Ham To Ham #28 - January 1998
73's Ham To Ham column
c/o Dave Miller, NZ9E
7462 Lawler Avenue
Niles, IL 60714-3108
USA
Lightning protection - what your mother never told you!
Moderator's note: This month we begin a series put together by Roger Block, president
and chief engineer of PolyPhaser Corporation of Minden, Nevada, and his brother Ron Block,
owner of W.R. Block and Associates (representatives for PolyPhaser) of Clarksboro, New Jersey. Roger and Ron have done a tremendous amount of research into the whys and wherefores of lightning protection for their commercial customers, but most of their research has equal relevancy to our own amateur radio station antenna installations. Lightning doesn't discriminate between amateur or commercial antenna sites, we're all vulnerable! Commercial installations are often located on out-of-the-way mountain tops, so they may have more vulnerability to lightning strikes from that standpoint, but the rolling hill country and flatlands that we hams live in can be equally appealing targets for Mother Nature's fury, when a highly charged thunderhead moves across the sky over our QTH.
Admittedly, not all of the tips and suggestions made by Roger and Ron Block will be able to be
totally implemented by each of us, but the more understanding that we have of the mechanics and safe-handling of lightning events, the more of these suggestions can be incorporated into our own backyard antenna installations. The material to be presented here each month in this ongoing series can become a bit involved, from both the technical and ease-of-installation standpoints. I'll do my best to break it down into manageable chunks so that a better understanding can take place. If you find that you don't quite understand a certain concept, it may pay to reread it a couple of times, then let it rest for a while, going back to it later for still another reread. I've found this to be a successful approach in my own case when there's a great deal of information to absorb in any complex subject. It usually works. If you'd like to see the original, unabridged version of Roger's and Ron's work, contact PolyPhaser Corporation, Customer Service Department, 2225 Park Place, P.O. Box 9000, Minden, NV 89423-9000 and ask for their Special Bulletin, "Protection to Keep You Communicating" (copyright 1995). You can also pay a visit to PolyPhaser's home pages on the world wide web at: http://www.polyphaser.com/. PolyPhaser's web site also supports text downloads of the original material that's going to be condensed here plus other related texts on the subject. The PolyPhaser Tech Line telephone BBS at (702)782-6728 is also available to interested readers. The communications parameters are: Data bits - 8, Parity - None, Stop bits - 1, Baud rate - 300 to 14400. If you are dialing in for the first time, the Tech Line requests your name, address and telephone number. You will also need to create a password. Once you've logged-on, just follow the menus to navigate around the Bulletin Board. Many thanks to Roger Block and Ron Block for their kind permission to disseminate the information in this series via 73's Ham To Ham column.
By the way, this series is not intended to be just one giant ad for PolyPhaser. The fact that it
comes from a commercial source, in my own opinion, adds credibility to the tips found
throughout the discussion. Roger and Ron have done their homework; you don't get away with
selling hype and fairy tales in the commercial communications world ... at least not for very long.
If the techniques that the authors will lay out for you weren't effective, word would quickly make
the circuit and the business would have disappeared (it's been active since 1979). Commercial
customers expect to be served with scientific facts and effective solutions. I'll keep the references to PolyPhaser itself to a minimum, but obviously the name will pop up from time to time. Again, this is not a paid or otherwise compensated advertisement. It's good, sound, proven information that hasn't been disseminated nearly enough in the popular amateur radio press ... until now.
Lightning protection - what your mother never told you! - Part 1
Did You Know?
ï A National Weather Service survey shows that lightning kills more people annually than
hurricanes, tornadoes or floods, up to 300 people a year.
ï More than 100 lightning flashes occur every second in the atmosphere or 8.64 million every day. Fewer than 20 percent hit the earth's surface; the rest jump from cloud to cloud.
ï About 18,000 homes and buildings are damaged yearly by lightning-caused fires. Many more
people are killed and injured this way too, but their cases are rarely reported as lightning-related,
so they are not included in the lightning statistics.
ï Deaths and injuries from lightning can be caused by direct strikes, side splash or the spread of
ground voltage after the strike. Blood vessels in the head have the least resistance to electrical
charge, and strikes frequently result in eye damage. Lightning can also cause paralysis, heart
stoppage and other traumas.
ï The Guinness Book of World Records recorded that a former Shenandoah National Park ranger was struck by lightning seven times and survived. By the way, he lived in Dooms, VA.
Introduction
The concept of lightning protection can be summed up in just a few words: we don't have control
over Mother Nature, but we do have quite a bit of control over how a lightning strike's energy is
dispursed and dissipated. That's an important point to remember. It means that we can maintain control over the destructive nature of lightning by providing a path to earth for the strike's energy, and by not simply allowing that energy to choose a random path. Stating it yet another way, it's not possible to stop a strike in progress, nor is it possible to prevent a strike from occurring in the first place, but we can make certain preparations to divert the energy in the strike safely to earth, via a deliberately planned and controlled path, so that damage can either be eliminated or at the very least, minimized. Here's another key issue: building or structural protection is more forgiving than modern-day electronics. A building can handle usually 100,000 volts, while solid-state
electronics will often be damaged by just a few volts over the intended safe operating voltage.
The Golden Rule
The primary rule for protecting your ham radio equipment against damage from a lightning strike
is in the interconnection of all of the station elements to a single, low impedance ground system.
Included in this low impedance ground system are: the antenna, the antenna support and all of the input/output lightning protection devices within your system. We'll expand on these points greatly throughout the remainder of the series.
Antenna location
There's an old joke in the real estate business that goes something like this:. what are the three
most important factors in placing a value on a piece of property? Answer - location, location,
location! The same thought applies to an antenna tower's ability to dissipate a lightning strike ...
location, location, location. The antenna's location and the effectiveness and location of it's
planned grounding system, will determine how fast and effectively the energy in a lightning bolt
will be able to be carried away from the tower structure and dispersed into the surrounding soil, and, perhaps most importantly, how much of your expensive electronics will survive .
Here's a picture that you might want to keep in mind: most lightning strikes will carry huge
charges of like polarity. But being of like polarity, those charges will naturally repel each other
and want to disperse. The easier the path that you give the lightning charges to disperse in
safely, the more likely your equipment will survive the hit. An antenna ground system comprised
of a number of ground rods, interconnected below grade by large, bare radials, will have a better chance of dispersing the strike's energy than a lesser system would have. So the golden rule for surviving a lightning strike is the same no matter which of the many possible variations you may have, ie., all equipment elements must be connected to a single, low impedance ground system. This includes the antenna, the antenna support (the pole, the tower, etc.), and all of your station's input and output lightning protectors ... transmission line protectors, power line protectors, telephone line protectors, rotor control cable protectors, etc.). By the way, the term radials, as applied here, doesn't refer to the thin wire radials that might normally be used by amateur operators to provide a better "phantom" ground for an HF quarter-wave vertical antenna. When dealing with lightning, don't think small, think big. The radials we're referring to are wide, below-grade copper straps what will be used to actually help disperse a powerful lightning strike into the earth's surrounding soil, though they may also contribute to a lowered feedpoint impedance for your HF quarter-wave vertical as a bonus.
Let's examine the significant elements of a good grounding and protection scheme to help you
construct a "bullet proof" installation that will have a reasonable chance of surviving a direct
lightning strike.
We begin by choosing the antenna's location. This, and the type of antenna, will dictate the size
and layout of the earth ground system needed to reasonably disperse the strike's energy.
Remember, the faster the ground system is able to spread out and absorb the strike's energy, the better the chances of preventing it from traveling to your equipment. The antenna ground system is part and parcel of the antenna's "location", in the sense that we'll be using that word.
As we'll detail later on, the primary ground system is represented by a set of copper-clad ground rods, interconnected below grade, with bare copper radials.
Also fundamental to a good protection scheme is the creation of a single point ground within the ham shack. This single point ground will be used to mount all of the I/O protection equipment and to provide a ground for all of the equipment cabinets at the station's operating position. This interior single point ground is connected to the external ground system (composed of those radials and ground rods) by the lowest impedance copper strap that you can manage. The tower ground system outside and the single point ground system inside must be solidly interconnected with a low impedance metallic strap, so that your coax cable's shield is not the only interconnection conductor between these two ground points. Keeping as much of the lightning's energy off of the coax shield as possible is essential to minimizing damage from a direct hit. For larger strikes, it's best to incorporate a grounding kit prior to the protector, to save your expensive coax connectors from arcing damage. An effective (good quality) coaxial in-line protector can then be used to handle smaller strike currents that may be tempted to travel down the cable itself.
That's all from Roger and Ron Block for this month. Be sure to check back next month for more of their advice on keeping your ham station safe from the devistating effects of a lightning strike ... their series will continue here throughout the rest of this year. You can also read the entire text immediately, by calling up the Special Bulletin, "Protection to Keep You Communicating", at PolyPhaser's home page on the world wide web at: http://www.polyphaser.com/.
Keep whittling away at it!
Here's a handy tip to keep in mind from Stephen Reynolds N0POU: "I found myself in need of a special 12-volt DC power cord recently, and not having the exact female end (to match the male connector on the equipment in question), decided to 'whittle the problem down' to match what I did have on hand.
The connector configuration that I needed is roughly illustrated by Figure 1. The configuration of the cord on that I had on hand is shown in Figure 2. A reasonably short amount of time with a sharp hobby knife provided me with the resultant connector end shown in Figure 3 (the dotted outline being the material whittled away).
With so many different plug configurations showing up on equipment these days, it can often prove useful to keep a small stock of various cords on hand (ala the next hamfest), and whittle away at them when something special (and unavailable) is needed in a hurry. If the cord is carrying DC voltage, double, triple and even quadruple check to make sure that you've matched the positive and negative pins on the equipment and the power cord correctly. For AC needs, polarity is generally much less critical. I hope that others will find this approach as useful as I have."
New life for an old drill
Here's a timely tip from Max Holland W4MEA: "Hate to just throw away your old cordless drill simply because the built-in NiCd batteries have seen better days? Many times it will cost nearly as much to replace the defunct NiCds as it would be to buy a completely new drill ... and you'd still have an old drill! Low-cost cordless drills often have a bad track record when it comes to battery life; the better ones have huskier batteries and generally use more than just a transformer cube and a diode in their charging circuitry. But what can you do with the old drill?
Why not recycle it (a very 90's thing to do) for use at your workbench, powered from an inexpensive and easy to construct dedicated power supply? The motor in your old cordless drill is just a DC motor and it doesn't really care whether its power comes from a battery pack or from a simple AC-to-DC power supply.
Begin by removing (and recycling at a NiCd battery collection center) the drill's defunct cells, counting the number of NiCd cells used. These drills generally use between 6 volts DC and 12 volts DC for full power, so 10 NiCd cells would indicate that the motor is roughly rated at the higher 12 volt DC figure. NiCds will charge up to 1.4 volts DC per cell, but quickly drop to 1.2 volts per cell under load. Their effective charge-life (usable time before recharging) remains fairly constant up to 1.0 volts DC per cell, after which they drop toward zero pretty fast (time to stop demanding power from them). So just counting the number of cells and multiplying by 1.2 will give you a close enough voltage rating of the drill's motor (if it's not marked on the motor housing in any way).
The schematic diagram in Figure 2 shows how you can use an easily obtained Radio Shack (reg. trade name) #273-1511, 12 volt, 3 amp transformer (see moderator's note below), along with a #276-1181, 25 amp, 50 volt bridge rectifier unit to whip up an unfiltered cordless drill power supply for use with drills in the 12 volt to 9.6 volt range. You can house the parts, along with a safety fuse holder, into any metal housing that you might have on hand. Photo 1 shows how the one that I made up turned out, certainly close enough for ocassional bench work! Figure 3 shows the schematic diagram of the same Radio Shack parts, but configured for a drill motor in the 6 volt to 7.2 volt DC range. If you need a two-speed option for a 9.6 to 12 volt unit, you can simply switch the negative lead between the transformer's center tap and the bridge rectifier's negative terminal for a HI/LO speed option. The switching can be done either at the power supply, or in the drill itself if it happens to have a speed selector switch already built into it. A third wire back to the power supply would do the trick in the latter case. Be sure to use a cable from the drill, back to power supply, that will carry the motor's current safely, and that will stand up the flexing and kicking-around that it might receive on your workbench. That's all there is to it!
By the way, don't try to utilize any of the parts that were originally supplied with the drill for its charging circuit. They're no doubt rated only for supplying charge current to the battery pack (usually one-tenth of the pack's rated amp-hour capacity), and won't be capable of supply nearly enough current for the application I'm describing. Stick to the parts described or equalivents from a reliable parts source."
Moderator's note: To be on the safe side, measure the actual current drawn by the DC motor in your own cordless drill before building up the supply shown either Fig. 2 or Fig. 3. Some of these little cordless drill motors draw deceptively high currents under load, as much as 10 amps or more! If you intend to use the drill and bench supply with any of the more power-hungry of the cordless drill motors, the transformer specified in the drawings will probably not be sufficient to meet the necessary current demands of your motor. Higher current 12-volt transformers can be obtained from several of the suppliers who regularly advertise in 73 and would be a better investment.
Murphy's Corollary: Only once all 18 cabinet scews have been completely replaced, will you remember about the blown internal 5 Amp fuse!
Warmest wishes for the holiday season to all and many thanks to this month's contributors, including:
Roger Block, President
PolyPhaser Corporation
2225 Park Place
P.O. Box 9000
Minden, NV 89423-9000
Stephen Reynolds N0POU
510 S. 130th. Street
Omaha, NE 68154
Max Holland W4MEA
7333 Valley Lane
Hixson, TN 37343
If you're missing any past columns, you can probably find them at 73's Ham To Ham column
home page (with special thanks to Mark Bohnhoff WB9UOM), on the world wide web, at:
http://www.rrsta.com/hth
Note: The ideas and suggestions contributed to this column by its readers have not necessarily
been tested by the column's moderator nor by the staff of 73 Magazine, and thus no guarantee of
operational success is implied. Always use your own best judgment before modifying any
electronic item from the original equipment manufacturer's specifications. No responsibility is
implied by the moderator or 73 Magazine for any equipment damage or malfunction resulting
from information supplied in this column.
Please send any ideas that you would like to see included in this column to 73 Magazine's Ham
To Ham column, c/o Dave Miller NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. We will make
every attempt to respond to all legitimate ideas in a timely manner, but please send any specific
questions, on any particular tip, to the originator of the idea, not to this column's moderator nor
to 73 Magazine.