Fixed Station Lightning Arrestors

DC-Blocking Lightning Arrestors

Prefer a DC-blocking arrestor unless you need DC current to pass along the coaxial cable.

Non-DC blocking arrestors are used when DC current needs to pass along the coaxial cable, such as when feeding power to a tower-mounted pre-amplifier. Non-DC blocking arrestors are inferior because they cannot divert all of the strike voltage to ground without sharing a significant portion of it with the equipment that is being protected.

DC blocking arrestors use a capacitor to block DC current flow along the center conductor of the coax. There is no center conductor continuity from connector pin to connector pin. This internal capacitive coupling prevents the sharing of low-frequency surge current with equipment, and limits the throughput energy to only the amount that can be coupled by the electrostatic field in the capacitor. The result is far less surge current than with a non-DC blocked arrestor.

If DC power on the coax is needed, there are combination devices which couple a DC blocking arrestor with a “bias T” (DC injector/pick-off). This allows the DC to be injected/picked off without giving up the advantage of a DC blocking arrestor. When DC power is not needed to feed tower-mounted devices, DC blocking arrestors are usually preferred.

Lightning Arrestor Types

The two most common devices for relieving the potential difference that can build up between the center conductor and the shield are a gas discharge tube and an inductor. For most applications, an inductor-type arrestor is preferred.

A gas discharge tube arrestor contains a tube of gas connected between the center conductor and the shield. The gas tube acts as an on/off switch. When the potential difference across the tube is high enough, the gas is energized and turns “on,” shorting the center conductor to the shield, and allowing the current to flow to ground. When the voltage falls back below the turn-on threshold, the gas tube turns “off,” allowing the coaxial cable to operate normally.

Gas discharge tube arrestors do not provide a path to ground for DC and low frequency components. They allow static electricity caused by wind and precipitation to build up in non-DC grounded antennas. This static causes noise. When the static voltage exceeds the turn-on threshold of the gas tube, the gas tube will turn on, shunt the static charge to ground, and turn off again, but then the cycle will repeat over and over.

Whenever the gas tube turns on, some discharge can reach the radio, especially if the arrestor is non-DC blocking. Some commercial radio manufacturers strongly recommend against gas discharge tube arrestors.

The gas in a gas discharge tube arrestor has a limited number of energized cycles. If the gas tube is repeatedly energized, such as to eliminate a build-up of static electricity in the antenna, the arrestor can become degraded over time and will not adequately protect the equipment when needed.

An inductor-type arrestor uses a high current inductor between the shield and the center conductor. The inductor effectively grounds DC and low frequency lightning components, while allowing the desired RF frequency range to pass through.

Inductor-type arrestors will bleed off static electricity that can build up in non-DC grounded antennas. Some inductor-type arrestors also include a gas tube to provide even more current-carrying capability for high-surge events.

DC-Grounded Antennas

Prefer DC-grounded antennas because they do not build up static electricity. If you must use a non-DC-grounded antenna, use an inductor-type arrestor with it.

A DC-grounded antenna displays a short when an Ohmmeter is connected between the center conductor and shield of the connector. Examples of antenna types which are DC-grounded include folded dipoles and shunt fed Yagi antennas. An advantage of DC-grounded antennas is that they do not build up static electricity since the charge is shunted to ground.

A non-DC-grounded antenna displays an open when an Ohmmeter is connected between the center conductor and shield of the connector. Many common collinear ground plane antennas are not DC-grounded. This includes the Comet CX-333 antenna used at our county network radio sites and at many EOCs.

Gas discharge tube arrestors should not be used with non-DC-grounded antennas, because they will allow static electricity to build up in the antenna, resulting in extra noise. When the static build up is high enough, the gas discharge tube will fire and short the static to ground. Some transients can occur at the equipment during discharge. The constant cycle of static build-up, followed by discharge, followed by build-up will prematurely age the gas discharge tube, potentially leaving the equipment unprotected.

Lightning Arrestor Installation

Lightning arrestors won’t work unless they are connected to a properly installed ground system. Lightning arrestors should be placed in the coax as close to where the cable enters the building as possible. Typically, a ground bus bar exists inside the building, at the cable entry point. The coaxial cable coming into the building from the outside should be terminated at that point just inside the building and connected to a lightning arrestor which is grounded to the bus bar. The cable can then continue to the equipment rack.

Ground wires must be low impedance to provide the best possible path to ground. Use at least #6 AWG wire. #2 AWG is even better. Copper strap is best. Keep the length as short as possible. Minimize the number of bends. Where bends are necessary, keep bends to a minimum radius of 8 inches.

Ensure that the single point ground for your equipment is also bonded to the cable entrance bus bar and the building electrical ground. Establishing a proper ground system for your home or building is beyond the scope of this web page. See the references for detail.

Lightning Arrestor and Grounding References

The following references include more information about lightning protection, including how to properly ground your station.