Reflector Connects 18.7-GHz Backhauls

Antennas can take many forms for many different applications. One of the less "visible" parts of a wireless network is the backhaul connection, such as between a central routing station and a cellular base station. To serve such needs a low-cost, high-gain reflector was developed for 18.7-GHz links. It employs a compact circular waveguide feed energized by a coaxial cable and bonded to the radome wall. Its relatively simple geometry is inexpensive to produce, yet provides high-gain performance.

The backhaul portion of a wireless communications system typically connects a backbone, such as a central routing system, and an edge node, such as a cellular telephone base station.1 Backhaul communications have also found applications in the public safety network which enables the real-time transmission of vital data, voice, and video information for public agencies and public-service personnel.²

Backhaul communications can take the form of unlicensed wireless point-to-multipoint bridges operating within frequency bands at 900 MHz, 2.4 GHz, 5.3 GHz, 5.8 GHz, and 60 GHz, Alternately, they can manifest in licensed microwave links (also known as point-to-point microwave radios) and operate in bands at 6 GHz, 11 GHz, 18 GHz, 23 GHz, and 80 GHz. Data throughputs in a backhaul system range from 100 Mb/s to 1 Gb/s full duplex.³

Point-to-point backhaul applications require antennas that have high gain and a tight sidelobe envelope. To achieve high gain, antennas in the form of a solid or wire-mesh reflector of the prime-focus type are often used. In a prime-focus reflector antenna, the feed is usually supported by a tripod or by a bent waveguide energizing the feed itself, commonly called a gooseneck.^4,5 Back-fire feeds connected at the end of a straight hollow waveguide or a coaxial cable are also widely used.^7-9 While offering the simplest feed-supporting method, back-fire feeds have two drawbacks. They require sophisticated design skill to implement effectively, and often lead to a large aperture blockage and a high level of the first few sidelobes. In this article, the authors present a simple reflector configuration that is easy to design and can be realized at low cost for backhaul applications.

Figure 1 shows the antenna structure along with a computer-aided-design (CAD) model for the purpose of performing computer simulations of performance. The notable feature of the design is that a small feed is held at the reflector's focus by bonding it to the inside wall of the radome. The feed's energy is routed to the back of the reflector via a short length of a low-loss coaxial cable. With this arrangement, the precise placement of the feed by either a tripod or a gooseneck waveguide is not required. This greatly simplifies the antenna geometry, which in turn significantly reduces the production cost at expense of a small reduction in antenna gain due to the cable loss (around 0.5 dB at 18.7 GHz, when low-loss cable is used).

A prime-focus reflector based on the above concept was designed for operation from 17.7 to 19.7 GHz. For design verification, the researchers used a ready-made set of a parabolic reflector and a radome purchased from a commercial vendor.⁹ The parabolic reflector has a diameter of 314 mm (D/λ = 19.6 at 18.7 GHz) and a focal length of 110 mm (F/D = 0.35, feed's half-subtended angle = 71 deg.). The radome is composed of 2-mm-thick acrylonitrile butadiene styrene (ABS) thermoplastic material with a relative dielectric constant (εr) of 2.79 and dielectric loss tangent of 0.008 at 10 GHz. The radome's center portion of diameter 40 mm is transformed from a concave shape into a convex form to reduce the reflection from the radome toward the reflector surface and then into the feed. Figure 2 shows dimensions of the designed reflector antenna.

A critical part of developing any reflector antenna is the design of a high-performance feed structure. The properties sought for the feed of a prime-focus reflector antenna include:

1. a small diameter to minimize aperture blockage
2. an axisymmetric radiation pattern with specified beamwidth at a level of -13 to -10 dB
3. low level of back-radiation
4. good impedance matching to the system impedance

Among the many types of feed structure suitable for a prime-focus antenna,¹⁰ one of the simplest and most compact is the dielectric ring-loaded circular waveguide radiator proposed by Raghavan and associates.¹¹ The dielectric loading generates hybrid modes which make the circular waveguide's radiation pattern symmetric around its axis.

For the design of the backhaul reflector, the feed consisted of a circular waveguide loaded with MC nylon material, which features a relative dielectric constant of 3.0 and loss tangent of 0.01 at 10 GHz. To reduce the feed's back -radiation, a quarter-wave (at 18.7 GHz) choke was formed around the aperture of the feed. The diameter of the circular waveguide was adjusted so that the feed provides a -12-dB taper at the reflector edge for a sidelobe level of -23 dB.

Figure 3 shows the dimensions of the designed feed. It was designed with the aid of the Microwave Studio electromagnetic (EM) simulation software from Computer Simulation Technology. Figure 4(a) shows a CAD model of the feed, while Fig. 4(b) shows the fabricated feed. The optimized waveguide diameter is 12.84 mm and the total diameter of the feed including the choke is 16.84 mm (1.05? at 18.7 GHz). The height and distance from the back short of the coaxial probe in the feed are adjusted for low reflection over the range of 17 to 20 GHz. The length and thickness of the nylon dielectric are adjusted for axisymmetric radiation pattern. The feed length is adjusted so that the feed's phase center will be accurately located at the reflector's focal point when the feed is bonded to the radome wall.

Figure 5 shows the measured E- and H-plane field patterns for the feed at 18.7 GHz. The feed has 9.0 dB gain and a front-to-back ratio of approximately 20 dB. The E- and H-plane taper levels at 71 deg. are -13 and -12 dB, respectively. Combined with the 1/r space taper of -0.85 dB at the reflector edge, the total edge tapers are -13.85 and -12.85 dB, respectively, on the E and H planes. This will yield a first sidelobe level at -23 dB below the main beam peak, assuming a parabolic-on-a-pedestal type distribution for the reflector's aperture field. The feed's phase center is 0.5 mm away from the waveguide aperture plane in the probe direction. The feed's far-field phase patterns are flat over its reflector-edge subtended angle of 142 deg.

Prior to constructing a prototype, Microwave Studio simulation software was used to verify the operation of the antenna design according to the CAD model shown in Fig. 1(b). The simulation was simplified by taking advantage of a four-fold symmetry in the structure, with the circular waveguide feed excited with its TE₁₁ mode rather than a coaxial probe. Figure 6 shows the simulated E- and H-plane gain patterns. From experience, it is known that large reflectors are not amenable to accurate simulation by Microwave Studio software using standard computer resources due to the large number of meshes required in the simulation. As a result, the simulation results must be interpreted with the knowledge that they are not fully converged EM solutions. While they may not represent simulations with the highest accuracy, they provide a fair indication of the proper operation of the reflector antenna. The simulation shows that the antenna delivers 33.0-dBi gain at 18.7 GHz, with E- and H-plane beamwidths of 4 deg., and shoulder-type sidelobes of -21 dB (in the E plane) and -28 dB (in the H plane). The antenna's increased sidelobe levels of -1.5 dBi gain in the E plane and -3.5 dBi gain in the H plane at far-out angles near 90 deg. from boresight (theta = 180 deg. in Fig. 6) are caused by diffraction at the reflector rim.

Figure 7 shows the prototype antenna that was constructed for evaluation. The feed is bonded on the inside wall of the radome using an epoxy adhesive. The feed is energized by low-loss cable (about 0.5 dB at 18.7 GHz) of 25-cm length, which is then routed to the rear of the reflector in a shape of minimum obstruction to the antenna's wave. The cable is connected to an SMA-SMA adapter attached on the fixture plate at the rear of the antenna.

Figure 8 shows the measured reflection coefficient of the prototype antenna. The antenna's VSWR is less than 1.50:1 over the frequency range from 17.7 to 19.7 GHz. The antenna's gain and radiation pattern were measured using a planar near-field facility. The table shows the antenna's gain from 17.7 to 19.7 GHz. At 18.7 GHz, a gain of 33.4 dBi translates into aperture efficiency of 57.8%. When the sum of the cable loss (0.5 dB) and radome loss (0.5 dB, estimated) are taken into account, the antenna's efficiency is 72.7%.

Figure 9 shows the antenna's measured gain patterns at 18.7 GHz, along with radiation pattern envelopes for Class 1 and Class 2 antennas per the European Telecommunications Standards Institute (ETSI).¹² The antenna has sidelobes of -22.3 and -20.0 dB, respectively, in the E and H planes, with E- and H-plane beamwidths of 3.74 and 3.62 deg., respectively. The antenna meets the radiation pattern envelope of the ETSI Class 1 antenna (for use in low-interference environments) and nearly satisfies that of the ETSI Class 2 antenna (for high-interference situations). For further improvements in the sidelobe performance, one can apply the shield and absorber lining techniques that have been described in an excellent article by Wojtkowiak¹³ in the May 2004 issue of Microwaves & RF.

In conclusion, this prime-focus reflector antenna uses a compact feed bonded to the radome wall to effectively serve wireless backhaul applications. Its simple design can be constructed inexpensively as a viable low-cost alternative to existing prime-focus reflector configurations.

Jae-Hoon Bang, Research Professor

Tsend Shirirbaatar, Graduate Student

Bierng-Chearl Ahn, Professor

Eun-Jong Cha, Professor

Chungbuk National University, San 48 Gaesin-Dong, Heungdeok-gu, Cheongju-si, Chungcheongbuk-do, 361-763, South Korea, +82 (43) 261-2114, FAX: +82 (43) 263-0612; e-mail: [email protected], www.cbu.ac.kr

Acknowledgments

The authors would like to express their appreciation to the financial support by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0001045), and by the Korea Healthcare Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (A100054).

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