This article was posted 03/28/2006 and is most likely outdated.

Grounding of a Horizontal Antenna
 

 
Topic - Lightning and Surge Protection
Subject - Grounding of a Horizontal Antenna

March 28, 2006  

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Grounding of a Horizontal Antenna

 I have a very basic question, and although I'm worried about showing my ignorance on this subject, let me try. I have a horizontal receive only antenna strung about 20 feet (6 m) high in the back yard.  Simple wire (not coax), that runs into a window and then into my radio.

When a lightning storm is nearby, I unplug it from the radio. Besides being concerned about a direct hit on the wire antenna, I am also concerned about voltage surge on the wire from a nearby strike.

There are two options:  I can leave the end of the antenna just dangling, or somehow ground it. My questions relate to the pros and cons of grounding, or not grounding the antenna. Everything I've read says to ground unused items, antenna towers, etc.

I guess part of this question is also if you do ground it, is it that now there is less likelihood of an actual strike on it, or is it that if there was a strike, it would have an easy path to ground and not damage the antenna?  

So, does grounding (a) help prevent strikes, or (b) save the item?

Thinking about this, I guess I should ask the purpose of the lightning rods on barns in the same context: do these help prevent, or...? But, doesn't grounding "just" provide the path to ground that the lightning stroke is "looking for"? Wouldn't having it floating be better?

Thanks,

Bob

Answer by Abdul M. Mousa:  We receive many basic questions of this type.  In such cases, we usually provide personal replies rather than post them to avoid disturbing the many experts among our 1800 members.  I am making an exception this time as posting may satisfy the needs of many others.

First, let me say that the probability that lightning will strike a 6 m (20ft) high, short horizontal wire in the backyard of a house is extremely small.  Hence, from the practical point of view, it may not make any difference whether the wire is grounded or not.  Also, a nearby strike to such a short wire is unlikely to produce a significant surge magnitude.

However, if the wire receives a direct strike, then leaving one end "dangling" inside the house could lead to a spark that may start a fire or injure a person.  Hence it would be prudent to ground it.  This way, same as in case of lightning rods, lightning would be safely discharged to ground.

Assuming that the wire was long enough (say several kilometers) and other factors (height, keraunic level, etc...) were such that the wire  receives a significant number of lightning strikes, then the frequency of lightning strikes will be independent of whether the wire is  grounded or floating.

By the way, the overhead shield wires on power lines used to be bonded to each tower, i.e. grounded.  This, however, generates circulating currents via 60 Hz electromagnetic induction.  The related power loss is significant.

For several decades, many utilities have been insulating the shield wires to avoid the power loss and or to serve other purposes, e.g. using the shield wires as communication circuits.  It is established, both theoretically and via field observation, that such a floating condition neither affects the frequency of lightning strikes nor impair the shielding function ability of those wires [1].

Reference:

[1] F. Iliceto et al. (1989).  "New Concepts on Medium Voltage Distribution from Insulated Shield Wires of High Voltage Lines", IEEE Trans., Vol. PWRD-4, No. 4, pp. 2130-2144.  Includes a discussion by A.M. Mousa

Thank you.

Abdul M. Mousa, Ph.d., P. Eng., Fellow IEEE Co-moderator
Visit the Lightning and Power Quality group at Yahoo: http://groups.yahoo.com/group/LightningProtection/

 Comment by R.T. Hasbrouck: Growing up in the humid Northeast offered many opportunities to experience the unbridled exuberance of summer thunderstorms.  As a teenager, there was my own unplanned lightning "experiment." When my amateur radio equipment wasn't being used, I generally made a practice of disconnecting and grounding the antenna lead-in wire. Awakened one summer night by sounds of distant thunder, I was startled to see the corner of my darkened bedroom, where the ham station was located, being illuminated periodically by a blue glow. Upon closer investigation I saw arcing between the inadvertently unterminated lead-in wire and a nearby point that was grounded. From the thunder that followed shortly after each spark, I estimated ("flash to bang" interval of five seconds equals about one mile) the storm to be from 10 to 15 miles away. My antenna was picking up enough of the lightning’s' horizontally-polarized radiated energy to produce sparks an inch long.

Grounding provides a preferential path for lightning current to follow. So, provide a good pathway, and pay attention to where it runs.

I strongly suggest getting a "lightning arrestor device" (LAD---my term) designed for antennas. The company with which I am familiar is Alpha-Delta---there are others as well. The LAD is located outside your house, and its body is connected to a proper earth grounding point by a suitable conductor. The feed line passes through the LAD before entering your house. Under normal conditions, the LAD is "invisible," i.e., it does not affect your radio reception. When a sufficiently high voltage appears on the feed line, an arc over occurs in the LAD, providing a conductive path to its grounded body. Current will then be conducted to earth. Note that high transient electrostatic voltages, capable of damaging the solid-state front end of a modern receiver, can appear on your antenna from distant lightning, nearby lightning, windblown sand, or snow. In the event of a direct strike to your antenna, the LAD will most likely be destroyed, and other collateral damage may occur. In addition to having an LAD, leaving the receiver disconnected (and unplugged from the ac outlet) will provide the highest level of equipment protection.

Regarding the other question---a lightning strike is a random event. When a structure lacking any kind of lightning protection system is struck, current will follow all conductive paths present---wet brick work, metal water pipes, metal conduit, electrical wiring, etc. The result is unwanted and most likely damaging currents flowing where you wouldn't want them to flow. A structure that incorporates properly designed and installed lightning conductors provides low-impedance, preferential paths directly to earth---external to the structure itself. Damage to the structure will be minimal or non-existent.

Good luck.

R.T. Hasbrouck, EE, PE

 
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Comments

  • Wire antenna strung between trees or structures that may receive a direct attachment or side-flash from lightning can easily deliver destructive energy to communications equipment, and interior wiring beyond them.

    Modern equipment provides a stand-off protection of about 2kv from the AC to DC components in the radios. But both house wiring and coax feedlines can deliver up to 5kv of surge energy from a nearby lightning event. Besides the fact that a receiver is designed for only millivolts of input, you are not protected by only grounding such an antenna to any interior component, or leaving it "dangling". All entrance and exit paths for lightning must either have proper bonding and grounding and surge protection, or else be completely isolated (disconnected and grounded outside) from the equipment and structure.

    Good luck.

    Jack Painter Communications Staff Officer Fifth Coast Guard District United States Coast Guard Auxiliary

    Jack Painter
    Reply to this comment

  • In the Navy on carriers we had many horizontal antennas 70 ft above the water. The top of the radar mast is another 100 feet up. Lightning will easily dance on those horizontal HF antennas before it will use the mast. Lightning does not pick the highest point it picks more often the point of least resistance. The HF antennas have a pass through insulator by the before the shielded coax cage. On the insulator there is a spark gap set to have a lower airgap impedance than the equipment's input impedance. Lightning then took the path around the antenna insulator to the ships hull. Never leave a RF antenna floating free. As the lightning builds in the area the antenna will build a charge that is quite lethal without an actual strike.

    Kid Stevens
    Reply to this comment

  • The LAD (self described term) that R.T. Hasbrouck, P.E. referred to is by IEEE and UL definition a Secondary Surge Arrestor (SSA).

    Currently, many of these devices use Metal Oxide Varistors, which are susceptible to failure during sustained over voltage conditions, which can occur due to utility distribution problems and loose or open neutral conditions. Although both rare, these do occur, and can lead to a hazardous condition known as fire.

    Changes to NFPA/NEC and the upcoming UL 1449 3rd Edition will effect changes to such devices in that fusing and thermal protectors be added to the designs. These devices will then be referred to as Type I Surge Protective Devices, which will afford the low impedance path to ground that such an application begs for.

    Additionally, users of surge protection must give prudent attention to the effectiveness of their current grounding system to determine quality of bond, level of impedance (should be well below 25 ohms) and that all grounds are common on the structure. Many details of this can be found on NEMA's website for surge protection... www.nemasurge.com. Good luck and pay heed to the other advice. If a storm's brewing and you're concerned - unplug it.

    J.D. Howell OE Applications Engineer Advanced Protection Technologies, Inc. Clearwater, Florida

    J.D. Howell
    Reply to this comment

  • I too had a similar experience with ungrounnded antennas. I had a long wire short wave receiving antenna. I typically grounded the antenna lead wire when not in use, but one time, I too awoke to a thunder storm. The thunder was still a long way off so I assumed there would be no problem and I immediately jumped up to disconnect and ground the antenna. As the lead came close to the ground connection, a spark jumped about 1/2". I was smart enouogh not to hold the ground and antenna at the same time, but the experience was nonetheless interesting. After that, I installed a coax cable type lightning arrestor. I too was a youngster of about 12 years old.

    Jim Sherbundy
    Reply to this comment

  • I feel that the first paragraph by Dr. Mousa was not necessary: " We receive many basic questions of this type. In such cases, we usually provide personal replies rather than post them to avoid disturbing the many experts among our 1800 members. I am making an exception this time as posting may satisfy the needs of many others."

    As many of my teachers in the past have stated, "there are no stupid questions, only poor answers". If Dr. Mousa wished to provide a personal reply, he could have just done so; no need say as much in his reply and make the requester feel out of place.

    I am an electrical engineer (somewhat of an expert in some electrical areas) and was not "disturbed" at all by Bob's question. It is always refreshing, informative and instructional to see how others perceive and understand electricity and electrical phenomena. I hope Dr. Mousa remembers all of his audience for his next reply.

    Mary Todd
    Reply to this comment

  • This has been very informative and shows a lot of profesionalism on the answers to this question of groundin.

    William Runkle
    Reply to this comment

  • For information about grounding antennas during lightning storms (or anytime the antenna is not in use) go to: http://www.arrl.org/tis/info/lightning.html.

    Bob Bennett
    Reply to this comment

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