Anritsu HFE0702 Kneeland

48 High Frequency Electronics

High Frequency Design

MULTI-SIGNAL CABLES

Another Look at Combined
Microwave-Optical
Cables and Connectors

By Foster Kneeland
Haverhill Cable & Manufacturing Corp.

C

ables that combine
optical and micro-

wave signal trans­mission have been avail­able for several years, but
it seems that this is a
good time for a review.
With so many new wide
bandwidth communica­tions systems, the ability

to carry both types of signals on a single cable
may have significant advantages.

Figure 1 shows the construction of these
cables. At the center is an optical fiber, sur­rounded by its cladding, which is then metal­lized on the outside. The metallization layer
becomes the inner conductor of a coaxial cable.
A typical coaxial outer conductor and dielec­tric material completes the cable.

A number of design issues must be consid­ered when manufacturing this specialized
dual-mode cable:

• The optical fiber should be chosen for the

application, whether single-mode, multi­mode with stepped index or multi-mode
with graded index, with the desired wave­length and attenuation characteristics.

• To use standard coaxial cable materials and

tooling, the outside diameter of the optical
fiber, after metallization, should be the same
as a standard coaxial cable inner conductor
(e.g. 0.011 in. diameter for a 0.047 in. semi­rigid cable.)

• The physical strength of the clad and metal-

lized fiber, as well as its bending behavior,
must be included when determining cable
specifications for pull strength, bending
radius, repeated bending, etc. These charac-

teristics can be significantly different from
the typical copper center conductor.

• The metallization thickness and the choice
of material must be appropriate for the skin
depth at the operating frequency range of
the coaxial cable.

Connectors

Connectors and launchers for microwave­optical cable are relatively simple in concept:
The center pin of the connector is tubular, per­mitting the optical fiber to pass through to its
termination at an optical source or detector
(Figure 2). Although straightforward in con­cept, the mechanical alignment must be
appropriate for an optical connector. In addi­tion, the conductor dimensions and dielectric
properties must be appropriate for the desired
microwave characteristic impedance.

SMA connectors have dimensions that
allow modification for use with this type of
cable. The center conductor can be adapted for

New applications in

microwave and optical

communications may

benefit from coaxial cables

that combine an optical

fiber with a conventional

center conductor and

outer conductor/shield

Figure 1 · Cutaway diagram of a combined
coaxial and fiber optic cable.

Coaxial Cable

Outer Conductor Dielectric Metallization

Cladding Fiber

Optical Fiber

From July 2002 High Frequency Electronics.

Copyright © 2002 Summit Technical Media, LLC

High Frequency Design

MULTI-SIGNAL CABLES

an optical fiber to pass through, while
providing an electrical connection to
the metallization of the fiber’s
cladding. The connection to the outer
conductor is unchanged from normal
coaxial cable.

Performance

Measurements on the MOST
(Microwave Optical Simultaneous
Transmission) cable from Haverhill
Cable & Manufacturing show that
this design is practical as a
microwave cable. The cable tested
was compatible with standard 0.047
in. diameter semi-rigid coax. A multi
mode step index fiber was used, with
a core diameter of 183 µm, a clad
diameter of 220 µm and a metallized
diameter of 275 µm, which is equal to
the normal inner conductor diameter
of 0.011 inch.

An 18 inch long test cable with
modified SMA connectors demon­strated a maximum VSWR of 1.28
from 0.5 to 18 GHz. Attenuation of
the cable assembly at 1 GHz was
equivalent to 58 dB/100 ft., rising to
260 dB/100 ft. at 20 GHz. To extend
the frequency range of the cable, bet­ter connectors are required, and work
has been done to develop 2.4 mm and

2.9 mm interfaces.

Applications

These MOST cable assemblies can
be used in phased array antenna sys­tems, signal processing networks,
and systems that require extremely
high isolation. The latter include
remotely-located sensors or up- and
downconverters that can be driven
with one transmission mode, return­ing a signal on the other.

About the Author

After more than 30 years in the
microwave industry, Foster Kneeland
founded Haverhill Cable &
Manufacturing Corp. in 1984. He
studied Mechanical Engineering at
Northeastern University and holds
three US patents. Contact Haverhill
Cable at 978-372-6386 or visit
www.haverhillcable.com

Figure 2 · Basic design of connectors and launchers for these cables.

Center contact
with fiber-optic
center

Two-hole flange
SMA jack

RF contact

Optical source
or detector