NASA Tech Briefs - Hilbert-Curve Fractal Antenna With Radiation-Pattern Diversity

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Hilbert-Curve Fractal Antenna With Radiation-Pattern Diversity

John H. Glenn Research Center, Cleveland, Ohio

Sep 30 2007

Two radiation patterns are attainable without active switching.

A printed, folded, Hilbert-curve fractal microwave antenna has been designed and built to offer advantages of compactness and low mass, relative to other antennas designed for the same operating frequencies. The primary feature of the antenna is that it offers the advantage of radiation-pattern diversity without need for electrical or mechanical switching: it can radiate simultaneously in an endfire pattern at a frequency of 2.3 GHz (which is in the S-band) and in a broadside pattern at a frequency of 16.8 GHz (which is in the Kuband). This radiation-pattern diversity could be utilized, for example, in applications in which there were requirements for both S-band ground-to-ground communications and Ku-band ground-to-aircraft or ground-to-spacecraft communications. The lack of switching mechanisms or circuitry makes this antenna more reliable, easier, and less expensive to fabricate than it otherwise would be.

Figure 1. This Multilayer Microstrip Antenna radiates in an end-fire pattern (maximum gain in plane) at 2.3GHz and in a broadside pattern (maximum gain perpendicular to plane) at 16.8 GHz. Dimensions shown hereare typical and can be adjusted to optimize performance, change operating frequencies, or both.

Fabrication of the antenna begins with etching of its Hilbertcurve fractal conductor pattern onto a single 5-mil (0.127-mm)- thick substrate of a dielectric material that has a relative permittivity (εr) of 2.2. The conductor is formed as a microstrip 0.5 mm wide. Notches are cut into the substrate to facilitate folding. Then the patterned, notched substrate is folded, along with 1-mm-thick separation layers made of a dielectric foam having εr = 1.07, to form the multilayer structure shown in Figure 1. This multilayer structure is mounted onto an aluminum ground plane.

Figure 2. Two Radiation Patterns are attainable, as shown : (a) end-fire radiation pattern at S-band and (b) broadside radiation pattern at Ku-band.

The antenna is excited via a probe feed. At 2.3 GHz, the antenna presents a matched reactive load to the probe feed and functions as a miniature dipolelike antenna that produces the endfire radiation pattern [see Figure 2(a)]. The antenna can be tailored at this frequency by adjusting the length of the probe feed in conjunction with the location of the probe connection and by choice of strip-line width, strip-line spacings, and interlayer spacings. At frequencies in the vicinity of 16.8 GHz, the antenna resembles a square patch antenna having dimensions close to a half wavelength, resulting in a broadside radiation pattern characterized by gain and bandwidth comparable to those of a microstrip patch antenna designed for operation in the same frequency range [see Figure 2(b)].

This work was done by James A. Nessel, Félix A. Miranda, and Afroz Zaman of Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17927-1.

This Brief includes a Technical Support Package (TSP).

Hilbert-Curve Fractal Antenna With Radiation-Pattern Diversity (reference LEW-17927-1) is currently available for download from the TSP library.

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