IARPA posts ESAs RFI
The Intelligence Advanced Research Projects Activity (IARPA) seeks information regarding innovative approaches to enhance the effective performance of antennas with an emphasis on electrically small antennas (ESAs) and antenna arrays with strongly coupled electrically small elements. Responses to this RFI are due by 5:00 p.m. Eastern on December 24. An IARPA virtual invitation-only workshop cosponsored with DARPA is being planned for January 19, 2021.
It is known from various fundamental theorems, such as the Chu Limit, that there is a fundamental limit on ESA bandwidth and gain-bandwidth product, provided that the system is linear, passive, and time-invariant. It is also known that surpassing these limits is possible by breaking any of the three aforementioned assumptions.
One way to circumvent these assumptions is to activate the antenna with non-linear and/or time-varying elements or materials. These techniques require active elements or materials which change the effective size, weight, and power (SWaP) of the passive antenna that is being replaced. Another common method of breaking the assumptions is by employing non-Foster circuits as antenna drivers.
Despite allowing for a greatly-enhanced ESA bandwidth in theory, such configurations have several drawbacks that prevent one from reaching these performances in practice. Current electromagnetic theory, numerical solvers, and measuring techniques for passive antennas are not ideally suited for non-LTI antenna-like structures, and commercial active elements have performance limitations. Non-Foster circuits are plagued by a number of issues, such as power instability, generation of unwanted circuit harmonics and inter-modulations, large dispersion in their frequency-impedance curves, and practical limitations of commercial circuit components such as nonlinearities and parasitic capacitances/inductances. Additionally, the design and simulation of coupled antenna/load circuit systems is significantly hindered by the lack of integrated design tools capable of co-simulation of both the electromagnetic and circuit behavior of such systems as well as the complex interplay between the two.
Background & Scope
ESAs – i.e., antennas whose dimensions are considerably smaller than the antenna’s resonant wavelength – are of interest in numerous applications. Fundamental physics limits the performance of ESAs. This performance limit, explored by Harold Wheeler and refined over the years by Lan Jen Chu and others, has proven to be indomitable against passive antenna designs. One approach to beat this fundamental limit is to contravene the assumptions it is based upon, specifically linearity and time-invariance (LTI). Wheeler reasoned that the volume of a passive ESA limits the power it can extract from an incident field as indicated by a small radiation resistance (P = I2R) and a corresponding small bandwidth. However, the power extraction potential of antenna-like structures with time-varying resistive or reactive loads has yet to be established, theoretically, numerically, and/or experimentally. As an example, it is possible that an improved signal-to-noise ratio in a non-LTI antenna-like structure could be indicative of a larger effective bandwidth as related by Shannon’s theorem for channel capacity. There may be other possible performance advantages beyond bandwidth. Advances in the field of non-LTI antenna-like structures could result in a paradigm shift, significantly impacting how the Intelligence Community (IC) collects radio frequency (RF) information wirelessly.
Another goal of this RFI is to develop deeper insights into breaking the passivity assumption of the Chu limit through approaches such as non-Foster circuits. Specifically, this RFI aims to identify fundamental limits to gain-bandwidth product of broadband ESAs and identify ways to circumvent or surpass these limitations while maintaining power stability of the system.
This RFI seeks innovative approaches to improve the effective performance limits of ESAs and other antennas with active elements.
Full information is available here.