A Low Cost, High Performance Point-to-point Slotted Waveguide Array
A slotted waveguide array intended for point-to-point radio link applications has been designed for the 23 GHz frequency band with a total bandwidth of more than 10 percent. (Resonant slot antennas typically have a lower percentage bandwidth.) The antenna shows a very low profile and consists of four waveguide layers with coupling slots in between. The antenna also has been designed for easy manufacturing, which is very important for this type of application. The measured performance shows good SWR performance and low sidelobe levels even close to the main beam, which is an advantage compared to traditional reflector antennas. The antenna aperture efficiency is superior compared to a microstrip array antenna.
Bengt Svensson, Göran Snygg and Per Holmberg
Ericsson Microwave Systems AB
Mölndal, Sweden
There is a growing trend to seek low profile solutions for commercial microwave and mm-wave antennas. The ever-increasing number of antenna installations calls for solutions with dimensions that are as small as possible and a landlord-friendly design. Radio point-to-point and point-to-multipoint link applications are no exception and new antenna concepts must be developed. For a long time, the dominating antenna type for these applications has been the reflector antenna. However, the reflector and feed horn together present a considerable thickness.
Flat Plate Antennas
The need for flatter antennas leads to different solutions. One obvious and popular choice is the microstrip planar array.1 This technology is considered to be easy to manufacture and inexpensive. Looking at the detailed design, several layers are often required and special arrangements are sometimes necessary to prevent parallel plate modes from propagating between different layers. These characteristics together with the cost of low loss materials and the supporting structure make the total antenna cost less attractive. It is also difficult to reduce the losses for a microstrip array, especially at high frequencies. Microstrip solutions could easily result in a 5 to 6 dB loss for an ordinary high gain flat plate array at frequencies around 20 to 30 GHz. This loss may be 3 to 4 dB more than for a reflector antenna. As a result, the microstrip antenna must have more than twice the area of the reflector antenna to achieve the same gain. The use of a lossy microstrip array will certainly reduce the thickness of the antenna, but will also produce a large increase in the antenna's area.
Another solution to this problem is to use waveguide horn elements and a waveguide feed network.2 This configuration offers a low loss solution but does not reduce the thickness of the antenna to the same extent as the microstrip array.
A concept related to the waveguide horn antenna is a slotted waveguide array.3 This type of antenna is well known for radar applications but less used for communication purposes. The reason may be that the slotted waveguide array is considered to be expensive to manufacture and the usable bandwidth normally is a few percent of the center frequency for resonant arrays. Nonresonant slot antennas have a wider bandwidth but add more losses to the design.
Slotted Waveguide Array Development
A slotted waveguide array has been developed for application as a point-to-point antenna for the European Telecommunication Standard Institute's (ETSI) 23 GHz radio link frequency band. The aperture is diamond shaped with approximately 300 slots, as shown in Figure 1 . The outer dimensions are 200 mm X 200 mm and the thickness is only 10 mm. The radiating slots are longitudinal shunt slots cut in the broad wall of the top waveguide layer. This top layer is fed from feed waveguides that run perpendicular to the radiating slots. The feed waveguides are, in turn, connected to a summation feed network.
Aperture Distribution
The diamond shape was chosen to achieve low sidelobe levels in the cardinal planes. However, the technique used for the antenna does not put any restriction on the possibilities of amplitude tapering in order to further reduce the sidelobe level.
A circular front face with an aperture amplitude distribution designed for equal sidelobe levels in all angular cuts also would be feasible. An even more attractive concept would be to use all of the available space and build an antenna that covers the entire radio link cabinet (quadratic in most cases). This configuration would offer approximately 45 percent more area, which corresponds to increased directivity of 1.6 dB. However, the aperture illumination would have to be tapered to reduce the sidelobe levels. Depending on the desired sidelobe levels, the resulting gain increase would be 0.5 to 1.0 dB.
Radiating Aperture
The ETSI 23 GHz frequency band extends from 21.2 to 23.6 GHz, which provides a total bandwidth of 10.7 percent. This bandwidth is normally considered to be rather extreme for a resonant slotted waveguide antenna. In order to meet this requirement, special attention must be given to the design over the frequency band.
One of the parameters that affect the frequency behavior is the width of the radiating and coupling slots. It becomes necessary to use a slot width that is approximately 15 to 20 percent of the slot length in order to achieve the desired SWR bandwidth. Accurate slot models are required due to this rather extreme width-to-length ratio. The self-admittance of the radiating slots has been modeled for a number of slot lengths and displacements using the method of moments. These data have been stored in a three-dimensional matrix (length/displacement/frequency) and used directly for the self-admittance in the slot synthesis procedure. To obtain admittance data for parameter values between the calculated points, multidimensional interpolation was necessary. Even though each radiating slot element has good frequency behavior, the SWR bandwidth is still limited by the feed.
Slot antennas are usually divided into subsectors fed from a corporate feed network. This configuration means that a number of slots are fed in series, which reduces the overall bandwidth. Thus, sectors that are as small as possible must be used. Of course, this setup results in a trade-off between the bandwidth and complexity of the corporate feed. For a fixed number of series-fed slots, the bandwidth also depends on the chosen impedance level for the waveguide.4 Therefore, it is possible to optimize the SWR bandwidth using the slot displacements of the radiating slots and the rotation angles of the coupling slots. However, it should be noted that a good SWR bandwidth does not always correspond to nice radiation pattern behavior over the frequency band.
For this specific design, subgroups of 2 x 4 slots have been used, as shown in Figure 2 . This sector shape provides a parallel feed in the E-plane and only two series-fed slots in the H-plane. The complete radiating aperture consists of 36 such subsectors.
The design of the aperture requires several steps involving synthesis at a specific frequency and then analysis over the complete frequency band. Selecting the center of the frequency band is not an obvious choice. Rather, it is necessary to analyze the antenna at several frequencies and then decide whether or not to move the design frequency. For this case the antenna aperture has been designed for 22.7 GHz rather than 22.4 GHz, which is the center of the band.
Another band-limiting factor is the external mutual coupling frequency variation between the slots. It is a common practice to let the slot displacement alternate on each side of the waveguide center line in the same fashion for all waveguides. However, this configuration produces an almost regular element position grid, as shown previously. If the slot displacement for every second waveguide is changed in sign, as shown in Figure 3 , the coupling contribution is less severe from a bandwidth point of view. This configuration requires that the transmission phase sign be corrected in some other way further down in the feed network.
The input impedance for a center subsector of the complete antenna has been calculated for both of these cases. The result is shown in Figure 4 and includes mutual coupling for the complete array. It turns out that the case with the alternating slot displacements shows a more broadband behavior and that a dual resonance is formed.
Corporate Feed
The corporate feed network distributes the energy from the input/output port to the radiating slots, as shown in Figure 5 . It is divided into two layers and formed principally by H-plane, T-shaped power dividers, shown in Figure 6 , and divided into two layers with interconnections between them. H-plane bends of different angles and E-plane steps are used to connect these components. Finally, an E-plane T-junction connects the antenna to the input/output ports of the radio, as shown in Figure 7 . The T-junction also includes a transformer to standard waveguide flange dimensions.
The bandwidth requirement for the standard link frequencies (21.2 to 23.6 GHz) of 10.7 percent makes it necessary to pay special attention to the internal matching of the corporate feed network. The return loss of each component must be considerably better than the overall requirement in order to meet the specification. Each component has been designed using the HP HFSS high frequency structure simulator. The power dividers and other parts of the network were first designed individually to a reflection goal of less than -25 to -30 dB over the full 10 percent bandwidth. This performance was achieved by impedance transformers and with matching H-plane irises. An example of the return loss of one of the power dividers is shown in Figure 8 .
When assembled, the network components affect each other because of the compactness of the corporate feed and larger groups must be matched in the actual environment. Finally, the complete network was analyzed using HFSS S-parameter results for individual component (or group of components) connected by straight waveguides using the HP MDS microwave design simulator software.
Measurements
The antenna was measured at a far-field test range and all important parameters such as sidelobe level and gain were evaluated. The measured H- and E-plane patterns for a prototype antenna are shown in Figures 9 and 10 , respectively. The result is given at the center frequency and shows that the sidelobe level close to the main beam is very low and far below the ETSI class 2 standard requirements for point-to-point applications. The far out sidelobes tend to increase due to the periodic nature of an array antenna. For the H-plane, the slot element factor suppresses the sidelobes, but in the E-plane they are slightly higher. It should be noted that the sidelobe level can be controlled by using a modified amplitude tapering.
The cross-polarization pattern for the E-plane is shown in Figure 11 . Due to the cross-polar purity of the slots, the level is lower than -48 dB below the copolar beam peak even in the main beam direction. The H-plane measurement shows similar performance.
The gain is measured at 31 dBi, providing a total efficiency of 55 percent, which compares very well with a reflector antenna of the same size. The antenna's input reflection is shown in Figure 12 . The return loss is > 14 dB for most of the band except at the 21.2 GHz low band edge where it is 12.5 dB. However, the applications intended for this antenna do not include this low end of the band (21.2 to 21.6 GHz) and the design requirement was therefore relaxed for this part of the band.
Conclusion
A resonant slotted waveguide array for a point-to-point application at 23 GHz has been designed and measured. It was demonstrated that it is possible to achieve a bandwidth of more than 10 percent (-14 dB return loss) with this type of antenna. The antenna profile is very low with a thickness of 10 mm.
The sidelobe levels close to the main beam are considerably lower than those of a similar reflector antenna and in the same order at far out angles. The cross-polarization level is extremely low even in the main beam direction. It should be noted that the sidelobes are controllable by changing the antenna's aperture distribution. Thus, it is possible to use all available space on the radio cabinet and still be able to control and keep the sidelobe levels at very low levels. It has also been shown that a low loss waveguide feed network can be used to achieve a very good gain performance.
References
1. C.A. Balanis, Antenna Theory and Design, John Wiley & Sons, pp. 722-784.
2. J.F. Johansson and N.D. Whyborn, "The Diagonal Horn as a Sub-millimeter Wave Antenna," IEEE Transactions on Microwave Theory and Techniques, Vol. 40, No. 5, May 1992.
3. S.R. Rengarajan, L.G. Josefsson and R.S. Elliot, "Waveguide-fed Slot Antennas and Arrays: A Review," Electromagnetics, Vol. 19, No. 1, January/February 1999.
4. A. Derneryd and R. Petersson, "Bandwidth Characteristics of Monopulse Slotted Waveguide Antennas," Proceedings of the Fourth International Conference on Antennas and Propagation (ICAP 85).
Bengt Svensson received his MSEE degree from Chalmers University of Technology, Gothenburg, Sweden, in 1984. He then joined Ericsson in Molndal, Sweden where he has held several different positions. Currently, he is an antenna specialist in the company's antenna R&D department. His major research interests are waveguide and slotted waveguide antenna systems for communication and radar applications. Other areas of interest are antenna near-field diagnostics, phased-array calibration and active-array antennas. Svensson can be contacted via e-mail at bengt.svensson@ericsson.com.
Göran Snygg received his MS degree in electrical engineering from Chalmers University of Technology, Gothenburg, Sweden, in 1995. He then joined Ericsson where he is now with Ericsson Product Unit Microwave Solutions. Snygg's major research interests are base station antennas, waveguide and slotted waveguide antennas, waveguide filters and multichip modules. He can be contacted via e-mail at goran.snygg@ericsson.com.
Per Holmberg received his MS degree in electrical engineering and applied physics from Linköpings University, Sweden in 1997. He then joined the antenna design department at Ericsson. His major research interests are waveguide and slotted waveguide antennas. Holmberg can be contacted via e-mail at per.r.holmberg@emw.ericsson.se.