IEEE Microwave Magazine - May 2016 - 64

(a)

(b)

Figure 4. A high-power RF module with R32 (WR-284)
waveguide ports for S-band primary radar applications: (a)
the complete module and (b) the details of a doorknob-type
waveguide transition with a hollow center conductor [1].
the waveguide with dry air or with sulfur hexafluoride, an insulating gas.

Radar Channels: A Medium-Power
RF Module with a Large Center Opening
The key requirement for medium-power RF modules
is a relatively large hollow shaft because surveillance
radars usually incorporate multiple medium-power
radar channels. This can be accomplished by using

Z1, l1
TEM

TEM
Z1, l1

Z2, l2

TEM

an overmoded coaxial transmission line with an
extremely low characteristic impedance (often, only a
few ohms) as the center rotational section of the rotaryjoint module. Usually a binary strip-line power splitter
is used on the stator side to feed the input power onto
the outer conductor of the low-impedance coaxial line
(see [11]-[13]).
The feed points (which number a power of two) are
placed equidistantly on the circumference of the outer
conductor to ensure the excitation of a pure, rotationally symmetric TEM mode and to suppress higherorder modes that would cause instability over rotation.
After the power has passed choke-loaded rotational
gaps, an identical strip-line power splitter is used on
the rotor side to combine the RF power back onto the
coaxial output of the module. The central conductors
of the strip-line divider networks are suspended midway in the air between the ground planes and held
there using small dielectric beads.
Figure 5 illustrates this design using a two-tier
power splitter (i.e., four feed points on the outer conductor of the low-impedance coaxial waveguide [12]).
The 50-Ω coaxial input is converted into a symmetric strip-line waveguide. The binary power divider is
constructed with specific characteristic impedances
and line lengths (in Figure 5, tier 1: Z1, l1; tier 2: Z2,
l2 with 50 Ω > Z1 > Z2 > Z3) to match the input onto
the low-impedance coaxial waveguide (Z3, l3) and to
supply the RF power onto the feed points in phase.
The TEM wave traveling through the power splitter is
inverted onto the outer conductor of the low-impedance coaxial line (Z3, l3) by using inductive coupling
at the feed points.
Figure 6 shows an L-band medium-power module that follows this design. The module features a
relatively large center opening of 70 mm in diameter.
It is operated in the frequency range of 1.0-1.1 GHz.
With its typical insertion loss of 0.5 dB, it can handle
an average power of up to 500 W.
Figure 6(a) provides an inside view of the module,
illustrating its four-fold strip-line power splitter and
the choke system that inhibits RF power from leaking
through the rotational gap. As the operating frequencies are low, the corresponding quarter-wavelength

Z2, l2

Z2, l3

Z2, l2

Z3, l3

Z3, l3
TEM

Z2, l2

TEM

Z2, l2
Z2, l2
Z2, l2
Z2, l2

Z1, l1

TEM

Z1, l1

Figure 5. A schematic representation of the medium-power rotary-joint module with a simplified rotational gap (stator: blue;
rotor: red). The TEM wave inversion onto the outer conductor of the central low-impedance coaxial line (Z3, l3) through
inductive coupling is best emulated as a coupled transmission line section.

May 2016

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