Microwave rotary connectors are essential components used to transmit high-frequency signals between a stationary platform and a rotating one. These devices enable continuous signal transmission between structures that move relative to each other, ensuring uninterrupted RF communication. They are widely applied in fields such as air traffic control radar, missile defense systems, medical imaging, satellite communications, and camera systems. By eliminating the risk of cable entanglement during 360-degree rotation, they significantly enhance system reliability. Microwave rotary connectors can be categorized into three main types based on the frequency range they support: (1) contact-type slip rings, which typically handle frequencies up to 120 MHz; (2) coaxial contact rotary connectors, suitable for frequencies up to 50 GHz; and (3) non-contact rotary connectors, which use either coaxial or waveguide technology and offer longer service life with a broader frequency range—usually within 20% of their maximum operating frequency. Current market demands emphasize compact design, low VSWR, minimal insertion loss, smooth rotation, and high channel isolation.
In RF coaxial theory, common characteristic impedances are 50Ω and 75Ω. For a 50Ω connector using an air medium, the ratio of the outer conductor's inner diameter (D) to the inner conductor's outer diameter (d) is determined by the formula:
$$
Z_0 = \frac{138}{\sqrt{\varepsilon_r}} \cdot \log_{10}\left(\frac{D}{d}\right)
$$
When ε₀ = 1 (air), the D/d ratio for a 50Ω connector is approximately 2.3. If PTFE is used as the dielectric material (ε₀ = 2.04), the D/d ratio increases to around 3.29. Most RF coaxial connectors use air or PTFE as the dielectric, with inner conductors made from copper alloys like beryllium bronze or tin bronze, and outer conductors typically made from steel or brass. Based on these principles, a rotatable and reliable microwave rotary connector can be designed, often using SMA connectors as a standard interface.
The design process involves several key steps: selecting appropriate inner and outer conductor dimensions based on the operating frequency, designing the groove layout and size, choosing an input/output structure such as SMA, simulating performance using software like HFSS or MWS, analyzing power capacity if the return and insertion loss meet requirements, manufacturing the component, testing, and conducting environmental and reliability tests.
An example design includes a small-sized microwave rotary connector operating at frequencies above 10 GHz. Using the SMA connector as a base, the cutoff frequency is calculated using the formula:
$$
f_c = \frac{c_0}{2\pi \sqrt{\varepsilon_r}} \cdot \frac{1}{\sqrt{\ln(D/d)}}
$$
With ε₀ = 2.04, d = 1.27 mm, and D = 4.1 mm, the calculated cutoff frequency exceeds 20 GHz, meeting the required performance. The inner conductor's outer diameter is kept consistent with the SMA connector to minimize reflection. The outer conductor’s inner diameter is set to 2.92 mm, while the rest of the structure follows the SMA standard. The final design features a miniature bearing and end caps to secure the inner and outer rings.
Testing results show that the connector operates effectively from 0–15 GHz, with a VSWR ≤ 1.25, insertion loss ≤ 0.5 dB, peak power of 1000 W, and rotation speed up to 100 rpm. It also functions in a wide temperature range (-40°C to +70°C) and uses SMA-F-SMA-F interfaces.
During the design phase, several critical considerations must be addressed: the choke slots should not be on the same plane, the gap width should be less than 0.05 mm, and transitional grooves should be added when transitioning from 4.1 mm to 2.92 mm. Gold alloy plating with a thickness of at least 3 μm improves conductivity and wear resistance, and a lubricant like DJB-823 is applied for protection. Pulse plating offers better uniformity and gloss compared to conventional electroplating. Dimensional tolerances must be tightly controlled, within ±0.005 mm.
In conclusion, this microwave rotary connector offers a compact, cost-effective, and efficient solution for high-frequency signal transmission. Its small size allows easy integration with slip rings, enhancing overall system performance. While it is ideal for low-speed applications due to its contact-based design, it remains a reliable choice in various industrial and technical settings.
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