IEEE Microwave Magazine - May 2016 - 67

subcomponents, the overall efficiency would be less
than 95%. The heat generated by the power loss needs
a special cooling design depending on the power class.
The best solution is removing the heat by liquid cooling. Presently, contactless electrical power transfer
with hollow-shaft modules is limited, in practice, to a
few kilowatts.
Figure 11 shows two contactless dc/dc power modules with a hollow-shaft design. Figure 11(a) presents
a 100-W module for 4.2 A at 24 V (efficiency: 88%). Figure 11(b) shows a 300-W module for 12.5 A at 24 V (efficiency: 90%). Both modules are increasingly integrated
into radar rotary joints.

Nonradar Channels:
A Gigabit/Ethernet Module
The core of an Ethernet module is most commonly
formed by capacitively coupled annular transmission lines to transfer electrical signals between the
stator and rotor (see [20] and [21]). The input signal
(in this case, an Ethernet signal) is converted into a
balanced signal and amplified before being applied
onto two concentric annular transmission lines on
the stator side (see Figure 12). This balanced signal
is picked up through capacitive coupling on the
rotor side of the module, where the coupled signal
is again amplified and converted back into the input
signal form.
The main advantage of this type of contactless
data transmission is the larger feasible inner diameter of such a module compared with contacting
solutions or contactless transmission using inductive coupling, which also has a relatively low bandwidth compared to capacitive links. In slip rings, the
maximum transferable data rate is limited by the
circumferential half-wave resonance and so by the
diameter of the annular lines. Resonances are not a
limiting factor for data rate in this type of capacitive
signal transmission. Rather, a maximum diameter
limit is imposed by small, balanced signal-delay differences caused by the slightly different radii of the
concentric annular transmission lines. In addition,
the signal-delay difference between co- and counterrotating waves on a raceway-a standard problem in
conventional slip rings-is avoided in a contactless
design. This allows the realization of a drastically
increased free inner diameter for the module, with
no reduction in performance.
A module for contactless bidirectional data transmission with four Ethernet channels is shown in
Figure 13. The module supports the transmission standards 10BASE-T, 100BASE-TX, and 1000BASE-T [22].
It automatically recognizes and selects the connected
devices' current Ethernet standard and duplex mode
(full or half). The module has an outer diameter of
165 mm, a clear center diameter of 16 mm, and a total
length of 139 mm. Modules of this kind with different

May 2016

Figure 10. A rotary transformer for contactless power
transmission, showing its split core with primary and
secondary windings. (Stator: blue; rotor: red.)

(a)

(b)

Figure 11. Two contactless dc/dc power modules with a
hollow-shaft design: (a) a 100-W module for 4.2 A at 24 V
(size in mm: Ø112 x Ø4 x 31.5) and (b) a 300-W module for
12.5 A at 24 V (size in mm: Ø160 x Ø32 x 54).

Figure 12. A simplified schematic of a capacitively
coupled Ethernet module with signal-processing (SP) and
amplification (A) blocks. (Stator: blue; rotor: red.)

clear diameters are standard building blocks for modern and future MRJs. Figure 13(c), for instance, shows
the core of a module with an exceptionally large clear
diameter (internal diameter: 300 mm; data rate: 1 Gb/s).

Nonradar Channels: A Fiber-Optic
Multichannel Module
As pointed out in [23], microwave photonic technologies represent a promising solution for the generation,

Table of Contents for the Digital Edition of IEEE Microwave Magazine - May 2016