High-Isolation RF Power Splitter/Combiner Technology and Application Analysis

　　I. Core Requirements and Scenario Drivers for High Isolation

　　The core value of high-isolation RF splitters/combiners lies in suppressing crosstalk between ports and preventing useless signals from occupying effective channel resources. This requirement stems from the stringent constraints of three scenarios: "multi-system coexistence," "weak signal transmission," and "high-power transmission":

　　* **Multi-channel independent signal transmission requirement:** In systems such as wireless intercoms and multi-carrier base stations, multiple transmitters need to transmit signals through a single antenna. Insufficient isolation (e.g., below 20dB) can lead to crosstalk between channels, causing communication interruptions or audio quality degradation. For example, in a large-scale event security intercom system, when eight channels are operating simultaneously, the combiner isolation must be ≥25dB to ensure clear transmission of instructions from each department.

　　Weak Signal Fidelity Reception Requirements: In scenarios such as satellite communication and radar reception, target signal strength is typically as low as -90 to -120 dBm. Adjacent channel crosstalk can easily mask effective signals. Therefore, splitter isolation needs to be increased to 35 dB or higher, and for Ka-band high-throughput satellite systems, it needs to reach 40 dB to ensure that uplink signals from multiple users do not interfere with each other.

　　High-Power Transmission Anti-Interference Requirements: In scenarios such as industrial heating and radar transmission, transmission power can reach tens or even hundreds of watts. High isolation can prevent reflected power from flowing back to other ports, avoiding damage to front-end devices (such as power amplifiers), and reducing interference to the receiving link.

　　II. High Isolation Core Performance Indicator System

　　1. Basic Isolation Indicators (Classified by Scenario)

　　General Communication Scenarios: 2-channel splitter/combiner isolation ≥ 20 dB, 4-channel device isolation ≥ 18 dB. A typical example is an 800-2000MHz wireless intercom system, where a broadband combiner can meet the basic isolation requirements.

　　Professional communication scenarios: For 2-channel devices in the 2-8GHz band, the typical isolation value is ≥28dB, with a minimum of 20dB; for 16-channel splitters in the 800-2000MHz band, the typical isolation value is 26dB, ensuring independent transmission of multi-channel signals.

　　High-end precision scenarios: In satellite communication and radar systems, the isolation of 4-channel combiners is ≥35dB, and the isolation of Ka-band phased array splitters needs to reach ≥40dB, with phase difference controlled within ±1° to avoid a decrease in beamforming accuracy.

　　2. Isolation-related performance indicators:

　　Frequency stability: Wide-band devices (e.g., 0.5-18GHz) must ensure isolation fluctuation ≤5dB across the entire frequency band. For example, a 500-6000MHz 1-to-2 power divider should have an isolation value ≥18dB in the 500-600MHz band and ≥20dB in the 600-6000MHz band.

　　Temperature Adaptability: After cycling at -55℃ to +125℃, the isolation change is ≤2dB, avoiding increased crosstalk in extreme environments. This is a core requirement for spaceborne and automotive applications.

　　Power Handling Stability: After 1000 hours of continuous operation at rated power (e.g., 30W CW), the isolation decay is ≤1dB, preventing crosstalk runaway due to device aging.

　　VSWR Coordination: Input/output port VSWR ≤1.5 (typical value ≤1.25). Poor impedance matching will indirectly reduce the effective isolation; for example, a VSWR of 1.45 will reduce the actual isolation effect by 3-5dB.

　　III. High Isolation Technology Solution Design and Implementation

　　1. Topology Optimization (Core Enhancement Path)

　　Wilkinson Topology Enhancement Solution: Adding a multi-stage isolation network to the traditional λ/4 transmission line, with each stage containing a series thin-film resistor and coupling capacitor, increases the isolation of the 2-way splitter from the basic 20dB to over 28dB. For example, a 2-8GHz 1-to-2 splitter, through a dual-section isolation design, achieves 32dB isolation at the center frequency of 5GHz.

　　Dedicated cavity combiner solution: Utilizing a combination of a cavity resonator and a circulator, it leverages the frequency selectivity of the resonator and the unidirectional transmission characteristics of the circulator to achieve a balance between high isolation and low insertion loss. Its insertion loss is ≤3.5dB, and the isolation increases with increasing frequency spacing, reaching ≥30dB at frequency spacings above 250kHz, suitable for high-power intercom systems.

　　Broadband branch line coupler solution: Extending bandwidth to 0.5-18GHz through a multi-section matching network, combined with LTCC multilayer integration technology, it three-dimensionally arranges transmission lines and isolation components, reducing electromagnetic coupling between ports. An 8-way splitter achieves a typical isolation of 25dB in the 2-8GHz band, with a 60% reduction in size compared to traditional solutions.

　　2. Material and Process Isolation Enhancement

　　Shielding Structure Design: An aluminum alloy sealed cavity is used, with internal metal compartments (thickness ≥1mm) to physically separate the transmission paths of each port, reducing radiated crosstalk. Combined with silver plating (thickness ≥3μm), the shielding effectiveness is improved to over 60dB.

　　Transmission Line Optimization: High-conductivity oxygen-free copper strips (conductivity 5.8×10⁷ S/m) are selected, with gold plating to reduce skin effect losses. Simultaneously, the spacing between transmission lines between ports is increased (≥λ/4, where λ is the lowest operating frequency wavelength), for example, ≥7.9cm for devices in the 950-2150MHz band, reducing capacitive coupling.

　　Isolation Component Selection: High-frequency thin-film resistors (operating frequency ≥20GHz, temperature coefficient ≤50ppm/℃) are used instead of traditional carbon film resistors to avoid the resistor's own noise exacerbating crosstalk. The power redundancy design of the isolation resistors in the 4-way splitter is 3 times the actual power consumption, ensuring long-term stability.

　　3. Integrated Crosstalk Suppression Design

　　Filtering - Integrated Splitting: A bandpass filter is integrated at the splitter input port. By setting a specific passband frequency (e.g., 145MHz or 435MHz), adjacent channel interference signals are suppressed by ≥40dB, making it particularly suitable for multi-band radio antenna systems.

　　Circulator-Assisted Isolation: A circulator (forward loss ≤0.8dB, reverse loss ≥20dB) is connected in series at the high-power combiner input to guide reflected crosstalk signals to the matched load, improving the combiner's isolation by 10-15dB, suitable for continuous wave power scenarios above 50W.

　　IV. High Isolation Testing, Verification, and Certification Standards

　　1. Core Testing Methods and Procedures

　　Accurate Isolation Testing: Using a vector network analyzer (such as Keysight N5247A), the testing process follows a "port calibration - load matching - multi-point sampling" procedure: First, perform SOLT calibration on the test cable to eliminate the influence of cable loss; connect all ports except the port under test to a 50Ω matched load to avoid reflected signal interference; take one test point every 100MHz within the operating frequency band to ensure that the isolation meets the standard across the entire frequency band. For example, when testing a 2-18GHz two-way splitter, more than 20 test points are required to ensure that the minimum isolation value is not lower than 20dB.

　　Environmental Stability Testing:

　　Temperature Cycling Test: 100 cycles at -55℃ to +125℃, with 2 hours of heat preservation per cycle. Isolation change ≤0.5dB after the test.

　　Vibration Test: 8 hours of vibration at 10-2000Hz/20g rms. No structural deformation. Isolation fluctuation ≤1dB.

　　Damp Heat Test: 1000 hours of continuous operation at 95% RH/+85℃. No plating corrosion. Isolation attenuation ≤1dB.

　　High Power Verification Test: 1000 hours of continuous operation at rated power (e.g., 30W CW). Real-time monitoring of isolation changes to ensure no sudden degradation. Suitable for high-power applications such as industrial heating and radar.

　　2. Key Certification Standards

　　General Standards: Complies with GJB 360B, MIL-STD-202G military standards, IEC 61169 RF connector standard, and RoHS 2.0 environmental requirements.

　　In the communications field: Wireless intercom system components must comply with ETSI EN 302 307 standards, and satellite communication components must be FCC Part 25 certified.

　　Industry-specific applications: Radar system components must meet the GJB 150A-2009 mechanical environment testing standard to ensure stable isolation under strong vibration.

　　V. Typical Application Scenarios and Practical Cases

　　1. Wireless Intercom Multi-Channel Combining System

　　Application Requirements: A large venue security system needs to combine the signals from 8 repeaters (covering the 800-900MHz frequency band) to a single antenna. The combiner isolation must be ≥25dB, insertion loss ≤3.5dB, and power handling capacity 10W.

　　Technical Solution: A cavity combiner topology is used, consisting of 8 sets of cavity resonators and circulators. The passband frequency spacing is above 250KHz. The housing is made of 316L stainless steel with an IP66 protection rating.

　　Application Results: Crosstalk suppression ≥28dB across all channels, noise-free call quality, supports 24-hour continuous operation, shows no performance degradation after 500 hours of salt spray testing, and is suitable for harsh outdoor environments.

　　2. Satellite Communication Multi-User Uplink Combining

　　Application Requirements: Low-Earth Orbit (LEO) satellite ground stations need to combine uplink signals from four users (Ka band 27-30GHz) before transmission. The combiner must have an isolation ≥40dB, insertion loss ≤0.6dB, and phase consistency ≤±1.5°.

　　Technical Solution: Wilkinson combiner based on LTCC technology, with an Al₂O₃ ceramic substrate (tanδ<0.001), silver-plated transmission lines, integrated bandpass filter to suppress adjacent channel interference, and titanium alloy encapsulation.

　　Application Results: Full-band isolation ≥42dB, signal-to-noise ratio loss due to crosstalk ≤0.3dB, suitable for simultaneous multi-user access, and already applied to LEO broadband satellite constellations.

　　3. Radar Array Signal Splitting System

　　Application Requirements: A phased array radar needs to distribute one transmit signal to 16 antenna elements (2-8GHz band). The splitter isolation must be ≥25dB, insertion loss ≤1.5dB, and power handling capacity 20W.

　　Technical Solution: Employs a multi-section branch-line coupler topology, LTCC multilayer integrated design, metal partitions between ports, and 2W high-frequency thin-film resistors with impedance matching ≤1.35Ω.

　　Application Results: Typical port isolation is 26dB, phase difference ≤±2°, beamforming accuracy meets radar detection requirements, and it passed a 1500g/0.5ms impact test without performance failure.

　　VI. Technological Development Trends

　　Intelligent Dynamic Isolation Optimization: Built-in miniature power sensors and AI algorithms monitor port crosstalk intensity in real time. Dynamic compensation of isolation is achieved through an adjustable capacitor array, ensuring wide-band isolation fluctuation ≤3dB, adapting to 6GHz multi-band coexistence scenarios.

　　Ultra-wideband high-isolation integration: Developing 0.5-18GHz ultra-wideband devices using a "multi-section matching network + filter integration" technology to achieve ≥28dB isolation and ≤1.0dB insertion loss in a two-way splitter, replacing traditional multi-device combination schemes.

　　High-power isolation upgrade: Developing a combiner capable of handling 100W power with ≥35dB isolation using a GaN-based circulator combined with a cavity structure, suitable for next-generation high-power radar and industrial heating systems.

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