High-power millimeter-wave rotary joint
T. H. Chang (張存續͒
a͒
and B. R. Yu (余博仁͒
Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan
͑Received 16 December 2008; accepted 8 February 2009; published online 4 March 2009͒
The rotary joint is a useful microwave component that connects a fixed part to a rotatable part. This
study systematically analyzes the effect of the discontinuity on the interface of a rotary joint for
several waveguide modes. Simulation results indicate that the transmission of the TE
01
mode is
independent of the geometry of the joint, and thus is ideal for such application. A rotary joint
consisting of two identical TE
01
mode converters, clasped each other by a bearing, is designed,
fabricated, and tested. Back-to-back transmission measurements exhibit an excellent agreement to
the results of computer simulations. The measured optimum transmission is 97% witha3dB
bandwidth of 8.5 GHz, centered at 35.0 GHz. The cold measurement shows that the results are
independent of the angle of rotation. In addition, a high-power experiment is conducted. The just
developed rotary joint can operate up to a peak input power of 210 W with a duty of 18%. The
working principle, although demonstrated in the millimeter-wave region, can be applied up to the
terahertz region where the joint gap is generally critical except for the operating TE
01
mode. © 2009
American Institute of Physics. ͓DOI: 10.1063/1.3089827͔
I. INTRODUCTION
A microwave rotary joint that connects a stationary part
to a rotatable part is an essential device for tracking radars. A
good rotary joint is characterized by high transmission, low
reflection, broad bandwidth, and high-power capability. Va-
rieties of rotary joints have long been invented and are com-
mercially available in coaxial and waveguide forms.
1–4
How-
ever, few studies have addressed the effect of the joint gap.
The shape of the gap significantly affects the performance of
a rotary joint. It is especially critical in the millimeter-wave
or even the terahertz region where the gap effect is signifi-
cant.
Azimuthal symmetry of the field pattern is preferred to
ensure rotatable features. It exists in the coaxial TEM
mode,
2,4
the circularly polarized TE
mn
modes,
1,5
TM
0n
modes,
5
and TE
0n
modes.
3
The selection of the operating
mode depends on how easily the desired mode is excited.
Thus TEM, TE
11
, and TM
01
modes are generally chosen.
Figure 1 shows the electric field patterns at the end and the
corresponding surface currents on a cylindrical waveguide.
The surface current of the TE
01
mode is polarized in the
azimuthal direction and has low transmission loss, suggest-
ing that the TE
01
mode is a good candidate for rotary joint
since no surface current flows across the gap. Recently, a
high-performance TE
01
mode converter was reported.
6
Therefore, a rotary joint is constructed herein using two
identical TE
01
mode converters joining back-to-back by a
simply sliding contact.
This work presents a rotary joint based on the TE
01
mode converter with the emphasis on the effect of the joint
gap. The gap geometry between two converters and the ef-
fect of misalignment is analyzed in detail. The principle and
design are presented. A cold test using a vector network ana-
lyzer and a hot test using high-power millimeter-wave source
are to be presented.
II. PRINCIPLE AND SIMULATION
The rotary joint consists of two identical TE
01
mode con-
verters that are joined back to back. A rectangular TE
10
wave
is converted into a circular TE
01
wave. The as-generated cir-
cular TE
01
mode is converted back to a rectangular TE
10
mode. The two converters align with the central axis. They
can be rotated about each other. This achieves the fundamen-
tal function of the rotary joint. The contact between the two
mode converters is very important. Thus this study focuses
on the effect of the contact. Computation simulation is con-
ducted using a full-wave solver—high frequency structure
simulator ͓
HFSS
͑Ref. 7͔͒.
A. Effect of circular slot
Contact can be achieved using various techniques, such
as the use of a ridged waveguide section
8
and a coupled
ring.
9
This study simply uses a sliding contact. The inset of
Fig. 2͑a͒ shows such a configuration. A circular slot appears
to be present between the two cylindrical waveguides. The
circular slot is characterized by the gap width d and the gap
length L as shown in the inset. A radiation boundary condi-
tion is assumed at the outer strip of the slot. Figure 2͑a͒
shows the transmission versus the gap width d for a fre-
quency of 35 GHz with a radius of 6 mm. Six modes are
depicted-TE
11
,TE
21
,TE
31
,TM
01
, and TM
11
, in addition to
the desired TE
01
mode. A longer gap length d corresponds to
a lower transmission due to the circuit mismatch. Notably,
the transmission of the TE
01
mode is independent of the gap
width d up to 1.5 mm because no surface current crosses the
a͒
Electronic mail: thschang@phys.nthu.edu.tw.
REVIEW OF SCIENTIFIC INSTRUMENTS 80, 034701 ͑2009͒
0034-6748/2009/80͑3͒/034701/4/$25.00 © 2009 American Institute of Physics80, 034701-1