General VHF/UHF Lightning Arrestor Considerations

A high-level summary is presented here in quick, bulleted form.  More details are provided below. It is important to understand that the recommendations for lightning arrestors apply to any simplex FM station, whether it is used for voice or data communications.

• Don't operate your station while there is a chance of lightning nearby
  • This means that temporary field stations generally don't need lighting arrestors. Instead, shut down the station and get indoors.
• DC blocking arrestors typically share much less surge current with the protected equipment than non-DC blocking arrestors
• Inductor-type arrestors help to bleed static electricity from non-DC grounded antennas
  • Colinear ground plain antennas are generally NOT DC grounded. Be sure to use an inductor-type arrestor with these antennas.
  • The Comet CX-333 tri-band antenna used at the county network radio sites and at many EOCs is NOT DC grounded.
• Avoid the use of a gas discharge tube arrestor with a non-DC grounded antenna.
• In general, a DC blocking, inductor-type arrestor is usually preferred for most FM simplex and repeater operations.

DC Blocking vs. Non-DC Blocking Arrestors

• Non-DC blocking
  • Non-DC blocking arrestors are used when DC current needs to pass along the coaxial cable. An example use case is to feed power to a tower-mounted pre-amplifier, such as may be used in contesting and other weak signal work.
  • A disadvantage of non-DC blocking arrestors is that 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
  • DC blocking arrestors use a capacitor to block DC current flow along the center conductor of the coax.
  • An advantage of DC blocking arrestors is that 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 desired, 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.

 

Inductor vs. Gas Discharge Tube

• The two most common methods for relieving the potential difference that can build up between the center conductor and the shield are inductor and gas discharge tube.
• Gas discharge tube
  • 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.
  • One disadvantage of the gas discharge tube arrestor is that it does not provide a path to ground for DC and low frequency components. So, it allows static electricity caused by wind and precipitation to build up in some types of antennas (non-DC grounded antennas -- see below). 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. So, some commercial radio manufacturers strongly recommend against gas discharge tube arrestors.
  • Another disadvantage of gas discharge tube arrestor is that the gas has a limited lifespan. It can only support so many energized cycles. So, 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.
• Inductor
  • 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.
  • An advantage of the inductor type is that is bleeds off static electricity that can build up in some types of antennas (non-DC grounded antennas).
  • Some inductor-type arrestors also include a gas tube to provide even more current-carrying capability for high-surge events.
• For most applications, an inductor-type arrestor is usually preferred.

 

Suitability for DC Grounded Antennas vs. Non-DC Grounded Antennas

• Antennas are either DC grounded or not.
• DC grounded antennas
  • A DC-grounded antenna displays a short when an Ohm meter 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.
• Non-DC grounded antennas
  • A non-DC grounded antenna displays an open when an Ohm meter is connected between the center conductor and shield of the connector. Examples of antenna types which are not DC grounded are many collinear ground plane antennas which are commonly used for FM.
    • The Comet CX-333 tri-band (144/220/440 MHz) antenna that is used at our county network radio sites and at many EOCs is NOT DC grounded.
  • A disadvantage of non-DC grounded antennas is that wind and precipitation will cause static electricity to build up on the antenna.
• A gas discharge tube arrestor should NOT be used with a non-DC grounded antenna. The gas discharge tube allows 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.
• An inductor-type arrestor should be used with non-DC grounded antennas and is also suitable for DC grounded antennas.

 

Installation

• Your coaxial 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 below.

 

Notes on Some Specific Arrestors in Use

• Polyphaser VHF50HN
  • The SCCo BBS sites use Polyphaser part number VHF50HN.
  • The VHF50HN is a DC-blocked, inductor-type arrestor. It has a frequency range of 100 MHz - 512 MHz and a maximum power handling capacity of 750 W. So, it is suitable for typical 144/220/440 tri-band antennas, even if all radios are operating at high power.
  • Part number VHF50HN has two female N connectors. Other variations exist with slightly different part numbers, depending on the mounting flange and connector type.

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

• White Papers
  • "Coaxial Cable Lightning Protectors", Polyphaser.
  • "Coaxial Cable Protection", Polyphaser.
  • "Ham Radio Station Protection", Polyphaser.
• Books
  • Silver, H. Ward. Grounding and Bonding for the Radio Amateur. ARRL, 2017
  • NFPA 70: National Electric Code