In the frequency band greater than 10GHz, PCB microstrip printed antenna has obvious advantages over waveguide slot antenna, lens antenna, reflector antenna and other antennas. Mature PCB processing technology can effectively control the production cost of microstrip antenna. The multilayer hybrid technology of microstrip antenna circuit board, microwave RF board and low-frequency digital analog circuit board also makes the whole RF system highly integrated. The microwave RF board ro4350b produced by Rogers has excellent high-frequency performance and low production cost. It has been widely used and verified in commercial RF systems. The author has successfully designed a series of 24GHz microstrip array antennas using Rogers circuit board Rogers ro4350b, which have been applied to the listed products of the company. Therefore, some design skills are summarized for its application.
The thickness is mainly selected according to the working bandwidth of microstrip antenna, feed network design and antenna efficiency.
1. The thickness of PCB affects the impedance bandwidth of microstrip antenna. The smaller the thickness of PCB, the larger the array size, and the smaller the working bandwidth of microstrip antenna.
2. The thickness of the medium affects the conductor loss of the microstrip line, and then affects the efficiency of the microwave RF antenna. Based on the above factors, the author’s design experience is that the thickness of small array is 10mil or 20MIL, the thickness of large array is 20MIL, and the thickness of microwave RF board is 10mil.
3. The thickness of PCB determines the linewidth of microstrip line in the impedance variation section of feed network. For ro4350b plate, the thickness is 20 mils, 50 Ω and 100 Ω. The linewidth of Ω microstrip line is 1.13MM and 0.27mm respectively, and the resonant length of the corresponding microstrip antenna at 24GHz is about 3mm. If the impedance of the microstrip conversion section in the feed network is too small or too large, the microstrip antenna line will be too wide or too narrow, and the microstrip antenna line will be too wide, which is easy to cause structural interference. If the line of microstrip antenna is too narrow, it will cause processing difficulties.
Microstrip array antenna can be divided into parallel feed array and series feed array according to the feeding mode. The parallel feeder array has longer feeders, resulting in greater losses in the feeder network. For large-scale arrays, the antenna efficiency is often limited, so the series feed array with simpler wiring is generally selected. The series feed array is a resonant antenna, and its working bandwidth is smaller than that of the parallel feed array, but the series feed structure is easier to realize weighted excitation. A series fed microstrip array antenna with different scales is designed by the author. They all use 20 mil thick ro4350b. As the array size increases, the impedance bandwidth decreases gradually. The bandwidth is 1.2Ghz, 1.2Ghz for 16 elements, and only 0.75ghz for 324 elements. Generally, the frequency modulation bandwidth of 24GHz radar using continuous wave system is less than 250MHz, so the impedance bandwidth of series feed array can meet most system design requirements.
Interconnection between antenna and RF chip
At present, chip manufacturers at home and abroad have mass produced 24GHz RF chips in the market. In the zero if radar architecture, the pin of the RF chip is directly connected to the antenna port of the microstrip transceiver. When using antenna circuit board (high frequency board) + multilayer FR4 + microwave RF board (high frequency board), the antenna and RF chip are interconnected through metallized vias. In the 24GHz band, the discontinuity introduced by metallized vias with a length greater than 1mm will be very obvious. The solution is to add several symmetrical metallized grounding vias around the metallized vias to form a coaxial transmission structure. When the antenna and the RF chip are located on the same side of the PCB, the RF chip and the transceiver antenna are directly connected through microstrip line or coplanar waveguide. This design can minimize the insertion loss of the transmission line.
Low sidelobe design
The sidelobe level of pattern is an important design index of array antenna. The low sidelobe design can reduce the environmental interference outside the radar main beam. Its function is equivalent to a spatial filtering, which is very effective to improve the signal-to-noise ratio of radar. The sidelobe level of uniformly distributed array antenna is greater than – 13dB. In order to obtain a lower sidelobe, the power fed into each array element forms a certain weighted distribution of low sidelobe through the feed network. The commonly used weighted distribution methods of equal phase and unequal amplitude low sidelobe include Chebyshev cloth and Taylor distribution. According to the sidelobe level and the number of array elements, it is easy to synthesize the ideal weighted distribution. The remaining work is to repeatedly optimize the feed network to make the power fed into each array element close to the ideal distribution.
Xinfeng Mingxin Electronic Technology Co., Ltd