# A 77 GHz Transmit Array for In-Package Automotive Radar Applications
## 2 Transmit Array Design
### Antenna Feed  
A planar antenna configuration is used as the primary feed to realize a low-cost, integrated, 77 GHz RF, front-end package for automotive applications, integrable into a transceiver module. The transmit array fed can be either monolithically integrated onto the chip [18] or printed on a standard substrate. In this paper, a standard printed antenna is employed (Figure 2). A conventional patch antenna with an inset feed has been designed to resonate at the central frequency of the band. The patch is printed on a 0.25 mm Rogers RO3003 substrate with DK = 3 and a dissipation factor of 0.0010.  
One of the main problems associated with this type of configuration is the excitation of surface waves, which cause a deterioration of the radiation pattern. It is well known that the suppression of the surface waves improves the antenna performance, increasing the energy radiated in the broadside direction. To suppress the propagation of surface waves, the radiating patch has been surrounded by a mushroom-type EBG (Electromagnetic Band Gap) structure [19, 20, 21].  
The EBG has been designed to provide a band gap able to cover the whole antenna bandwidth. In this manner, the surface waves launched from the antenna inside the substrate are efficiently suppressed. The mushroom-type EBG [22] is composed of metallic patches with central vias, as shown in Figure 3. This type of structure exhibits unique electromagnetic properties with a bandgap characteristic that is mainly determined by the patch and gap width, substrate permittivity, substrate thickness and metal vias' radius.  
To evaluate the band gap proprieties of the EBG, a dispersion diagram of the mushroom-type structure's unit cell has been generated. The parameters have been tuned to create a band gap able to cover the band of interest. An LC filter array can be used for the explanation of the operation mechanism of the EBG structure. An increment in patch width \(Wp\) and decrement in gap (\(Dp\)-\(Wp\)), while keeping the height and permittivity of the substrate fixed, causes an increment in capacitance, which results in the decrement of the structure's resonant frequency. Figure 3 shows the dispersion diagram obtained with \(Dp\) = 1.05 mm; \(Wp\) = 0.7 mm; and via a diameter of 0.254 mm. A large stop band is created on the periodic surface from 60 GHz to 82 GHz.  
In order to improve the transmit array's performance, a flat radiation pattern over a large angular range is preferable. To this end, the patch antenna has been surrounded by three rows of mushroom-type EBGs, as shown in Figure 2. The normalized radiation patterns of the patch antenna with and without EBGs are compared in Figure 4. A noticeable  
Figure 2: Feeding patch antenna with EBG structure. a = 4.55 mm; b = 1.6 mm; c = 0.98 mm; d = 0.15 mm; e = 0.5 mm; s = 0.25 mm.  
improvement in terms of the radiation pattern's flatness is observed: the \(-3\) dB beamwidth has been increased from \(14^{\circ}\) to about \(90^{\circ}\). The smoother pattern is more suited to being a feed for the transmit array. The feeding patch has a gain of 7.5 dBi.