IEEE Microwave Magazine - May 2016 - 65

(a)

(b)

(c)

Figure 6. A medium-power RF module for L-band secondary radar applications: (a) the inside view of a single module, (b) an
assembled single module (size in mm: Ø214 x Ø70 x 72; insertion length: 45 mm) [1], and (c) an axial stack of four modules [1]
exhibiting the four stator-side coaxial cable inputs on the outer circumference.

sections of the choke system would become rather
large. A typical method to reduce the required space
is to fold the quarter-wave sections as choke rings with
interleaving grooves (see Figure 7).
Figure 6(b) shows an assembled single module. Two
adjoining modules in an axial stack need a torsionally
rigid mechanical connection. On the modules' outer
rings, this is accomplished by direct bolting, while
on the inner rings, a clutch coupling avoids geometric
overdetermination [note the clutch dogs around the
center opening in Figure 6(b)].
The presented module is a universal building block
for secondary radar applications such as secondary
surveillance radar (SSR) and identification of friend or
foe (IFF). Rotary joints of surveillance radars are often
equipped with a stack of between two and four such
modules as depicted in Figure 6(c), generally providing a sum (R), a difference (D), and an omnidirectional
(Ω) signal.

Nonradar Channels: A Slip-Ring Module
There are three basic contact-ring techniques for passing electrical currents from a stator to a rotor: slip-ring,
roll-ring, and liquid-metal contact-ring technologies,
as shown in Figure 8. The slip-ring technology uses
sliding blocks or wire brushes to contact a circular
raceway (the literal slip ring [14], [15]). The roll-ring
technology inserts rolling metal rings between two
concentric raceways [16]. The liquid-metal contact-ring
technology connects two concentric raceways by a
layer of liquid metal (mostly mercury).
While liquid metal contact rings are generally not
employed in surveillance radars, the other two types
are common. Slip-ring technology is by far the most

May 2016

Figure 7. A comparison of a conventional outer conductor
choke (left) with a folded outer conductor choke (right).
Folding the choke greatly reduces its radial extension from
the axis of rotation but leaves the effective length of the
quarter-wave sections unchanged. (Stator: blue; rotor: red.)

commonly used contact-ring design and traditionally
serves as the workhorse for the transmission of electrical power and signals.
Two slip-ring types are in use for power transmission: 1) graphite brushes on raceways made of copper or silver alloys and 2) precious-metal brushes on
precious-metal raceways. Signals are usually transmitted only by the latter technology. Due to their sliding
contacts, slip rings suffer from contact wear and so

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

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