Array of Waveguide-Fed Microstrip Antennas B. Yu D.Wu
K. Seo
Jiangsu Key Laboratory of Wireless Communications Nanjing University of Posts and Telecommunications Nanjing 210003, P.R.China Email:
[email protected]
Development 2ND Dept. Nippon Pillar Packing Co. Ltd. Sanda, Hyogo 669-1333, Japan Email:
[email protected]
waveguide to microstrip transition is necessary. Various waveguide to microstrip transition have been investigated theoretically and experimentally. Davidovitz [12] proposed an equivalent circuit model of a waveguide to microstrip, and designed a waveguide to microstrip line transition and power divider where return loss is less than 15dB and average insertion loss is around 0.5dB over the frequency range. Wu [13] presented a new type of waveguide to microstrip line transition and power divider of K-band, which has a bandwidth 10% and an average insertion of 3.5dB for less than 15dB return loss.
Abstract—A low-cost Ku-band array of microstrip antennas, which is fed by standard waveguide, is presented in this paper. The 72 corner-fed square patches of the array are arranged in a 6-by-12 configuration. A novel waveguide to microstrip line transition with power divider is employed as a centre fed for the array in order to reduce the loss of the microstrip feed line. The resonant frequency of the array is 12.5GHz and the gain was measured to be 24.3dBi. The measurement results show good agreements with full-wave simulation. This array of waveguidefed microstrip antennas can be used in many modern communication systems, such as a mobile radar sensor, in which the transition can be fixed as a connection between the module with a front-end waveguide and the antennas. Keywords-array antenna; waveguide feed; transition
The present work concerns with the realization of Ku-band array of corner-fed square patches profiting from low loss centre waveguide feed network. The network comprises of standard rectangular waveguide, a novel waveguide to microstrip transition and power divider. The 72 patches of the array are arranged in a 6-by-12 configuration and the novel transition is employed as centre fed for the array in order to reduce the loss of the micrstrip feed line. This array of waveguide-fed microstrip antennas can be used in many modern communication systems, such as a mobile radar sensor, in which the transition can be employed as a connection between the module with a front-end waveguide and the antennas.
I. INTRODUCTION Microstrip antenna arrays are exploited in a vast number of engineering applications owing to their ease of manufacturing, low cost, low profile, and light weight. A quasi-square patch fed at one corner has been used as an element of arrays for a long time. Daniel et al [1] designed a low-cost linearly polarized microstrip antenna array of corner-fed square patches first. Cruz and Daniel [2] presented an experimental analysis of the corner-fed square patch element. A closed-form expression for the input impedance of a corner-fed patch antenna was reported by Lim et al [3]. Many microstrip arrays of these elements have been developed for various demands, such as low side-lobe, shaped beam, and multi-beam, etc. [4][6]. In many practical designs, these arrays are fed by a coplanar corporate microstrip feed network in order to keep the overall constructional complexity at a minimum and maintain compact size [7]. At higher microwave frequencies, this approach, however, suffers from ohmic and dielectric losses of the connecting microstrip lines, as well as the undesired radiation of the feed network [7], [8]. Realization of a high-efficiency microstrip antenna array having a large number of elements can thus be challenging, unless a low-loss low-radiation feed network replaces the coplanar one. Among all microwave transmission lines, waveguides feature extremely low losses up to very high frequencies provided that their internal surfaces are finished smoothly [9]. For this reason, they have been intensively utilized in the special feed system of planar slot arrays [10], and in the beam forming network (BFN) of satellite antennas [11]. To realize waveguide feed system of planar array antennas, a low cost
To introduce components of the realized planar array, the paper is organized as follows. In Section II, the transition configuration is in focus, while the array of corner-fed square patches is discussed in Section II-B. Radiation measurements conducted on the realized array are presented in Section III.
II.
A. Waveguide to microstrip transition with power divide The novel waveguide to microstrip transition with power divider as shown in Fig. 2 consists of a top and a backside conductor pattern on both surfaces of a dielectric substrate. Fig. 3 depicts the top view, the bottom view and the cross-section
This work was supported in part by the Scientific Research Foundation of Nanjing University of Posts and Telecommunications under Grant No.NY207016, in part by Jiangsu provincial research scheme of natural science for higher education institutions under Grant No.07KJB510080, in part by the National Basic Research Program (973) of China under Grant No. 2007CB310607.
1-4244-2424-5/08/$20.00 ©2008 IEEE
CONFIGURATION
Fig.1 illustrates the constituting components of the waveguide-fed planar array. It is composed of a one-layer substrate (1) for supporting the array elements and a waveguide to microstrip transition (2). The 72 elements of the array are subdivided into six series feed sub-array of elements (3). Each element of the array (4) is a corner-fed square patch. The electrical connection of the array to the waveguide feed (5) is established through a waveguide to microstrip transition.
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(a)
(b)
Fig.1 Constituting components of the 6h12 array
(c) Fig.3 Waveguide to microstrip line transition and power divider (a) bottom view (b) top view (c) cross-section view
Fig.4 Configuration of sub-array TABLE I DIMENSIONS OF ARRAY AND TRANSITION Fig.2 Configuration of waveguide to microstrip line transition and power divider
Symbol a b d Ɏ W S
view through the plane of symmetry AA’. On top of the substrate, to prevent radiation from the end of the waveguide, a conductor pattern with two notches, named a waveguide short pattern, is printed. Two mictostrip lines are located inside the two notches of the short pattern. On another side of the substrate, a rectangular conductor, named a frequency adjust element, is printed in the centre of an etched window in the ground plane. The operation frequency is determined by length Lf of the frequency adjust element, which is equal a half resonant wavelength. Some via holes with diameter Ɏ and distance d between the two holes are used in order to make an electrical connection between the waveguide short pattern and ground plane.
Value(mm) 15.8 7.90 1.10 0.60 1.50 0.30
Symbol Lf Wf Ȝg ȡ h n
Value(mm) 6.98 6.71 17.90 2.09 0.50 7.70
along the feeding line equals one guided wavelength. III.
SIMULATION AND MEASURED RESULTS
The array and waveguide to microstrip transition with power divider were simulated using the Ansoft HFSS software, and measured using the Agilent 8720E vector network analyser and a far field measurement system. A Rogers 5880 dielectric substrate of İr=2.2 at 10 GHz was used. The dimensions of the array and transition are shown in Table I. A photograph of the realized waveguide-fed array is shown in Fig.5. The measured and simulated return loss of the waveguide-fed array was measured from 10 to 14 GHz, and the result is shown in Fig. 6. The patterns of the waveguide-fed array have been measured in far field chamber at a distance of about 5m (in order to be in a far field configuration). The transmitting antenna is the ridged horn BJ140, a pyramidal
B. Array of corner-fed square patches The 72 elements of the array are subdivided into six series feed sub-array as shown in Fig .1. Each element of the array is a corner-fed square patch. When the patch is excited at one corner, the main part of the internal field is the sum of two degenerate modes with equal amplitudes, i.e. modes (0, 1) and (1, 0). As shown in Fig.4, quarter wavelength impedance transformers are used in each sub-array, which produce a Chebyshev amplitude distribution while keeping the simplicity of the series feeding. And the spacing between two elements
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Fig.8 The measured H-plane patterns of the array
Fig.5 Photograph of the waveguide-fed array antennas
is used as a standard antenna for measuring the gain of this array. The measured E-plane and H-plane patterns at 12.5GHz are shown in Fig. 7, Fig. 8, respectively. The waveguide-fed array directivity was simulated to be 25.4dBi and the corresponding gain was measured to be 24.3dBi. The difference is mostly due to mismatch, conductor, dielectric losses in the feed network of the array. IV. CONCLUSIONS A low-cost Ku-band waveguide-fed array antenna has been presented. A novel waveguide to microstrip line transition with power divider is employed as a centre fed for the array. This array of waveguide-fed microstrip antennas can be used in many modern communication systems, such as a mobile radar sensor, in which the transition can be employed as a connection between the module with a front-end waveguide and the antennas. The resonant frequency is 12.5GHz and the gain of the array is more than 24dBi. The measurement results show good agreements with full-wave simulation.
Fig.6 Return loss of the waveguide-fed array antennas
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[5] [6] Fig.7 The measured E-plane patterns of the array [7]
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