High-power RF circulators and isolators for radar systems

　　As the core equipment for detecting and monitoring targets, radar systems are widely used in military, aerospace, meteorology and other fields. Its working process involves the transmission of high-power RF signals and the reception of weak echo signals, which places extremely stringent requirements on the circulators and isolators at the RF front end. As key components in radar systems, high-power RF circulators and isolators need to have high power tolerance, excellent isolation performance and stable operation in complex electromagnetic environments to ensure that the radar system operates accurately and efficiently.

　　I. Material selection under high power requirements

　　(I) High temperature resistant, high saturation flux density ferrite materials

　　The output power of radar transmitters can reach several kilowatts or even higher. For example, the peak power of shipborne radars can reach megawatts, which requires the ferrite materials in circulators and isolators to withstand high temperatures and maintain stable magnetic properties. Garnet ferrite with higher saturation flux density (Bs), such as modified YIG ferrite doped with Bi, Y and other elements, can be selected. Its Bs value can be increased to 0.2-0.3T, which can maintain normal operation at higher power compared with conventional ferrite. At the same time, this type of ferrite has good thermal stability, and the Curie temperature (Tc) can reach 250℃-350℃. In the high temperature environment caused by long-term high-power operation of the radar (the operating temperature can reach 80℃-120℃), the magnetic permeability (μ) changes very little, ensuring the performance stability of the device.

　　(II) High thermal conductivity heat dissipation material

　　In order to effectively dissipate the heat generated by high-power operation, high thermal conductivity materials are required in the structural design of circulators and isolators. Copper tungsten alloy (CuW) is used as the heat dissipation base, and its thermal conductivity can reach 180-220W/(m・K), which is 2-3 times that of ordinary aluminum alloy, and can quickly conduct the heat generated by the ferrite. At the same time, thermal conductive silicone grease (thermal conductivity ≥ 3W/(m・K)) is filled between the inner conductor and the outer shell to further reduce thermal resistance, ensure uniform temperature distribution of the entire device when working at high power, and avoid local overheating leading to performance degradation.

　　II. Structural design to adapt to high power and complex environment

　　(I) Enhanced RF transmission structure

　　The radar system operates in a wide frequency band, from L band (1-2GHz) to Ka band (26.5-40GHz), and has a wide signal bandwidth. To meet this demand, high-power RF circulators and isolators use optimized transmission line structures. For example, in a coaxial structure, the inner conductor is solid oxygen-free copper (purity ≥ 99.99%) and the outer conductor is silver-plated copper alloy, which reduces conductor resistance and reduces RF signal transmission loss. At the same time, by precisely controlling the size and spacing of the inner and outer conductors, and optimizing the structure using electromagnetic simulation software (such as HFSS), the characteristic impedance is kept stable at 50Ω in the entire frequency band, ensuring efficient signal transmission, and the standing wave ratio (VSWR) is ≤1.2.

　　(II) High reliability sealing and shielding structure

　　Radars usually work in harsh environments, such as salt spray corrosion at sea, sand and dust erosion in the desert, and complex electromagnetic interference environments. To this end, the circulator and isolator use a fully sealed metal shell, such as stainless steel or aluminum alloy, which is precision machined and surface treated (such as anodizing, chemical nickel plating) to achieve a protection level of IP67, effectively resisting external environmental erosion. At the same time, the shell, as a good electromagnetic shielding body, combined with the internal grounding design, can shield external electromagnetic interference, ensure that the internal RF signal is not affected, and enhance the radar system's anti-interference ability in complex electromagnetic environments.

　　III. Performance parameter optimization to meet the strict indicators of radar

　　(I) High isolation to suppress transmission and reception interference

　　In the radar system, the transmission signal power is strong. If it leaks to the receiving end, it will seriously interfere with the weak echo signal. Therefore, the port isolation of the high-power circulator (such as the transmitter to the receiver) needs to reach more than 30dB, and the reverse isolation of the high-power isolator should be ≥35dB. By optimizing the shape and size of the ferrite and adjusting the magnetic field distribution of the permanent magnet (using high remanence NdFeB permanent magnets, remanence Br = 1.2 - 1.4T), the finite element analysis software is used to accurately calculate the magnetic field distribution, so that the magnetic field of the forward transmission path is uniform and the magnetic fields of the reverse path cancel each other, thereby achieving efficient isolation and effectively suppressing the interference of the transmitted signal on the received signal.

　　(II) Low insertion loss guarantees signal strength

　　The radar echo signal itself is weak, and every increase of 0.1dB of insertion loss may shorten the target detection distance. The forward insertion loss of high-power RF circulators and isolators must be strictly controlled at ≤0.5dB (L band) and ≤0.7dB (Ka band). By using low-loss ferrite materials, optimizing the transmission line structure and fine process manufacturing, the energy loss of the signal during transmission is reduced, ensuring that the transmitted signal can be efficiently transmitted to the antenna, and the received signal can be losslessly transmitted to the back-end processing module, thereby improving the detection sensitivity and range of the radar system.

　　(III) High power capacity adapts to radar transmission power

　　The pulse power output by the radar transmitter has a high peak power and a large average power. High-power RF circulators and isolators need to have high power capacity and be able to withstand high-power signals with a peak power of ≥10kW (pulse width 1 - 10μs) and an average power of ≥1kW. In terms of structural design, the cross-sectional area of the internal conductor is increased, and thick film conductor technology (conductor thickness ≥100μm) is used to improve the current carrying capacity; at the same time, the heat dissipation structure of the ferrite is optimized to ensure that the device performance is stable under high power and long-term operation without damage or performance degradation.

　　4. Collaborative design with other components of the radar system

　　(I) Matching design with antenna

　　There are various types of radar antennas, such as parabolic antennas, phased array antennas, etc., and their input impedance and radiation characteristics are different. High-power RF circulators and isolators need to be precisely matched with the antenna. By adjusting the output port impedance of the circulator/isolator, an impedance transformer or gradient line structure is used to accurately match it with the input impedance of the antenna, reduce signal reflection, and improve the radiation efficiency of the antenna. For example, for phased array antennas, the design of the circulator/isolator needs to consider the mutual coupling effect between antenna elements, and optimize the structure through electromagnetic simulation to ensure that the signal transmission is stable when the entire antenna array is working, and there is no abnormal signal reflection and interference.

　　(II) Integrated design with transmitters and receivers

　　In radar systems, circulators and isolators are closely connected with transmitters and receivers. In order to achieve efficient integration of the system, a modular design concept is adopted. The high-power circulator and the final power amplifier of the transmitter are integrated in the same module to reduce the length of the transmission line and reduce signal transmission loss and interference. At the same time, the isolator is integrated with the front-end low-noise amplifier of the receiver to effectively protect the low-noise amplifier from the influence of reflected signals and improve the sensitivity and stability of the receiver. Through this integrated design, the overall performance and reliability of the radar system are improved, the system volume and weight are reduced, and the development needs of miniaturization and lightweight of modern radar systems are met.

　　V. Testing and verification to ensure that radar application standards are met

　　(I) High-power durability test

　　Simulate the high-power environment in which the radar actually works, and conduct long-term durability tests on high-power RF circulators and isolators. Under rated peak power and average power, the device works continuously for more than 1,000 hours, and the isolation, insertion loss, standing wave ratio and other performance parameters of the device are monitored during this period. The performance parameter change before and after the test is required to be ≤5%, ensuring that the device has stable and reliable performance under long-term high-power operation.

　　(II) Complex electromagnetic environment simulation test

　　Use electromagnetic compatibility test equipment to simulate the complex electromagnetic interference environment that the radar may encounter in actual work, such as strong electric field, strong magnetic field, broadband interference signal, etc. Place the high-power circulator and isolator in this environment to test their performance under interference conditions. It is required that under the specified interference intensity, key performance indicators such as isolation and insertion loss can still meet the working requirements of the radar system to ensure the normal operation of the radar system in a complex electromagnetic environment.

　　The high-power RF circulators and isolators that have been strictly designed, tested and verified can play a key role in the radar system, provide solid guarantees for the accurate detection and reliable operation of the radar, help the continuous development of radar technology, and meet the growing application needs in multiple fields such as military defense and civilian detection.

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