Coaxial Cables Lightning Protection

Nearby lightning strikes usually generate a short duration high current (thousands of Amps) pulses. These impulses contain components with very diverse frequency ranges i.e., both low and high frequency. Typically, the VHF and UHF components can be picked by outdoor communication antennas leading to generation of very high voltages and currents.

                      

Fig 1: Gas Discharge Tube Lightning Protection Device

 

Generally, the strike creates a potential difference between the center conductor and the cable shielding of the coaxial cable. This then forces a high voltage differential and damaging current flow through the equipment. A lightning surge protector is therefore needed to shunt this very high current to the ground without transferring it to the connected devices.

Fig 2: Connecting LSPD to the Coax cabling

It is important to note that these are not direct strike devices but only handle the effects of nearby strikes. In the event of a direct strike your protection device would most likely be permanently damaged.

What to consider for Coaxial Lightning Surge Protector Devices (LSPDs):

·           1.  RF Performance

This covers the operational frequency, insertion loss and VSWR. These parameters will indicate what portion of the transmitted signal will reach the output and what portion would be lost during transmission. The goal is to keep the losses low, thus low values of VSWR and IL are ideal. Typically, the surge protector primarily operates as a passive component during most of its lifetime only coming into action in the event of surges.

·            2.   Connector type (for instance N, TNC, SMA etc.)

In most cases protectors offer bidirectional protection, meaning you can reverse inputs and outputs without affecting operation. Connectors bring different designs for both the inner and outer conductors; usually the outer conductor has a significantly greater current capacity than the center pin to help dissipate the surge current. The current rating of most center pins is rated as the survivability of ten 8x20μs ampere pulses. 

 

Table 1

 

Of these connectors, each has an inherent maximum voltage, current, and RF power limitations due to the physical design and materials used.

·               3. DC pass/ DC block protector

Indicates whether the connector allows or blocks DC power to equipment alongside the RF signal. If equipment is to be powered, you need DC power through the center pin of the COAX cable to connected devices.

·               4. Type of technology

There are two dominant technologies in surge protection:

Gas Discharge Tubes (cost effective and very common, derate over time usually about 5 years; can build static electricity over time which can cause equipment damage when discharged)