Machine Design

Keeping coax safe

Special protectors dissipate potentially dangerous transient currents in coaxial cable.

George M. Kauffman, P.E.
NexTek Inc.
Westford, Mass.

Edited by Robert Repas

When voltages on the center conductor exceed a specific value, the gas in the gas-discharge tube ionizes to discharge the buildup to ground.

Even though the quarter-wave stub is electrically grounded, it presents a high-impedance circuit to a signal within the frequency range specified by the protector. Any transient current on the center conductor is immediately bled to ground through the stub.

Any outdoor wire suffers the risk of lightning damage. Protectors reduce that risk by suppressing transient voltages imposed on the wire. Though transient suppression is relatively straightforward in ordinary wires, there are complications that arise when protecting coaxial cables. Protectors must limit the transient voltages on both the outer shield and center conductor. And this protection must not affect the signal carried by the center conductor.

Principle factors when selecting a coaxial protector are the connector type and operating frequency. Connector choice depends on size, durability, and speed (frequency and bandwidth) of the connection with the most popular connectors being N, THN, and SMA.

Many protectors come as coaxial-adapters — two coaxial connectors at either end of a central housing that holds the protection circuit. The protection circuit attempts to remove the bulk of excess-electrical energy from the center conductor and outer braid without destroying the desired signal. Protection circuits typically take the form of gasdischarge tubes or quarter-wave shorting stubs.

Gas-discharge tubes (GDT) connect between the center conductor and the outer housing. When voltage on the center conductor reaches a specific trigger value, the gas in the GDT ionizes to create a conductive path to ground. Any high-voltage buildup is effectively shorted to ground. Once the charge dissipates, the gas deionizes. This removes the short and lets the signal through. Better GDTs can handle 10 or more 20-kA pulses up to 20 usec wide before the protector must be replaced.

The GDTs in many protectors are permanently installed, though some are replaceable. The lifetime and reliability of GDTs make frequent replacement unnecessary, except for the highest exposure applications. GDT-based protectors typically work over a frequency range from dc to 3 GHz, though some models work as high as 12.5 GHz. The GDT trigger voltage must be greater than the normal RF voltage on the center conductor. Transient currents travel from the center conductor through the GDT into the outer housing and on to ground through the mounting panel, bracket, or grounding wire. The shield or outer conductor discharges any induced currents through the outer housing as well.

The second style of coaxial protector uses a quarter-wave shorting stub from the center conductor to the shield. Protectors using this method seem counterintuitive — the center conductor looks like it's shorted to ground! But the shorting stub is tuned to work at the frequency of the RF signal passing through the center conductor. At that frequency it presents a high-impedance path that effectively blocks the signal from ground and lets the RF energy pass safely through the protector. The high transient-current capacity of the stub shorts out any unwanted voltage.

These protectors specify the range of RF they'll pass by stating a minimum and maximum operating frequency within a range that spans from 400 MHz to 6 GHz. Do not use this protector where the coaxial cable is also handling a dcpower component because the stub shorts dc to ground. The transient current path from the center conductor travels through the stub connection to the housing and then to ground through the mounting panel, bracket, or grounding wire. While early protectors-were T-shaped, modern protectors totally enclose the stub within the outer housing.

The degree of protection offered by coaxial protectors hinges on good grounding. Mounting to a grounded bulkhead panel provides the best connection. Avoid using long ground wires as they limit current capacity and transient response because of conductor impedance.

Always use protectors when coaxial cables are brought into a building or structure from outdoors to reduce equipment damage and boost application uptime. Signs of high quality protectors include waterproofing, low signal loss, low signal reflection, high transient current capability, and good clamping performance.

NexTek Inc., (978) 486-0582,

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