In a previous report it was shown that the channel capacity of an in vivo communication link using microscopic antennas at radiofrequency is severely limited by the requirement not to damage the tissue surrounding the antennas. For dipole-like antennas the strong electric field dissipates too much power into body tissues. Loop-type antennas have a strong magnetic near field and so dissipate much less power into the surrounding tissues but they require such a large current that the antenna temperature is raised to the thermal damage threshold of the tissue. The only solution was increasing the antenna size into hundreds of microns, which makes reporting on an individual neuron impossible. However, recently demonstrated true magnetic antennas offer an alternative not covered in the previous report. The near field of these antennas is dominated by the magnetic field yet they don’t require large currents. Thus they combine the best characteristics of dipoles and loops. By calculating the coupling between identical magnetic antennas inside a model of the body medium we show an increase in the power transfer of up to 8 orders of magnitude higher than could be realized with the loops and dipoles, making the microscopic RF in-vivo transmitting antenna possible.

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Pushing the limits of radiofrequency (RF) neuronal telemetry

Recent experiments on high-efficiency permeable conformal antennas have drawn attention to a long-standing misconception on the theory of the ferrite rod antenna family. Deriving in closed form the radiation efficiency of these antennas and using full-wave simulations, we show that their radiation efficiency is not necessarily reduced by the loss tangent of the permeable material. It is shown that the key material parameter controlling the radiation efficiency of these antennas is not the loss tangent but the maximum of the Heaviside magnetic conductivity, a quantity proportional to Snoek’s product. This yields the clear design rules for obtaining permeable antennas with maximized efficiency-bandwidth product (EBWP). Because the radiating currents of these antennas are the magnetic polarization currents in the material, they can be placed close to a metal ground plane or inside a low impedance medium without performance degradation while having an EBWP much higher than the metal-and-dielectric antennas having the same conformal volume, in spite of the presence of the magnetic loss. As an example, an antenna operating from HF to VHF can attain an EBWP 10 dB higher than the corresponding metal antenna. Similarly, using the Best–Yaghjian formalism, it is shown that these antennas are easier to match and can attain wider bandwidths than the equivalent conformal metal antennas.

Why the Magnetic Loss Tangent Is Not a Relevant Constraint for Permeable Conformal Antennas

A conformal permeable toroidal antenna operating in both a vertically polarized monopole mode and circularly polarized (CP) dipole mode in the UHF frequency range is designed based on theory of magneto-dielectric radiators. This antenna has been fabricated using a new advanced ferromagnetic laminate composite material whose high impedance and anisotropic properties enable an antenna that radiates via magnetic currents. The antenna, less than 0.5 m in diameter and less than 2 cm thick, was fabricated and tested. Including the imperfections of the feed network and a simple matching scheme, the antenna operates from 200 to 500 MHz with an input match of better than -10 dB, CP dipole realized gain as high as +2 dBi, and monopole mode vertically polarized gain as high as -1 dBi.

A Wideband Multimode Permeable Conformal Antenna Thinner Than $\lambda /75$ Using Advanced Ferromagnetic Laminate Composite Materials

The present disclosure provides systems and methods for enhancing the performance of permeable antennas. Further, the parallel solenoid feed system disclosed herein may be used to reduce or eliminate significant phase delays in antennas, which may lead to destructive interference. Moreover, use of the parallel solenoid feed in an antenna eliminates the need for multiple feeds, complicated feed networks, and elaborate matching circuits. Using the parallel solenoid feed in circular magnetic antennas may enhance the performance of the antenna through maintaining the flux. Finally, many adjustable parameters for further tuning and/or optimizing the performance of particular antenna design have been identified herein, which may allow those skilled in the art to utilize known systems, such as full wave simulation software, to determine the desired final design for an antenna utilizing a parallel solenoid feed.

Parallel Solenoid Feeds for Magnetic Antennas

Under SBIR-funded contracts [1 & 2], researchers at JEM Engineering and Arizona State University have demonstrated the critical components of a new class of physically small antenna that utilizes a magneto-dielectric composite material having a high magnetic flux conductivity [3]. The magnetic material is placed into flux channels which are made from a metallo-dielectric substrate. The substrate can be placed directly onto a conducting platform skin and its thickness is on the order of one hundredth of a wavelength or less. This paper introduces a novel realization of such a flux channel antenna by winding a thin composite tape about a mandrel [4]. The number of turns is potentially several thousand as the composite tape thickness is only a half mil (13 microns). Measured gains (IEEE) for a magnetic loop, which is equivalent to an electric monopole, of-5 to +4 dBiL were demonstrated from 225--600 MHz by a prototype antenna that measured 0.25" high by 16" diameter.

Tom Sebastian

Papers

Magneto-Dielectric Wire Antennas Theory and Design