Dodging Thor's Hammer - Part 3
How NOT to Have a QSO with the Clouds
By Tom Cox, KT9OM
A Reprint by Permission of
antenneX Online Magazine
http://www.antennex.com
Issue Number: 74 July 2003


 

Dodging Thor’s Hammer – How NOT to Have a QSO with the Clouds

Tom Cox, KT9OM

 

First, a Trip Through the Archives: The Nature of the Beast

 

Welcome back to Ham Work Shop. In the first episode of the “Dodging Thor’s Hammer” series, we talked about the nature of lightning, and how important it is to protect ourselves and our equipment against Mother Nature’s constant desire to reconcile differences in electrical charge by causing 20,000 to 50,000 amps of current to pass through an ionized channel of air between the volumes of opposite charge.

 

Nobody, whether they read the first installment or not (but you did, of course), wants to be standing in the spot where that ionized channel appears, super-heating the air hotter than the sun, expanding that air so fast that the shock wave actually exceeds the speed of sound, and getting your name into the headlines for the last time in your all-too-short life.

 

We discussed that the individual lightning strike, which actually consists of one or more “strokes” of electricity, resulting in that familiar lightning flicker, is actually very hard to protect against, if it hits you or your equipment directly. I described a direct strike as a low-probability, high-damage event, which is something of an understatement, but also compared it to a nearby strike, which is a much more frequent possibility, with less than catastrophic, but still quite serious consequences. Everything we do to protect ourselves against one type of event helps to protect us against the other.

 

Being Well Grounded in the Fundamentals

 

The first line of defense, we learned in the second part of the series, is good grounding. Grounding is the best way to drain off the most electrical energy from a strike, either direct or nearby, right at the tower, before it can get into your house and your equipment. A lightning ground is more than it seems, because lightning-induced energy covers a broad portion of the RF spectrum, and has its impact over fractions of a microsecond. A household electrical safety ground, while seeming to be a perfect conductor at 60 Hz, is actually a low pass filter where lightning is concerned. It offers significant impedance to the lightning energy, most of which is concentrated in the 1 MHz to 30 MHz range, with the energy share at higher frequencies dropping off rapidly.

 

Unnecessary impedance is the last thing we want to put between lightning-induced Electromagnetic Pulse (LEMP, or just EMP), and ground, because the energy that can’t get to earth through the intentional grounds will get there through unintentional grounds – your home entertainment system’s electrical ground, your home’s structural framework, your utility lines, etc.  Unnecessary impedance is introduced in lightning grounds by making them too long, or too thin, or by running the conductor with too many coils or sharp  bends, or by using connections that don’t make good, long-lasting physical and electrical contact between mating surfaces.

 

 I showed you some pictures of the ground system at El Rancho KT9OM, which protects a 120-foot tower less than 40 feet from my house. The Ufer ground, consisting of bonded rebar and anchor bolts inside the poured concrete slab, is interconnected with 4/0 wire, which “pigtails” out into the yard and connects with #6 AWG solid copper wire to a network of ground rods. The object of all of the above is to provide a “massively parallel” electrical circuit between the tower and ground, such that each, individual conductor in the circuit carries only a small fraction of the total energy that needs to be dissipated.

 

A book I picked up at the Dayton Hamvention (I know, it’s now correctly known as the Troutwood Hamvention, but I react slowly to change), called Lightning Protection and Grounding Solutions for Communication Sites, by Ken R. Rand (1st ed., 2000, Polyphaser Corporation), advocates strongly for using copper strap, as opposed to heavy wire, for ground connections. The rationale is that, since lightning energy is mostly RF, and RF travels mostly on the surface of a conductor -- and more closely to the surface as frequency increases (the “skin effect”), copper strap is better than wire because it offers far more surface area per unit of length than any but the biggest wire.

 

Rand describes current flow in the strap conceptually as equivalent to “an infinite number of very small wires spaced infinitely close together,” which is interesting and plausible, but not subject to imperial verification here at El Rancho KT9OM. All I can say is that I buy that strap is better than wire. Of course, now that I have about 200 feet of #6 AWG copper wire in the ground, I don’t anticipate digging it up and replacing it with strap. However, if I can find copper strap at reasonable prices, I will probably use it when I experience the impulse to expand the system by adding more ground rods.

 

The Polyphaser book, by the way, is very informative, and despite a few typos, well worth reading. The marked price is $20 US, but I got mine at Dayton (sorry, Troutwood) for $15. Had I known how much information it contained, I’d have paid $20. Of course, it should not shock anyone that the Polyphaser book suggests a Polyphaser product for just about any lightning grounding or protection situation, but there is still a lot of very useful information in that 90-page, large-format book, regardless of the brand name one chooses to use in the process of designing and installing lightning protection. Don’t neglect the reference list in the back of the book for more in-depth reading on lightning. Polyphaser makes and sells a great many devices and materials for lightning protection for amateur and commercial use, from DC and telephones through AM broadcast and deep into the microwaves, so they must be doing something right.

 

The Last Bastion

Once you have established the best lightning ground system you can afford, you have to decide among several types of lightning protectors – discrete devices that intercept the EMP that gets past your ground system. There are several manufacturers of lightning protectors, including, of course, Polyphaser, and I am not in a position to recommend one over the rest. Polyphaser’s approach to protector design uses “DC blocking.” They say this is the only way to keep the protected equipment from “sharing” the impulse with the protector. This design means there is no continuity between the center conductors on the antenna and equipment sides of the coax. The DC (and low-frequency AC) are blocked by a high-voltage capacitor that will still pass RF. The protectors designed for transmitting use include a gas discharge tube that breaks down and conducts to ground at somewhere in the 400 to 1,000 volt range.

 

ICE HF coaxial protector Model 300, front view

Protectors that don’t DC-block the center conductor use some sort of device to intercept the EMP energy on the center conductor and shunt it to ground, while passing the desired RF energy along in the antenna circuit. Among the possible shunt devices discussed in the are a gas tube spark gap and a tuned stub. The gas tube fires too late, passing a significant amount of the EMP before it conducts to ground. The tuned stub “softens” the impulse, but still shares it with the radio. 

 

Silver/Teflon connectors on Model 300

Not surprisingly, there is another manufacturer that disagrees with Polyphaser’s approach. Industrial Communications Engineers, Ltd., (ICE) Indianapolis , Indiana , publishes a paper that is critical pf the DC blocking approach, among other things, because it does not allow for the constant drainage of low-voltage static buildup that accumulates in coax as a result of precipitation and other atmospheric phenomena. The coax, itself, is a long, skinny capacitor, and is capable of strong significant electrostatic potential between the center conductor and shield, if the antenna is not DC-shunted. Leakage of this potential through the dielectric, the insulating material between the shield and center conductor, or across connector components, causes “hash,” noise in the receiver.

 

Model 300 with case removed; inductor (upper left), gas tube (upper right); bleeder resistor (lower right), neutralizes "tiny voltages that feed through the capacitor dielectric when the antenna side of the capacitor is active during an impulse event (according to the Model 300 manual);" stainless grounding bolts (upper left and upper right, through chassis)

The ICE paper expresses some of the same misgivings about the gas discharge tube the Rand book does, with some additional points. ICE says gas tubes may not handle the “stretching” of the EMP by the inductive component of a long coax run, causing it to puncture and fail, and requiring replacement. There is no easy way to tell that a gas tube has failed, except by observing the damage that results from the first EMP after the failure, which is, of course, too late.

 

Model 300, bottom view, showing blocking capacitor

The ICE devices use a large blocking capacitor, but with a beefy discharge inductor (a choke wound on a toroid form) on the antenna side, the latter providing the DC static drain ICE recommends. As a third level of protection, the devices also include a gas tube. The gas tube only is active if a strong “back-EMF” signal develops across the inductor as a result of taking an EMP hit. The gas tube conducts the back-EMF to ground, but it only works when the back-EMF occurs, and after the inductor has absorbed most of the EMP energy. I have no pictures of the inside of a Polyphaser device, but as I do use some ICE protectors, I did disassemble a couple different types of those devices and photographed the interior. Some pictures are included.

Model 309 HF balanced line arrestor

 

The ICE units are sturdy and well made, with very clean assembly and no stinginess in the ratings and wire sizes used. They are enclosed in a sturdy aluminum cabinet, as are the Polyphaser units. As I said, I can’t recommend one brand over the other, but I happen to own several of the ICE units. Polyphaser units are sold very often with digital wireless equipment we use in our wireless network at the school corporation, so they may have a better reputation for use in the 2.4 GHz band. Most of the ICE units I have seen or read about seem to be designed for LF, HF or the low VHF region. I don’t know whether this reflects engineering decisions, or just a particular marketing strategy.

 

Model 309 balanced line arrestor; note capacitor, inductor and gas tube for each side of line

I leave the decision on which brand of device to use up to the reader, and recommend checking out others, such as Alpha Delta, before coming to a decision. I just want to stress that you SHOULD pursue some sort of lightning protector for your valuable radio equipment. Keep in mind, though, that the best lightning protector made is no good without a good ground system to sink those currents into. The “big ball of dirt beneath our feet,” as I described it in Part 2, Earth, is the best place to send your excess lightning energy. Use it in good health.

ICE conductive lubricant with high copper content for secure grounding connections

 

 

Author's outdoor panel for one Model 300 and one model 309 — made from 1/8th-inch aluminum sheet mounted directly to a 10-foot piece of galvanized pipe driven 8 feet into the ground and tied to the rest of the ground system.

 

Lightning Protection on the Web

There is no shortage of free material on lightning protection available on the Web. Of course, manufacturers tend to promote their own brands, so keep that in mind. There is plenty to learn from many manufacturers’ sites, as well as the academic ones. I recommend searching Google for hundreds of good prospects.

 www.google.com

Put “lightning protection” (including the quotes) and stand back. You will get more than you could read in a week.

 www.polyphaser.com

Good reference (but not as good as the book), and plenty of product info.

 www.iceradioproducts.com

Includes most of the ICE product line *. Several popular amateur radio vendors also distribute ICE products, but Array Solutions has the  complete list I know of, other than the ICE catalog, itself. *


* Note from Array Solutions, we have the complete ICE catalog on-line now and it is in an easy to use shopping cart form.  We also have deleted the products ICE has discontinued and added some that are new and not in the manual.  

 

 * Many hams ask me what is a good way to install surge arrestors for a tower system.  Here is a nice example sent to us by an ICE customer. It uses several coaxial arrestors as well as a large number of control line arrestors for several SteppIR antenna and rotators.

 

Picture courtesy of AN Wireless