Leakywave Antennas

Research Projects

Introduction From Dr. DeLisio's website:

Guided electrical waves travel along coaxial cables, optical fibers, and metallic waveguides. Unguided, or propagating, waves travel through air. An antenna is a device that converts guided waves to unguided waves and vice-versa. Antennas are thus an integral part of many communications systems, especially today's popular "wireless" systems. Examples of systems that require antennas include radio and television systems, cellular telephones, radar, and satellite communication links. To understand antennas, one must consider the relation between voltages and currents in the guided wave to electric and magnetic fields in the unguided wave. This property makes antennas a very interesting subject of study.

Here at the University of Hawaii, we have begun some preliminary investigations of a leaky-wave antenna. Leaky-wave antennas radiate microwave and millimeter-wave energy from a series of metal strips mounted on a dielectric waveguide. The dielectric waveguide operates similarly to an optical fiber. The spacing of the metal strips relative to the guided wavelength determines the direction of radiation. Leaky-wave antennas are inexpensive and easy to fabricate, which makes them attractive for millimeter-wave systems.

We have developed an electronically switchable leaky-wave antenna, shown below. This antenna uses p-i-n diodes as switches to control the radiation from two different sets of metal strips. By varying the bias on the strips, we can alter the direction of the main beam.

Operation of the switchable leaky-wave antenna.

Below are some photographs of our antenna. The antenna has a rectangular cross-section, with dimensions of 2.75 by 1.25 inches. The antenna is 36 inches long. We measured a 35-degree change in beam direction at 3.5 GHz by switching the p-i-n diodes. The radiated beam could also be steered by varying the operating frequency. Our next step is to develop a millimeter-wave version that uses microelectromechanical (MEMS) switches.

An Electronically Switchable Leaky-Wave Antenna

Limin Huang, Jung-Chih Chiao, and Michael P. DeLisio

Abstract

We report an electronically switchable dielectric leaky-wave antenna. The main beam angle can be electronically steered using p-i-n diodes. The diodes are used as switches to control the radiation from two sets of gratings with different periods, thereby switching the main beam angle. Beam steering is achieved at a single fixed frequency; no frequency sweeping is necessary. A microwave prototype demonstrates a 35° change in beam direction at 3.5GHz. Measured antenna patterns agree with theoretical predictions. This approach should be scalable to millimeter-wave frequencies using diodes monolithically integrated on a semiconductor waveguide.

Introduction

Leaky-wave antennas based on a dielectric rod waveguide periodically loaded with perturbations have been investigated for some time [1-6]. These antennas are lightweight, easy to fabricate, and readily integrated into conventional millimeter-wave systems---properties that make leaky-wave antennas the subject of continuing interest today. Another intriguing property of leaky-wave antennas is the fact the main beam direction can readily be scanned. The angle \theta of the main beam measured from broadside will be given by the well-known expression

where Many researchers achieve scanning by changing the operating frequency, thereby changing both the free-space and guided wavelengths and in a prescribed manner [1,4,6]. Fralich and Litva [7] have reported a frequency-scannable leaky-wave antenna integrated with a voltage-controlled oscillator, effectively achieving an electronically controlled beam. For many applications, however, fixed-frequency operation would be preferable. Horn and co-workers [8] achieve electronic scanning by using p-i-n diodes to modulate the effective waveguide size. They demonstrate fixed-frequency electronic beam steering by effectively changing the guided wavelength . These researchers report a 10°-change in beam angle as the diode bias is varied. Finally, Maheri and others [9] achieve fixed-frequency beam scanning by varying a static magnetic field in a corrugated ferrite slab. This approach also effectively varies the guided wavelength .

Figure 1. Principle of antenna operation. Metal strips loaded on a dielectric waveguide radiate microwave energy. p-i-n diodes act as switches to change the strip spacing from d1 to d2, thereby switching the beam angle from theta_1 to theta_2.

We report fixed-frequency beam switching using a different approach: we electronically vary the perturbation spacing d. Fig.1 illustrates the idea. A rectangular dielectric rod is loaded with a grating of metal-strip perturbations. p-i-n diodes are loaded in these metal strips. Two grating periods d₁ and d₂ are present. The p-i-n diodes act as switches, controlling the effective grating spacing. A dc bias voltage controls the state of the p-i-n diode switch, thereby controlling whether grating d₁ or d₂ primarily radiates. This, in turn, switches the main beam angle from theta_₁ to theta_₂. The beam could be switched among many possible angles by using several grating spacings. This approach has a number of advantages. Beam scanning is achieved without changing the operating frequency. Electronic control allows rapid variation of the scan angle. Finally, the approach is amenable to MMIC fabrication techniques by monolithically integrating the switch diodes on a silicon, gallium arsenide, or indium phosphide waveguides.

Figure 2. Photographs of the leaky-wave antenna. The dielectric waveguide cross-section is 2 3/4 by 1 1/4 inches. The antenna is 36 inches long.

Antenna Fabrication

We designed, fabricated, and tested a microwave scale model as a proof of the concept. A microwave system has the advantage of being easy to fabricate and test, while the approach should be scalable to millimeter wavelengths. Fig.2 shows a photograph of the antenna. The dielectric rod waveguide is constructed out of Lucite, with a relative dielectric constant of 2.56. The cross-section of the rod is rectangular, machined to be 2 3/4 by 1 1/4 inches. These dimensions were chosen to be comparable to the cross-section of S-band waveguide as well as a standard size for easy fabrication. The entire rod is 36 inches long, with both ends tapered to a point to improve the input and output matching. Metal-strip radiators are positioned along the length of the antenna. Six strips are spaced 5.1cm apart d₁ and five strips are spaced at 7.8-cm intervals d₂. All strips are identical, with the length and width determined experimentally. M-pulse packaged p-i-n diodes are located midway along the strips. Thin magnet wire loaded with ferrite-bead chokes provide dc bias to the switch diodes.

Fig.3 shows a simplified schematic of the measurement setup. A sweep oscillator connected to a S-band pyramidal horn is used to excite the dielectric waveguide. The isolator helps to maintain the frequency stability of the generator. A copper plate at the mouth of the horn is used as a screen to block any direct radiation from the horn itself. The dielectric rod is excited through an aperture in the plate. The copper screen can also be noted in Fig. 2(b). Microwave absorbing material placed at the tapered end of the rod acts as a termination. The antenna pattern is measured with a small wideband horn antenna and a spectrum analyzer.

Figure 3. Simplified schematic of the measurement setup.

Measured Results

To test the beam switching, we excite our dielectric leaky-wave antenna with a 3.5-GHz signal. One set of p-i-n diodes is forward biased +1 V and the other set is reverse biased -25 V. Fig. 4 plots the antenna's measured H-plane radiation patterns. Fig. 4(a) shows the pattern with the diodes biased such that the 5.1-cm grating is the primary source of radiation. Fig. 4(b) shows the pattern with most of the radiation emitting from the 7.8-cm grating. Negative angles correspond to a beam radiated in the backward direction. The main beam shifts by 35° when the diode bias is switched. We note that the absolute peak radiated power for the two cases are within 1 dB of each other. We suspect that the broad flat sidelobe centered at -25° in Fig. 4(b) may be the result of reflections from the copper end plate. The measured cross-polarized power is low---more than 10 dB below the power in the main beam, as shown in Fig. 5.

Figure 4. Measured and theoretical H-plane radiation patterns. (a) corresponds to the 5.1-cm strip spacing and (b) corresponds to the 7.8-cm strip spacing.

The theoretical curves in Fig. 4 are generated using Marcatilli's method to determine the wavelength in the dielectric rod [10]. We also assume that the currents in the radiating strips have identical magnitudes. Ideally, the non-radiating strips would carry zero rf current. Practically, however, the diode switch cannot completely prevent these strips from radiating. It is this spurious radiation that causes the rather high sidelobe level. A good fit between theory and experiment is achieved when we assume the spurious power radiated by a single non-radiating strip is 12 dB less than the power radiated from a single radiating element. It may be possible to improve the agreement between theory and experiment by assuming a more complicated non-uniform radiating strip illumination.

Figure 5. Measured cross-polarized H-plane radiation pattern.

Fig.6. Measured and theoretical main beam angle for both strip spacings as a function of frequency.

It is also possible to scan the main beam for both grating spacings d1 and d2 by varying the operating frequency. This is illustrated in Fig.6. The operating frequency is varied from 3.0 to 4.0GHz. The theoretical curves are generated using equation (1), with determined using Marcatilli's approach. The agreement for both strip spacings is very good. With frequency scanning, the main beam angle can be swept over a considerable angular range.

Conclusion

We have demonstrated an electronically switchable leaky-wave antenna using p-i-n diode switches. The angle of the main beam changes by 35° when the bias on the diodes is varied. Measured radiation patterns agree very well with theoretical predictions. Although these measurements were conducted on a 3.5-GHz microwave model, the approach should be extendable to millimeter wavelengths using monolithic fabrication techniques.

At very high millimeter-wave frequencies, parasitic losses in the diodes will ultimately limit the antenna efficiency. An intriguing alternative is to use microelectromechanical (MEMS) switches to control the beam direction [11,12]. These tiny switches have very low series losses well into the millimeter-wave regime. The idea is proposed in Fig. 7. A cantilever-beam switch is formed by depositing a thin layer of dielectric material with evaporated metal patterns on sacrificial layers. The cantilever beam is supported by a thicker layer of electroplated metal. The dielectric layer serves two functions: it provides DC bias isolation for an electrostatic force to close the switch and it provides the required stress to separate the contact when the bias is off. Another interesting possibility is to use photo-sensitive devices as switches, controlling the beam direction optically [13].

Figure7. Proposed MEMS switch for millimeter-wave applications. The state of the switch is controlled by the bias lines.

References

[1] K.L. Klohn, R.E. Horn, H.J. Jacobs, E. Freibergs, ``Silicon Waveguide Frequency Scanning Linear Array Antenna,'' IEEE Trans. Microwave Theory Tech., vol. 26, pp. 764--773, Oct. 1978.

[2] T. Itoh, B. Adelseck, ``Trapped Image Guide for Milli-\break Meter-Wave Circuits,'' IEEE Trans. Microwave Theory Tech., vol. 28, pp. 1433--1436, Dec. 1980.

[3] S. Kobayashi, R. Lampe, R. Mittra, S. Ray, ``Dielectric Rod Leaky-Wave Antennas for Millimeter-Wave Applications,'' IEEE Trans. Antennas Propagation, vol. 29, pp. 822--824, Sept. 1981.

[4] T.N. Trinh, R. Mittra, R.J. Paleta, ``Horn Image-Guide Leaky-Wave Antenna,'' IEEE Trans. Microwave Theory Tech., vol. 29, pp. 1310--1314, Dec. 1981.

[5] F.K. Schwering, S.-T. Peng, ``Design of Dielectric Grating Antennas for Millimeter-Wave Applications,'' IEEE Trans. Microwave Theory Tech., vol. 31, pp. 199--209, Feb. 1983.

[6] M. Ghomi, B. Lejay, J.L. Amalric, H. Baudrand, ``Radiation Characteristics of Uniform and Nonuniform Dielectric Leaky-Wave Antennas,'' IEEE Trans. Antennas Propagation, vol. 41, pp. 1177--1186, Sept. 1993.

[7] R. Fralich, J. Litva, ``Beam-Steerable Active Array Antenna,'' Electronics Lett., vol. 28, pp. 184--185, Jan. 1992.

[8] R.E. Horn, H. Jacobs, E. Freibergs, K.L. Klohn, ``Electronic Modulated Beam-Steerable Silicon Waveguide Array Antenna,'' IEEE Trans. Microwave Theory Tech., vol. 28, pp. 647--655, June 1980.

[9] H. Maheri, M. Tsutsumi, N. Kumagi, ``Experimental Studies of Magnetically Scannable Leaky-Wave Antennas Having a Corrugated Ferrite Slab/Dielectric Layer Structure,'' IEEE Trans. Antennas Propagation, vol. 36, pp. 911--917, Nov. 1988.

[10] E.A.J. Marcatilli, ``Dielectric Rectangular Waveguide and Directional Coupler for Integrated Optics,'' Bell System Technical J., vol. 48, pp. 2071--2102, Sept. 1969.

[11] K.E. Petersen, ``Micromechanical Membrane Switches on Silicon,'' IBM J. Res. Devel., vol. 23, pp. 376--385, July 1979.

[12] J.-C. Chiao, D.B. Rutledge, ``Microswitch Beam-Steering Grid,'' 17th Int. Conf. Infrafed Millimeter Waves Dig., pp. 406--407, Dec. 1992.

[13] V.A. Manasson, L.S. Sadovnik, A. Moussessian, D.B. Rutledge, ``Millimeter-Wave Diffraction by a Photo-\break Induced Plasma Grating,'' IEEE Trans. Microwave Theory Tech., vol. 43, pp. 2288--2290, Sept. 1995.

Created by J.C. Chiao