Immunity to EMI makes copper-fiber cable attractive


We marveled at photos of the Titanic taken by the team headed by Dr. Robert Ballard, who located the shipwreck in 1986. But how many of us realized the photos were made possible by a special type of fiber optic cable?

Ballard’s team at Woods Hole Ocean-ographic Institute developed a unique type of undersea tether cable for remotely operated vehicles (ROVs) that combined steel for strength, copper for power and fiber optics for signals. Before that, ROVs were limited to tether lengths too short to find shipwrecks at the depth of the Titanic.

Using fiber to replace heavy coax cable for video and signals, and using high voltage (about 1,800 volts) for power to reduce current loads, allowed to make a cable much lighter and stronger than conventional cable. The lighter cable allowed the ROV called Jason to dive deeper than ever before, producing the historic photos of the Titanic and many more undersea sites.

Now all ROVs use this composite fiber optic cable to expand their range, but the idea of combining copper and fiber into one cable, called a “composite” cable, has not been limited to undersea applications. Composite cables may combine fibers for signal and copper for powering the device connected over the fiber or they may offer both fiber and copper signal cables for dual applications or future upgrades. Fiber optics, being an insulator, is not affected by electromagnetic interference (EMI) from the copper nor can it short out power conductors. Therefore, it becomes a simple, safe design for combining multiple cables.

Fiber’s immunity to EMI is a reason electrical utilities use lots of composite cables, mainly in the form of optical fiber ground wire (OPGW). OPGW is installed in most power distribution lines today, as the utility gets telecommunications fiber optic cable installed for free every time they install power lines.

Consider the antennas of cell phone systems. These remote antennas are usually connected over single-mode fiber optic networks, since radio frequencies are scarce and are more efficiently used for connecting to customer cell phones. Eventually the phone calls will connect to a fiber optic backbone anyway.

The electronics of a cell phone tower are divided into a base unit that connects to the backbone and remote radio antenna units on top of the tower. New generation systems connect the antenna electronics with fiber, so combining fibers for the antennas with power conductors in a single cable speeds installation and reduces tower weight.

Another application is remote video monitoring. Cameras often use fiber optics instead of coax for greater distance capability, but they still require power. If local power is not available, or if uninterruptible power is desired, delivering power over the same cable as the fibers may be an ideal solution. A simple cable with wires for power and fiber for signal is still lightweight and easy to install, and of course, total labor is less than if two cables are needed or local power hookup is required.

Structured cabling systems offer several opportunities to consider composite fiber optic cable. If you install a cable with both Cat 5e UTP and multimode fiber, you can allow for future upgrades to a centralized fiber system without having to install more cable.

The incremental cost of the fiber included in the composite cable is much less than the cost of removing UTP cables and installing fiber optics. You can probably even use the same cable to power fiber optic media converters using the power over Ethernet (PoE) standard for delivering power through UTP pairs.

A wireless access point connected into a structured cabling network requires both signal and power too. Many units use Cat 5e to carry signal and power using PoE. Adding fiber allows you to use the fiber to connect the access point and power it over one cable with no worries about signal interference from electrical noise.

Even home networks use composite cables. A standard “bundled” cable consisting of two Cat 5e cables, two coax cables and two optical fibers is installed in many new homes. The Cat 5e is used for connecting home computers to the Internet and each other, the coax distributes video from CATV or satellite dishes, and the fiber is available for future upgrades like digital high-definition TV.

Don’t expect to go to any fiber optic cable manufacturer and find composite cables in stock. They are specialty items, designed for each application and custom made. But most manufacturers will be familiar with them, as their applications are more common than you might think. Just make sure they know you mean a cable with copper conductors and fiber, not both single-mode and multimode fibers, which are included in what is called a hybrid cable.

HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.