Technical Field

Base station antenna for mobile communications

Background Art

There are some recent expansions in mobile wireless applications. Mobile TV and mobile WiMax are examples of such new applications. As the number of mobile applications increases, the need for broadband base station antennas increases. Arrays of crossed dipoles are commonly used as base station antennas for mobile communications [1]. This is because base station antennas of mobile communications are demanded to achieve two orthogonal polarizations. They are also required to generate narrow elevation beamwidths and wide azimuth beamwidths. However, such arrays have limited frequency bandwidths which are not sufficient for recent expansions in mobile applications. Furthermore, they are heavy in weight, high in coast and they suffer from high wind loads. Moreover, they are not easy to assemble and disassemble. Therefore, they are shipped and stored in their large-size assembled forms.

Disclosure of Invention

A low-coast, lightweight, low wind load, foldable / deployable, broadband base station antenna that overcomes all the above problems has been developed using dual parabolic cylindrical reflectors [2] with a novel small size broadband dual-polarized resonant feed. The new base station antenna covers a wide frequency band, which ranges from 450 to 960 MHz with 72% bandwidth. Thus, it can cover all bands of 450 MHz WiMax (450 MHz-470 MHz), 700 MHz WiMax (698-806 MHz), UHF mobile digital TV "DVB-H" (470-862 MHz), CDMA / GSM800 (824-894 MHz) and E-GSM900 (880-960 MHz).

The geometry of a dual parabolic cylindrical reflector antenna shown in fig.1. It consists of two parabolic cylindrical reflectors Si and S ₂ with focal lengths Fi and F ₂ and a feed F positioned on the focal line of S ₂ [2]. The aperture area of the main reflector Si is XiYi and the aperture area of the sub-reflector S ₂ is X ₂ Y ₂ . Dual parabolic cylindrical reflector antennas can generate arbitrary azimuth and elevation beam widths with an arbitrary ratio between them. However, they have some challenging problems when they are used as base stations in UHF band. The first problem in conventional UHF dual parabolic cylindrical reflectors is their heavy weight and high wind loads.

In order to significantly reduce the weight of the reflectors and also reduce their wind loads, several holes are punched in their surfaces as shown in Fig.2. The overall weight of the punched (grid) reflectors can be less than half the weight of the solid surface reflectors depending on the grid size. To minimize the antenna weight and wind loads, the total size of the punched holes are increased as much as possible compared to the overall size of the reflectors, provided that they do not have any effect on the

performance or the mechanical rigidity of the antenna. Although grid dual parabolic cylindrical reflectors have lightweights and low wind loads, they cannot be shipped or stored in a compact form because of their large sizes. With large size antennas, it is helpful to make the antenna deployable and foldable in a compact form. This can be achieved by replacing the solid parabolic cylindrical reflector surfaces by a group of narrow rectangular plane panels as shown in Fig. 3. A plurality of narrow plane panels are assembled edge to edge to form an approximation to parabolic cylindrical surfaces. Of course, simulating parabolic cylindrical reflectors by a group of plane panels will cause some reduction in the antenna efficiency. However, the accuracy of simulating parabolic cylindrical reflectors can be significantly increased by reducing the widths of the plane panels. Reducing the panel widths will also reduce their wind loads. The antenna wind load can be further reduced by increasing the separating distance between the panels as much as possible provided that they do not have a significant effect on the performance of the antenna.

The most challenging problem in conventional UHF dual parabolic cylindrical reflector base station antennas is finding a small size UHF feed with a broad bandwidth that is sufficient for the above applications. UHF feed antennas are large in size and, therefore, they cause severe blockage for the main reflector and/or the sub-reflector [3]. In this research, a novel small size broadband dual-polarized resonant feed is developed with the required bandwidth. Fig.4 shows the geometry of the new feed antenna. It consists of two narrow printed metallic arms connected together by a short metallic strip. The length of the short arm is Li and its width is W _\ while the length of the long arm is L ₂ and its width is W ₂ . The thickness of the antenna is T _a and the antenna is fed at a distance F _a from the shorted edge. Each arm has a set of slots having different shapes and locations which are optimized in order to maximize the bandwidth of the antenna. The new antenna is completely self-contained and it does not need an additional ground plane, a matching circuit or any other component. Furthermore, the new feed antenna is made of a flexible material and it can be bent and/or folded in different forms. The peak gain of the new feed antenna can be increased to more than 5 dBi over the whole band by adding EBG (electromagnetic band gap) structures to the new antenna [4]. The EBG structure also improves the return loss of the antenna and increases its efficiency to around 90% over most of the band.

Best Mode for Carrying Out the Invention

One of the Best modes for carrying out the invention is using it as a base station antenna which covers the frequency bands of 450 and 700 MHz WiMax, UHF DVB-H, CDMA and GSM. A UHF prototype of the grid dual parabolic cylindrical reflector antenna is designed and manufactured as shown in Fig.5. The Dimensions of the antenna are: Fl=80 cm, F2 = 19 cm, X, = 210 cm, Y, = 30 cm, X ₂ =32 cm and Y ₂ = 30 cm. The reflectors are manufactured from 1 mm thick aluminum sheets. The punched holes are square in shape with 1 cm side dimensions. The overall weight of the punched (grid) dual parabolic cylindrical reflector antenna is about 6 Kg. The new feed is located on the focal line of the sub-reflector [2]. Since the overall size of the new feed antenna is very small, it almost does not cause any blockage for the main reflector or the sub-reflector. A UHF prototype of the new broadband feed antenna is designed and manufactured as shown in Fig.6. The dimensions of the antenna are: Li =1 1.5 cm, L ₂ = 25 cm, Wi = 2.6 mm, W ₂ =3.5 mm and T _a = 2 mm. Thus, the overall size of the new UHF feed antenna is 25 x 0.35 x 0.2 = 1.75 cm ³ . The whole weight of the new feed antenna is about 1 gm. The performance of the new antenna is numerically calculated by a software packages that uses the moment method. It is also measured at IMST antenna labs in Germany [5]. Fig.7 shows the radiation patterns of the new antenna alone (without parabolic cylindrical reflectors). The radiation patterns are omni-directional with about 0 dBi peak gain and they are sensitive to only one polarization. To make the new feed antenna sensitive to two perpendicular polarizations, it is bent by 90° as shown in Fig.8. The measured patterns of the bent antenna are shown in Fig.9. The patterns of the bent antenna are sensitive to two perpendicular polarizations in the plane of bending the antenna as shown in the lower graph of Fig.9.

In order to further increase the peak gain of the new feed antenna, a rectangular EBG structure was added to the antenna. The EBG consists of 3 x 1 1 elements as shown in Fig.10. The dimensions of each element are 24 x 24 mm and the gap between each two elements is 4 mm. The dimensions of the EBG ground plane are 10 x 32 cm. Fig. l 1 shows the return loss of the new antenna with the EBG structure. The return loss is better than -10 dB over most of the band. Fig.12 shows the efficiency of the EBG antenna. The antenna efficiency exceeds 90% over most of the band. The peak gain of the antenna is shown in Fig.13, which is more than 5 dBi over most of the band.

Fig.14 shows the measured return loss of the new feed antenna with the grid main reflector behind it. The feed naturally resonates from 450 MHz to 960 MHz with a return loss lower than -7dB and without using matching circuits or any tuning components. On the other hand, the existence of the grid main reflector behind the feed antenna significantly modifies its radiation patterns. The radiation patterns of the feed alone without the main reflector were omni-directional with a peak gain of about 0 dBi. The calculated radiation patterns of the feed antenna with the grid main reflector behind it at 800 MHz are shown in Fig.15. The gain of the feed antenna in front of the grid main reflector is about 9 dBi.

The radiation patterns of the grid dual parabolic cylindrical reflector antenna with the new feed are calculated using GTD (geometrical theory of diffraction) [2]. A GTD software code was written for dual parabolic cylindrical reflectors with arbitrary feed patterns and its accuracy was verified experimentally [6]. Fig.16 shows the calculated radiation patterns of the new developed grid base station antenna with the new feed in two principal planes at 800 MHz. The gain of the antenna is close to 16 dBi and the beam widths in two principal planes are about 103° and 14°.

A UHF prototype of foldable / deployable dual parabolic cylindrical reflector antennas is also manufactured as shown in Fig.17. The parabolic cylindrical reflectors are replaced by a group of rectangular narrow plane panels. The width of each panel is 2 cm and the separation between them is also 2 cm. The same outer frame of the grid antenna is used in the foldable / deployable antenna and thus they have the same overall size. The overall weight of the foldable / deployable dual parabolic cylindrical reflector antenna is very close to the weight of the grid antenna which is about 6 kg.

The measured return loss of the new feed antenna while it is in front of the multi-panel main reflector is shown in Fig.18. The calculated radiation patterns of the feed antenna with the multi-panel main reflector behind it at 800 MHz are shown in Fig.19. The radiation patterns of the foldable / deployable dual parabolic cylindrical reflector antenna with the new feed are shown in Fig.20. The performance of the foldable / deployable antenna is close to that of the grid antenna with about 1 dB reduction in the antenna gain.

Industrial Applicability

This new antenna can replace the conventional base station antenna for mobile communications which consist of arrays of crossed dipoles. The new broadband base station antenna can cover all applications in the band from 450 MHz to 960 MHz. It can cover all the bands of 450 MHz WiMax (450 MHz-470 MHz), 700 MHz WiMax (698- 806 MHz), UHF DVB-H mobile digital TV (470-862 MHz), CDMA / GSM800 (824-894 MHz) and E-GSM900 (880-960 MHz).

On the other hand, this new antenna can also replace the conventional parabolic reflector antennas (the conventional dishes) in any of their applications such as satellite TV dishes as shown in Fig.21. The new broadband feed adjusted and used as feed for this new satellite TV dish. Furthermore, many other feeds can be used.

Brief Description of Drawings

Fig.1 A dual parabolic cylindrical reflector antenna.

Fig.2 Grid parabolic cylindrical reflectors.

Fig.3 Simulating parabolic cylindrical reflectors by a group of narrow rectangular plane panels.

Fig.4 Geometry of the new feed antenna.

Fig.5 A prototype of the grid antenna with the new feed.

Fig.6 The new broadband feed antenna.

Fig.7 Measured radiation patterns of the new feed antenna in two principal planes at 800 MHz.

Fig.8 Bending the new broadband feed antenna by 90°.

Fig.9 Measured radiation patterns of the bent feed antenna in two principal planes at 800 MHz.

Fig.10 EBG structure

Fig.11 Return loss of the new EBG antenna

Fig.12 Efficiency of the new EBG antenna

Fig.13 Peak gain of the new EBG antenna

Fig.14 Measured return loss of the new feed antenna with the grid main reflector behind it.

Fig.15 Radiation patterns of the new feed antenna at 800 MHz with the grid main reflector behind it. Fig.16 Radiation patterns of the new grid base station antenna at 800 MHz.

Fig.17 A prototype of the deployable / foldable antenna with the new feed.

Fig.18 Measured returns loss of the new feed while it is in front of the multi-panel main reflector.

Fig.19 Radiation patterns of the new feed at 800 MHz with the multi -panel main reflector behind it.

Fig.20 Radiation patterns of the new foldable / deployable base station antenna at 800 MHz.

Fig.21 A prototype of the new Satellite (DBS) TV antenna

References

[1] G. Deng and B. Vassilakis, "A Broad band Dual Polarized Azimuth Beamwidth Adjustable Antenna for Wireless Communications", IEEE Microwave Conference, Asia Pacific, APMC 2008

[2] M. Sanad and L. Shafai, Performance and Design Procedure of Dual Parabolic Cylindrical Antennas", IEEE Transactions on Antennas and Propagation, USA, Vol.36, No.3, pp.333-338, March 1988.

[3] S. Zhong, X. Liang and F. Yao, "Compact Broadband Slot Antenna for UHF-Band

Application", IEEE APS Symposium, pp.2567-2570, July 2006

[4] J. Liang and H. Yang, "Radiation Characteristics of a Microstrip Patch over an

Electromagnetic Bandgap Surface", IEEE Transactions on Antennas and Propagation,

VOL. 55, NO.6, pp.1691-1697

[5] http://www.imst.com/en/home.php

[6] M. Sanad and L.Shafai, "Generation of Elliptical Beams of Arbitrary Beam Ellipticity and Low Cross-Polarization Using Offset Dual Parabolic Cylindrical Reflectors", Canadian Journal of Electrical Engineering, Vol.13, No.3, pp.99-105,