Protecting Coax Cable

Protecting Coaxial Antenna Lines from Lightning Damage

Protecting coaxial lines from lightning requires careful consideration of three parameters; the layout of the system to be protected; the selection of the optimal protection components, and the grounding of the protectors and of the system as a whole. When all of these steps are taken, the results are not only cost-effective solution design, but that the maximum benefits are attained through acceptable installation practices.

The need to protect against lightning damage usually relates to either economics of costs of repair and cost of downtime with the associated risk or likelihood of damage. The risk of damage corresponds to equipment susceptibility and configuration as well as geographic location. Balanced against these costs and risks are the expense to protect, and the portion of failures which would be thus eliminated. In addition to these strictly economic factors are the standards for prevention of injury using contemporary techniques. The need to protect will not be discussed at great length here.

Protection mapped to system layout

The layout of protection systems depends upon whether the system uses single ended electronics, such as a simple antenna port, or double ended with active components on both ends of the coaxial cable, such as a tower top amplifier or antenna phase control. In the case of single ended system, the protection considerations usually result in protection near the sensitive equipment end, and sometimes protection at the antenna end. In figure 1, you can see that protectors are grounded, and are placed at both ends of the coax

If there are active electronics at each end of the coaxial cable, in addition to an antenna, then there is a need to protect both ends of the cable at the location of the electronics, as well as the antenna port. This need to protect is due to direct differential mode coupling to the cable, and the high levels of common mode currents, which are also converted to differential mode energy within the complex real world system. Figure 2 illustrates a typical layout with protection identified.

Selection of Protectors

Coaxial surge protectors are selected based on RF performance, protection ability, and physical configuration, primarily for connection and grounding/mounting. We will focus on the first two parameters, and discuss configuration as it related to grounding. Table 1, below, includes some of the most important parameters of protectors. The majority of protectors in the industry are based on the following protection technology. The typical performance parameters are listed, but each model needs to be compared to specific application needs. In any case, the choice of connector types is also important. Large external interfaces are desired (7/16 and N) which are more rugged and able to withstand transient input. The protected side connector is sometimes the same, or may be reduced for more compact and flexible internal cabling. In some cases a lead from the protector can eliminate a mating connection pair.

Coaxial Protectors Parameters and Selection

Parameter

PTR/PTC

BTL

QWS

Product
Description

Gas Tube Based Coaxial Protectors

Lightning Protected Bias T

Quarter Wave and filter protectors

Protection
Technology

Gas Discharge Tube

Gas Discharge Tube

l/4 wave shorting stub bandpass filter

Frequency

Dc-2.5 GHz+

1 MHz - 2.5 GHz

0.40 GHz Ð 8 GHz

Bandwidth

Entire range

50 %

5% to 50%

Insertion Loss

0.1 dB

0.1 dB

0.05 dB

VSWR

1:1.12

1:1.12

1:1.1

RF Power

30W to 1kW, according to GDT voltage, and limited by connector

Usually limited by RF Pass capacitor and GDT voltage

Limited by connector

Work with
dc injection

Yes

Yes, performs bias T function

No, shorts dc to ground

Protection Capacity
(8mµs x 10µs)

50 kA single pulse

50 kA single pulse

60 kA single pulse (higher for 7/16)

Protection Performance
(µJ or Vpk)

Good

Better

Very Good

Replacement
Elements

Yes (PTR)

Yes, at factory

Not needed

Configuration for Grounding

The protectors that are required need to be selected with the grounding requirements in mind. The most popular configurations for protectors, and the associated flow of lightning current are illustrated in the table below. It is clear that the mechanical configuration can either enhance or compromise the ease of connection and grounding.

Improper grounding of coaxial protectors is one of the most common ways to render excellent components useless. There are several important points in grounding protectors.

Requirement

Rationale and description

Use low impedance ground bonding.

Ground to a panel or grid directly or use a wire of minimum of 2x the shield area or greater than 10 AWG (3.5 mm2). Keep bonding jumper less than 18" (0.5m).

Suppress all copper conductors (power, data, signal, telco) to a common panel or grid.

Use adequate protectors to reduce all transients, to minimize common mode voltage internal to the system being protected.

Follow national, local and industry electrical codes for grounding, shielding and bonding.

These codes frequently specify the size, location, methods and materials, and earthing for structures, and can provide the best basis for suppression grounding.

Have a protection expert review sites and/or solution design.

Someone who can recognize and estimate risks based on experience better accomplishes lightning control

Finally, although lightning is a beautiful and intriguing natural phenomena, it represents a danger to people and property. Do not work on exposed cabling systems when there is a chance thunderstorm or lightning activity. Ground protectors prior to connection cabling. Use redundancy, with adequate maintenance, particularly for situations where personal injury is a risk. Consult an expert in lightning protection to design and examine your specific application.

Conclusion

Coaxial surge protectors can enable long term reliable operation of wireless communications systems, if they are properly selected and used. This article brought out the system configuration needs, component parameters, configuration and grounding which governs successful protection from lightning. This article introduced the topics, and provides a point to start the journey to develop the skills and knowledge to resolve lightning risks to systems.

 

© XL Systems 2014