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