Key points of RF microstrip array antenna design
In the frequency band greater than 10GHz, PCB microstrip printed antennas have obvious advantages over other antennas such as waveguide slot antennas, lens antennas, and reflector antennas. Mature PCB processing technology can effectively control the production cost of microstrip antennas. The multi-layer mixed pressure technology of antenna boards, RF boards, and low-frequency digital-analog circuit boards also makes the entire RF system highly integrated.
Thickness selection
The thickness is mainly selected based on three factors: the working bandwidth of the microstrip antenna, the feed network design, and the antenna efficiency. First, the PCB thickness affects the impedance bandwidth of the microstrip antenna. The smaller the PCB thickness and the larger the array scale, the smaller the antenna working bandwidth. Second, the thickness of the PCB determines the width of the microstrip line in the impedance change section of the feeding network. For a 20mil thick RO4350B board, the widths of the 50Ω and 100Ω microstrip lines are 1.13mm and 0.27mm respectively, while the corresponding resonant length of the microstrip antenna at 24GHz is about 3mm.
If the impedance of a certain microstrip transformation section in the feeding network is too small or too large, the microstrip line will be too wide or too narrow. If the microstrip line is too wide, it is easy to cause structural interference, and if the microstrip line is too narrow, it will cause processing difficulties. Third, the thickness of the dielectric affects the conductor loss of the microstrip line, which in turn affects the antenna efficiency. Based on the above factors, the author’s design experience is to choose a thickness of 10mil or 20mil for small arrays, a thickness of 20mil for large arrays, and a thickness of 10mil for RF boards.
Antenna type
Microstrip array antennas are divided into parallel-fed arrays and series-fed arrays according to the feeding method. The parallel-fed array feed line is longer, resulting in greater loss introduced into the feeding network. For large arrays, the antenna efficiency is often limited, so a series-fed array with simpler routing is generally selected. The series-fed array is a resonant antenna with a smaller working bandwidth than the parallel-fed array, but the series-fed structure is easier to achieve weighted excitation. Table 1 shows the series-fed microstrip array antennas of different sizes designed by the author, all of which use RO4350B plates with a thickness of 20 mil. From the data in the table, it can be seen that as the array size increases, the impedance bandwidth gradually decreases. The bandwidth is 1.2 GHz when there are 16 array elements, and only 0.75 GHz when there are 324 array elements. The 24 GHz radar frequency modulation bandwidth that usually uses a continuous wave system is less than 250 MHz, so the impedance bandwidth of the series-fed array can meet most system design requirements.
Interconnection between antennas and RF chips
Currently, domestic and foreign chip manufacturers have mass-produced 24 GHz RF chips launched on the market. In the zero intermediate frequency radar architecture, the pins of the RF chip are directly connected to the microstrip transceiver antenna port. When using a mixed pressure board form of antenna board (high frequency board) + several layers of FR4 + RF board (high frequency board), the interconnection between the antenna and the RF chip is achieved through metallized vias. In the 24GHz frequency 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 ground vias around the metallized vias to form a quasi-coaxial transmission structure. When the antenna and the RF chip are located on the same side of the PCB board, the RF chip and the transceiver antenna are directly connected through a microstrip line or a coplanar waveguide. This design can minimize the insertion loss of the transmission line.+
Low sidelobe design
The sidelobe level of the directional pattern is an important design indicator of the 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 in improving the radar signal-to-noise ratio. The sidelobe level of the uniformly distributed array antenna is greater than -13dB. In order to obtain lower sidelobes, the power fed into each array element is made into a certain low sidelobe weighted distribution through the feeding network. Commonly used equal-phase and unequal-amplitude low sidelobe weighted distribution methods include Chebyshev distribution and Taylor distribution. Based on 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 feeding network so that the power fed into each array element is close to the ideal distribution.