Ham To Ham #29 - February 1998
73's Ham To Ham column
c/o Dave Miller, NZ9E
7462 Lawler Avenue
Niles, IL 60714-3108
USA
e-mail: dmiller14@juno.com
Moderator's note: Roger and Ron Block of PolyPhaser Corporation have put together a well written series of tips and suggestions on how we can effectively protect our ham radio stations from the effects of a lightning strike. Part 1 of that series was begun last month, part 2 follows:
Lightning protection - what your mother never told you! - Part 2
In the first part of this series last month, we gave you some little-known lightning statistics, showed that while we can't control when or where lightning might strike, we can exercise some control over how much damage it will cause, stressed the importance of a single, low impedance ground
system, and briefly outlined how such a system can be implimented in your own ham radio installation. Much more detail will following in the coming months, so stay tuned!
Picking up where we left off last month, and changing gears slightly for a moment, remember that the energy in a lightning bolt covers a broad range of spectrum, from dc on up through the UHF frequencies. We hear lightning strikes in our HF receivers and we see it on our UHF television
sets ... it's RF. Whenever we're dealing with RF energy, simple resistance measurements are no longer adequate, system impedance must also be considered. System impedance certainly includes dc resistance, but it also takes into account inductive and capacitive reactances. The higher in frequency we go, the more the system impedance will be determined by these inductive and capacitive reactances. With this in mind, let's continue.
Three Techniques to Improve Conductivity
Every conductor has measurable inductance. Similarly, ground conductors (radials) exhibit normal inductance before they go below grade. Once in conductive ground, most of the inductance of bare conductors and ground rods is also shunted by the earth's conductivity.
If the soil at the grounding location is not very conductive, several techniques will improve the situation. First, increase the surface area of the conductor to decrease its normal inductance. Second, replace the soil with coke breeze, bentonite clay or other doped soil to increase its
conductivity and help shunt the inductance of the in-ground bare conductors. Third, install additional bare radial runs, with attached ground rods, to effectively parallel the inductance, thereby reducing the overall system inductance. Adding a drip system, to keep the soil moist, with plain water (or better yet, with a saline solution) can be effective. In some locations it may be necessary to utilize all of these techniques for the best results.
1. Conductor Surface Area
The most effective material for a ground system conductor is bare copper strap. Because lightning has a large portion of its energy in the UHF range, it will behave like an RF signal. Energy (the surge current) will primarily be conducted on the surface of a conductor (the RF "skin effect"). RF
currents within a round conductor will not make use of the round conductor's large cross-sectional area because of this "skin effect". The so-called "skin effect" forces RF signals mainly toward the surface of a conductor. With a 1-1/2 inch (or larger) flat strap (of at least 26 gauge or 0.0159
inches), both of the large, flat outside surfaces will be responsible for conducting the surge. So large, flat, bare copper strap is the better choice for your ground radial system.
2. Soil Doping
Water, in its purest form, is an insulator. lonic salts, mixed with water in the soil, create ionic soil. The earth is a conductor due to the number of ionic salts naturally present in the soil. So conductivity can be improved by adding more ions to the soil.
Soil doping is accomplished by either adding water or a saline solution to the soil around the grounding system. If the soil already has a sufficient amount of naturally occurring salts, adding water will free the ions in those salts, improving conductivity. The more ions (salts) available, the
less water needed to reach a given level of conductivity.
If few natural ions are present, additional salts such as Epsom (MgSO4) or rock salt (NaCI) can be added to the soil to increase its conductivity. Depending on the amount of rainfall, doping the ground system radials with four pounds of salt per 10 linear feet, and 10 pounds per ground rod,
can substantially improve the ground's natural conductivity. The length of time that this doping treatment will last, will depend upon the rainfall in your area and the percolation factor of your local soil. Note that adding too much salt can harm grasses and other plantings. The addition of salts
will also decrease the life of the metals used in the grounding system, due to natural corrosion factors, but these are simply facts of life and are mentioned only so that that don't come later on as a surprise.
3. Ground Radials
Radials are the most cost effective grounding technique when considering system impedance, material cost and labor for installation. If one radial gives "X" resistance, then two will deliver an equivalent "parallel rule" plus about 10 percent. The "parallel rule" is the same familiar formula
used for calculating resistors in parallel. The rule only applies, however, when the soil has the same conductivity over the entire radial area. Also note that after the first two radials, we must double the total number of radials each time to achieve the parallel-plus rule.
Radials have an effective length limit. If the surge energy has not been launched into the soil within the first 75 feet, the inductance of the radial will prevent any further effective propagation of the lightning stike. As a general rule of thumb, all radials should be at least 50 feet long, but no longer than 75 feet.
Ground rods should be placed along the entire length of each radial. The most cost effective spacing between rods for normal (grassy) soil is two times the length of a rod into the ground. For example, if 8-foot rods are used, they should be placed on 16-foot centers (8 X 2). If the soil is not
a good conductor (eg., very dry or sandy soil), additional ground rods should be used with closer spacing to reduce the subterraneanly natural inductive impedance.
Ground Measurement
Since most soils are stratified, the best way to determine the effectiveness of a ground system is to measure it ... at various depths. The simplest way to determine the sub-layer conductivity is to measure the first ground rod, one foot at a time, compared to a reference ground rod, using an earth resistance meter, as the new rod is hammered into place. This technique can provide a profile of the lower layers relative to the first foot.
Most earth resistance meters measure only dc or low frequency ac resistance of the ground system. Since the fast rise time of lightning strike energy is predominately RF energy, the inductance of the ground system is important. Without using expensive specialized test methods, the only way to ensure a low impedance ground system is to follow the suggestions previously
presented for conductors, doping and radials.
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 reasonably safe from the devistating effects of a lightning strike ... their series will continue in this column throughout the rest of 1998. 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/.
And now, a few updates to some past Ham To Ham columns ...
The mysterious capacitor ... update
Here's an e-mail note that I received from Wray Lemke KI4XS with regard to the piece in the August 1997 Ham To Ham column by Ken Guge K9KPM, and his experiences with and warning about the dangers of humid air being trapped inside of the connecting block of an antenna. Ken's conclusions were right on the money! "I read the piece from Ken, K9KPM and thought I'd offer our
experiences in the marine electronics field. Here's what we've found: Take an object with a volume of air inside, such as a ship's radar dome or some air dielectric coax. Now heat it up in the July sun, then rain on it and cool it off rapidly. The air inside the object will decrease in volume creating
a considerable vaccum which can suck in an amazing amount of water ... right past gaskets and other forms of weather sealant schemes. The solution (as Ken suggested) is to either make the open interior space truly airtight, or to leave a vent hole so that the air pressure can equalize during rapid temperature changes."
Moderator's note: Ken (K9KPM) originally suggested filling the void in a connector or connecting block with an RF innert material, to keep humid air out by simply not providing any room for it! Still a wonderfully simple solution.
More power to you ... update
Ariel Elam K4AAL sent in this follow-up note: "In the October 1997 Ham To Ham column, Stephen Reynolds N0POU offered a tip on using an abandoned electric range 240-volt breaker as a potential source for a dedicated power feed into the ham shack: '... when our electric stove finally gave up and we decided to replace it with a gas unit instead. I now had a 240 volt, 40 amp circuit on my current fuse panel that was available for other usage, namely my ham station! I ran #8 gauge wires to a new sub-panel in my shack, being careful to follow all of the electrical codes for my area, and I now have plenty of power conveniently available for just about anything imaginable,
of either a 240 volt or 120 volt nature' (quoted from Stephen's original tip).
Stephen's idea is a good one, but if the rated capacity of your main breaker panel isn't now at full capacity (ampere-wise), you may not have to wait until a major appliance is replaced to provide a separate feed to your ham shack. Lots of folks don't realize that most service panel manufacturers
now make 'tandem' or 'twin', half-size circuit breakers these days. The half-size breakers are two individual circuit breakers built into one standard-size breaker case. As a result, two tandem circuit breakers may be used to replace two full-size breakers, freeing up 2 spaces for a 2-pole breaker, that can then be used to feed a 240-volt ham shack sub-panel. Again, as Stephen suggested, make sure that your service panel is rated for the extra current that may be needed (particularly if you're adding a husky linear amp), and be familiar with, and always follow your own local electrical codes to the letter. Seek out the help of a licensed electrician if you don't know the electrical codes, don't have the experience needed to work with high-current AC or don't have the means to do-it-yourself."
Moderator's note: Thanks for the information Ariel, I wasn't aware of the half-size breakers myself. Even if you need to hire a licensed electrician to do the job, just knowing that the whole service box may not need replacing makes the task of a separate service feed for the shack seem a less daunting project.
Recycled tubing ... update
Thanks to Jon Seaver N8SUA who wrote in with this timely warning: "With reference to Bob Boehm's (NBEXF) contributions presented in the May 1997 Ham To Ham column, I'd like to share a few concerns. While there may be some practical ham radio uses for discarded medical tubing, etc., as a practicing paramedic and ER nurse, there are also some potential for danger in recycling
what is essentially 'medical waste'. This is, after all, the 'Age of Universal Precautions'.
'Universal Precautions', for those who are not in the sick-people-business, refers to the procedures taken by health care workers to protect themselves from acquiring blood borne disease causing agents. The highest profile among these is the agent causing AIDS, the Human Immunodeficiency Virus or HlV. We should always act as if any fluid originating from the human body is teeming with your least-favorite germ or virus. Never allow contact between your
skin and any potentially infectious material. If you don't think that you have any cuts or scrapes on your hands by which germs can enter, simply immerse them in alcohol for a few seconds. That stinging pain identifies all the little portals of entry which could be utilized by whatever germ
you might encounter.
So before.you undertake to recycle medical waste, consider that you might be exposing yourself and, worse yet your family, to a variety of undesirable organisms. Among the agents you may not want to invite into your home, are a species of Staph Aureus (which is resistant to every antibiotic currently available). Another is known to physicians and nurses as "VRE", for
Vancomycin Resistant Enterococcus which is also immune to nearly everything we have to throw at it. Is the utility of that piece of tubing worth the risk?
On the other hand, perhaps you may have access to supplies that are being discarded, simply because they're outdated and not considered fit for use on human patients. These should pose no undue risk, but make sure that you know what you're getting before you grab!
Examples of potentially useful items are UNUSED plastic bags containing IV solutions. Drained of their contents, you may be able to fashion a rain-guard for your handheld transceiver. I've cut off the end that contains the port for inserting the tubing, and then carefully made a small slit for the BNC connector of an HT to poke through. Another potentially useful item is the foil overwrap sometimes found on bags of pre-mixed IV solutions, or used to package "splint rolls" of plaster or fiberglass casting material. These might provide some protection against the dreaded Electro-Magnetic Pulse or could also find use as an "antenna attenuator" for near-field fox hunting.
But please, don't recycle anything if you have any doubts whatsoever about its exposure to disease causing organisms, it just isn't worth the risk."
Moderator's note: Jon brings up a very valid point with regard to attempting to re-use discarded medical supplies that come from unknown sources, and of unknown infectious state. In the May 1997 column, however, both Bob Boehm and I were referring to medical tubing that came from our own hospital experiences, and therefore of intimately familiar origin. I would hope that no one would ever attempt to scrounge such tubing from any other source, so it's worth noting the dangers Jon mentions.
Hex on them!
From time to time, original equipment manufacturers seem to put forth extra effort to thwart the home-service-person from gaining entry into their own equipment ... for whatever their reasoning. Odd-ball assembly screws are one of their favorite means! I recently ran into one such effort used by a manufacturer of computer power supplies. They used assembly screws that appeared to be of the torx variety (hex-sided), but yet an ordinary torx driver wouldn't fit them. Upon careful examination (dragging out my trusty magnifying glass!), the reason became clear ... the six-sided recess of the screws also contained a small male center pin! See Figure 1. Apparently the torx driver that the manufacturer uses to assemble the unit has a hollow shank, whereas most conusumer torx drivers have solid shanks ... so of course they won't fit.
Not about be outsmarted, I found that I was able to insert a small, straight-bladed screwdriver into the torx recess, immediately next to the erriant center pin (see Figure 2), and snap off the pin with a quick, calculated blow from a small hammer. A bit more primative than I prefer to be when trying to open up a piece of equipment, but it worked. I was then able to remove the screws with my standard torx bits. Just an idea to keep in mind if you happen to run into the same situation. By the way, there was also no way to "grab" the sides of the truss-head shaped screw with a locking
grip-pliers, another technique that sometimes works for odd-ball screws. A "hex" on manufacturers who get cute with their hardware!
DE NZ9E
Murphy's Corollary: A lost piece is always found in the last place you'd think of looking ... so next time, it might pay to look there first!
Many thanks, as always, to this month's contributors, including:
Roger Block, President
PolyPhaser Corporation
2225 Park Place
P.O. Box 9000
Minden, NV 89423-9000
Wray Lemke KI4XS
Box 688
Sullivans Island, SC 29482-0688
Ariel M. Elam K4AAL
1065 Barnes Road
Antioch, TN 37013
Jon Seaver N8SUA
RN BSN EMT-Paramedic
P. O, Box 278
Rapid City, MI 49676
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.
END
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