Small electronic devices (cell phones, mobile DTV receivers, etc.) demand small antennas, but it has been difficult to create electrically small antennas (lengths of around a tenth wavelength or less) without a huge loss in efficiency and bandwidth. Researchers at the University of Illinois at Urbana-Champaign used a conformal printing technique to print antennas on hemispherical substrates using metallic inks.
Jennifer T. Bernhard, a professor of electrical and computer engineering at the University of Illinois, explained, "Recent attention has been directed toward producing antennas by screen-printing, inkjet printing, and liquid metal-filled microfluidics in simple motifs, such as dipoles and loops. However, these fabrication techniques are limited in both spatial resolution and dimensionality, yielding planar antennas that occupy a large area relative to the achieved performance."
Jennifer A. Lewis, a materials science and engineering professor at the school, described the approach. "Omnidirectional printing of metallic nanoparticle inks offers an attractive alternative for meeting the demanding form factors of 3D electrically small antennas (ESAs). To our knowledge, this is the first demonstration of 3D printed antennas on curvilinear surfaces."
The antennas are typically one-twelfth wavelength or less in size. For reference, at 180 MHz, a wavelength is one and two-thirds meters. A twelfth wavelength is only 0.14 meter, or about 5.5 inches. This is still big, but a lot easier to add to a cell phone or cable case than a typical half-wave dipole, which would be almost 33 inches long, or even a quarter-wave whip (monopole) antenna (which would be compromised if there weren't a large enough ground plane) at half that size. The researchers said the 3-D conformal antenna delivers performance metrics an order of magnitude better than those realized by monopole antenna designs.
"There has been a long-standing problem of minimizing the ratio of energy stored to energy radiated—the Q—of an ESA," said Bernhard. "By printing directly on the hemispherical substrate, we have a highly versatile single-mode antenna with a Q that very closely approaches the fundamental limit dictated by physics (known as the Chu limit)."
The antenna's operating frequency is determined primarily by the printed conductor cross-section and the spacing between the meander lines within each arm printed on the outside of the hemisphere.
Details on the research were published in the cover story in Advanced Materials.