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The recent advent of millimeter-wave technology and the increasing demand for higher frequencies have stimulated a renewed interest in the design of lens antennas for radar and communication systems. For frequencies above K u -band the penalty in weight and size associated with a lens is relatively small. In fact, for many ehf applications, lenses are selected for reasons of economy, performance, and reliability.

To explain the mechanism of radiation from SPP TLs, this paper analyzes the radiation from the input mode converter and that from corrugated metallic strip of finite length separately, by regarding the SPP TL as a kind of surface wave antenna22. The field distribution indicates that radiation from the input mode converter resembles a pair of planar horn antennas23 with difference beam. This paper designs a loaded SSPPs TL with a series of resistors with gradual value to calculate radiation loss from the input mode converter. By observing current distribution on an SPP TL, we deduce that surface currents towards the direction of propagation are the main source of radiation. Single corrugated metallic strip of finite length could be regarded as a long wire antenna. A radiation model is proposed to validate our explanation, predicting numbers of radiation lobes well. Furthermore, based on the effective radiation section (ERS) theory24, 25, the concept of average radiation length (ARL) is proposed to estimate the relationship between radiation loss and frequency because ARL is proportional to radiation loss. It is found that at low frequency band, the corrugated metallic strip of finite length is the key factor of radiation loss; at high frequency band, the input mode converter is the main source of radiation loss.

The input mode converter transforms the EM field from the mode of coplanar waveguides (CPW) into the mode of the SSPPs, and it can be regarded as a pair of CPW flaring horn antennas23 split by the middle corrugated strip exhibiting a difference beam radiation characteristic. In this paper, a loaded SPP TL with resistors of gradual value from 10 ohms to 100 ohms is designed to estimate radiation loss brought by the input mode converter. The configuration of loaded TL is as same as that of the typical SPP TL, shown in Fig. 2(a), and the simulated S-parameters are shown in Fig. 2(b).

The effective radiation section (ERS) theory was presented to study positions of side lobes from leaky wave antennas in recent researches24, 25. But the radiation loss, in other words, the radiation efficiency has not been discussed. By taking the average value of ERS at angles from 0 to 180 degrees, the concept of average radiation length (ARL) is proposed to discuss radiation efficiency from the finite corrugated metallic strip. Note that the definition of axes in this paper is different from that in 24 and 25, some important formulas need to be rewritten before calculating ARL.

The configuration of the proposed SIW slot array antenna can be found in Figure 1, whose overall size is 47mm x 15mm (includes flange). There are 1 x 14 slots in total on the broad wall of the SIW, and the feed mode of the antenna in a rectangular waveguide converts to SIW. All the structures are designed on a dielectric substrate Rogers5880 ([[epsilon].sub.r] = 2.2 @10GHz, thickness h = 0.254 mm), and the metal used in the substrate is copper with a thickness of 17 [micro]m. Considering the antenna performance and convenience of testing, FR4 substrate material with thickness of 1 mm and dielectric constant of 4.4 was selected to strengthen the structure. For the bonding of antenna and substrate, the thickness of 0.127 mm and dielectric constant of 4.4 of FR4 solidified sheet were selected.

The configuration parameter of the SIW antenna array is shown in Figure 2. The thickness and width of the SIW are h and [a.sub.siw], respectively. The metallic post separated by a distance s is placed periodically with diameter d. To ensure that the resonant frequency, transmission loss, and transmission mode achieve the design requirements, an accurate analysis is important for a SIW structure size. The SIW is equivalent to a conventional rectangular waveguide filled with dielectric [9, 10].

The low sidelobe performance of antenna element is designed by Taylor synthesis method. The SLL performance of -35 dB is achieved by the array composed of 14 cells, and the current distribution of each unit is shown in Table 2.

The longitudinal slots on the top metallic plate are placed alternately on both sides of the centerline to perturb the surface current for radiation. Adjacent radiating slots are staggered and placed 180 degrees out of phase on both sides of the centerline. Therefore, the spacing (d) with half of the waveguide wavelength is designed in order to stimulate all slot elements in the same phase, and the distance ([e.sub.s]) between the terminal slot and the short-circuited wall is three-quarters of the waveguide wavelength at the design frequency. The antenna configuration is shown in Figure 6.

The extraction results can be served as initial values for the further accurate optimization by the full-wave simulations of the linear SIW array, thus the efficiency of the designed antenna could be improved by this process.

The traditional microstrip feed has a large loss in the high-frequency band of 79 GHz, so it is necessary to design a waveguide to SIW structure for processing and testing antenna [4, 16]. There is a pressing need for an effective transition between air-filled rectangular waveguide (RWG) and SIW. A WR-12 waveguide to SIW vertical transition is designed in Figure 7.

Figure 9(a) shows the effect of the upper reflector width on the impedance bandwidth of the antenna. The resonance point is negatively correlated with the value of a. When a = 0.93, 0.95, and 0.97 mm, the corresponding resonance points are 77.8, 77, and 76 GHz, respectively. From Figure 9(b), the return loss of the transition bandwidth from 73 GHz to 80 GHz is better than 10 dB, and the insertion loss is less than 0.8 dB over the frequency range of 73-80 GHz. The proposed transition structure provided with wide bandwidth, low insertion loss, and designed on one layer with a simple structure is demonstrated, and these advantages make it acceptable for use. 2b1af7f3a8