suspect it’s a solvable problem—that will be a big development for certain types of applications of small antennas,” he predicts.
Small antennas would be able to perform tasks currently open only to large antennas, Stuart points out. Similarly, a small antenna could operate over the same bandwidth as another antenna but with a much better signal-to-noise ratio. The result would be an improvement in both capability and construction, which would benefit military applications immensely.
