Keeping the Navy's cables shipshape

Complex and crowded cabling installations are hard to test and maintain, but probably none are more so than those the US Navy has on its vessels. Combine a critical need for system reliability with a tight working space, relatively unskilled workers, and an environment that can be damaging to wires and connectors, and you have a situation as challenging as any faced in the communications industry. But the Navy came up with an off-the-shelf answer to maintaining its cables: a portable cable and antenna analyzer.

Return loss measurement gives Navy technicians a way to find or predict cable failures. Courtesy of Anritsu.

The environment in which the Navy tests is harsh. The atmosphere is typically humid, and salt is an ever-present corrosive. Quarters are tight, and cabling is difficult or impossible to access. Testing must handle everything from RF coax to gigahertz waveguides. Further, there are between 50 and 100 RF sources generating interference over a broad frequency spectrum at power levels suitable for worldwide reception. Despite all this, the test task often falls to recruits with limited experience.

Instead of developing special hardware, the Navy was able to find appropriate commercial instrumentation: the Anritsu Site Master antenna and cable analysis systems. The systems come in three versions that collectively span the frequency range from 2 MHz to 20 GHz. These handheld instruments weigh less than 5 lbs, offer up to 17 dBm of interference immunity, and are battery-powered, making them usable in virtually any field condition, including the tight confines on a ship.

Because the three models operate similarly, technicians only need to learn how one instrument works rather than learning different instruments for different systems. This simplifies the training required for a new recruit.

The Site Master provides a number of diagnostic capabilities, including VSWR, power, and spectrum-analysis measurements. One useful capability for the Navy is the instrument's ability to measure return loss by generating a frequency sweep and monitoring reflected signals. The measurement provides a quick check of a cable's integrity and performance across its intended operational bandwidth. If the overall loss measurement is low enough, the cable passes inspection and the technician can move on to other cables. The quick check allows more frequent testing of the entire system.

When there is a break in the signal path, the return loss signal spikes due to the abrupt change in impedance. By measuring the timing of the return signal spike, the instrument can estimate the fault's position.

Because such estimates must be based on assumptions about the signal's propagation speed, the instrument offers a built-in database of propagation characteristics for several hundred cable and waveguide types. Users can also enter parameters for custom cable types. Knowing the cable's characteristics allows the instrument to automatically convert the return signal timing to distance, making it easier for technicians to locate and repair the fault.

Ideally, the Navy's systems should never fail. The ability to identify problems as they develop is an important aspect of system maintenance. Site Master can assist in that task by logging as many as 200 measurements and 10 test setups.

Suppose a cable connector that is exposed to the weather has been installed without protective waterproofing, or the protection has been compromised. The connector won't fail immediately but will gradually corrode. A simple pass-fail test can't provide an early warning of the failure. Even routinely applied distance-to-fault measurements may not be enough. The measurements will show a return signal from the corroding connection, but a technician may not realize the signal is anything other than the normal reflection that comes from such junctions.

Comparing the signal with a previously stored measurement, however, makes the problem more apparent. The instrument will display multiple measurements as overlapping traces, revealing the changes in the corroding connection. Routine use of the distance-to-fault measurement along with a comparison to earlier measurements enables the Navy to recognize and prevent impending cable failures. Downtime for a Navy communications system doesn't risk loss of revenue; it risks loss of life.