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In the frequency band greater than 10GHz, the PCB microstrip printed antenna has obvious advantages over other antennas such as waveguide slot antennas, lens antennas, and reflector antennas. The mature PCB processing technology can effectively control the production cost of the microstrip antenna. The multi-layer hybrid technology of the antenna board, the RF board and the low-frequency digital-analog circuit board also makes the entire RF system have a high degree of integration.
Thickness selection
The thickness of the microstrip antenna is mainly selected based on three factors: the working bandwidth of the microstrip antenna, the design of the feeding network 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 operating bandwidth.
Second, the thickness of the PCB determines the line width of the microstrip line in the impedance change section of the feeding network. For the RO4350B board with a thickness of 20 mil, the line width of the 50Ω and 100Ω microstrip lines are 1.13mm and 0.27mm, respectively, and the microstrip antenna corresponds to resonance at 24GHz. The length is about 3mm. If the impedance of a microstrip conversion section in the feed 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. will cause processing difficulties.
Antenna type
As shown in Figure 1, the microstrip array antenna is divided into parallel-fed array and serial-fed array according to the feeding mode. And the feeder line of the feeder array is long, which leads to a large loss introduced by the feeder network. For large arrays, the antenna efficiency is often limited, so a cross-feed array with simpler routing is generally selected. The series-fed array is a resonant antenna, and its operating bandwidth is smaller than that of 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 scales designed by the author, all of which use RO4350B plate with a thickness of 20 mil. As can be seen from the data in the table, as the scale of the array becomes larger, 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 FM bandwidth of the 24GHz radar that usually adopts the continuous wave system is less than 250MHz, so the impedance bandwidth of the cross-feed array can meet most of the system design requirements.
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 10mil or 20mil thickness for small arrays, 20mil thickness for large arrays, and 10mil thickness for RF boards.
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