META name="description" content="Lightning management, grounding methods and procedures for communications sites and ham radio facilities.">
One of the disciplines in which I specialized as a telecommunications engineer before retiring was the field of "electromagnetic compatibility" - which is a fancy way of saying "keeping electricity and radio energy from doing things you don't want." After having acquired some knowledge in this field, I have been startled, even shocked, at some of the misinformation out there in the ham radio community (and even among professional communications site engineers) about how to deal with the issues of lightning management, RFI and grounding issues and solving noise problems, and how ham radio operators routinely expect to lose equipment and expensive cables to lighting strikes from time to time, and think there is nothing practical that can be done to prevent it, or live for years with noise problems that are easily curable. And I have been shocked at some of the sloppy, even dangerous practices I have encountered at the hundreds of commercial communications sites I have visited over the years. In an endeavor to assist hams and communications site engineers in preventing some of these bad practices, I am offering this essay in best practices and practical ideas about how to do an effective job of proper grounding and lightning management as inexpensively as possible.
You will notice that I did not say "lightning suppression." There is a reason for that. You can't prevent lightning strikes. It is really as simple as that - forget all the "static hats" and "lightning deflectors" and such nostrums. The reality is that none of them have ever been shown to be genuinely effective in properly controlled studies. If lightning wants to strike your prize long-boom 5 element 40 meter yagi, it will happen and there is not much you can do to prevent it. It's the nature of lightning. But what you can and should do is make every effort to prevent a lightning strike from generating a surge that will destroy your coaxial cables and rotator, and enter your shack and destroy or damage much of what is in it. And for the vast majority of lightning strikes, it is possible and quite practical to do just that. Do you realize, for example, that most mountaintop microwave communications sites get hit by lightning quite regularly and remain quite undamaged? At certain peak times of the year, particularly in August and September, many mountaintop communications sites in the western U.S. may get hit once or twice a week, even every afternoon in some of the more extreme cases I have seen. Yet not only do these sites remain on the air, their customers expect and even demand that they remain on-air, and operate right through the thunderstorm, never being shut down for weather. I don't know of any communications companies that send technicians racing to the mountaintop to shut things down when they see dark clouds developing over the mountaintop. So how do they do it? Installing an effective lightning management/grounding system isn't as difficult or as expensive as you might think. In most cases, your investment will be limited to a few ground rods, some heavy gauge copper wire, a bunch of green-insulated electrical wire, and the installation of a bulkhead panel. And in many cases, your investment in a proper grounding system will pay performance dividends as well in the reduction of RFI problems in your shack. You may even end up with no more RFI or ground-loop problems at all!
The Basic Premise Of Lightning Management And Grounding - Bringing Everything To The Same Potential
I did not say bringing everything to ground potential. The reason I didn't say that is simply because it is not possible. Run the heaviest monster copper cable you want for your grounding system, and you still have resistance to actual earth ground potential. That is because 1) there is no such thing (at room temperature at least) as a perfect conductor, and far more importantly, 2) every linear conductor (such as a piece of wire, even a superconducting wire) has an inductance associated with it. Because it has an inductance, it also presents certain amount of resistance to the flow of any form of alternating current. Since the most damaging (and difficult to manage) component of lightning is RF, and not DC as most people suppose, even a heavy copper pipe can offer substantial reactive impedance to the flow of lightning energy through it. So as a result, no matter how hard you try, you can't get everything to ground potential.
So, if you can't get everything (or even anything) to ground potential, how do you prevent damage from occurring during a lightning strike? Well, there is a way, and that is simply to ensure that during a lightning strike, when everything is raised above ground potential, everything in the shack rises to the same potential, and at the same time. This is your goal. To ensure that when a strike occurs, that there are no significant differences in potential that can cause components in your shack to "talk to each other." If you take nothing else away from this essay, this is the principle that you should remember.
Also, you must keep in mind that everything is above ground potential, and the higher it is above ground potential, the more likely it is that an arc can occur between that component and another component (maybe even a building structural element, such as a piece of re-bar in the floor). Because of this, an effort must be made to keep the common-voltage level being sought, to be as close to ground potential as possible.
The way you keep everything at the same ground potential in a lightning strike is to have one grounding point and one grounding point only. This common grounding point needs to be kept as close to ground potential as possible, and then every coaxial cable, every waveguide, every electrical outlet, every equipment rack, every electrical panel needs to be grounded to it. And nowhere else!
In most communications sites, the way this is done is to create what is called an "entry bulkhead" - a large metal plate fastened to the outside wall of the equipment shelter, through which all waveguides and coaxial cables, firmly grounded to it, enter the building. This bulkhead should be placed as low to the ground as possible, and a heavy copper strap is then run to the common station grounding point (usually on a grounding halo) at ground level immediately below the entry bulkhead. This common station grounding point is where all grounds come together - a very heavy gauge, buried copper wire going straight to the tower, and the ground bus from the electrical panel, and technical grounds from the station equipment all find their grounding point here and only here. There are two things this will do for you - first, ensure that there are no ground loops, since all grounds are brought to the same electrical-system potential (no more hum and noise on your PSK signal!), and second, it will ensure that in a lightning strike, every ground is raised above ground potential by the same amount, and at the same time, if the ground wires and coaxial cables going to the technical equipment are the same length. We will presently see why this is important. You will need to run short coax jumper cables from your equipment to the bulkhead panel, and I would recommend that you spend a little extra money and make those cables out of Times Cable LMR200 or LMR400, or their Belden equivalents, RF200 and RF400 (Belden also makes RF245, an equivalent to RG8X). The Times cables are available from Ham Radio Outlet, and can be used with standard connectors. These cables have a foil shield, which means that the shielding effects (and their exclusion of noise and containment of RF) is absolute and complete - they are 100% shielded rated. It doesn't get any better than that.
Why can't I just cheap-out and daisy-chain my grounds together and run one ground wire (or braid) back to the grounding point? There are a number of reasons for this. First, when you daisy-chain grounds together, there is a difference in resistance and reactance between each piece of equipment and ground. This is of particular concern when one piece of equipment on the end of the daisy chain has more power-supply leakage to ground than the rest - you'll end up with a ground loop that will drive you crazy if you have a sensitive piece of equipment like a hard-wired PSK interface. Second, this practice increases the chances that one piece of equipment will end up at an odd multiple of a quarter-wave from RF ground - and if it does, it will be at a voltage point, which will almost guarantee that you will have RFI problems in the shack on that frequency. Third, during a lightning strike, there is a difference in propagation time from the piece of equipment nearest the ground point as compared to the one at the end of the daisy chain - and because electrical propagation isn't instantaneous, the difference in propagation time can result in an arc between the pieces of equipment, even though they are bonded together! From the standpoint of RFI problems in the shack, you should consider that the daisy-chain cable is itself an antenna, with a capacity hat on the end (the equipment itself) - and that can cause RF ground loops to appear between different rigs in the shack. The common practice of running a piece of ground braid along the back of the operating desk and grounding different rigs to it in different places, often causes more problems than it solves. The best practice is to run grounding braids, insulated from each other, or simple green-insulated stranded building wire, #12 or heavier, from each piece of equipment back to the common grounding point, and keep the ground wires all the same length, avoiding coiling any excess (you don't want to inadvertently create an inductor). Best to simply lap them back and forth, if necessary, to take up excess length. The length chosen for your grounding cables should be 1) as short as possible, and 2) carefully planned to avoid, as much as possible any quarter-wave or odd-multiple of quarter wave resonances at your HF operating frequencies. The ARRL Antenna Book shows lengths to be avoided for avoiding tower guy wire resonances, and the same chart can be used for this purpose. Why avoid multiples of a quarter wave? Because a quarter-wave section of wire, grounded on one end (at your grounding common point) becomes essentially an open circuit (at radio frequency) on the other end - at that frequency, it is no ground at all - and is even an antenna! And you want to avoid that. You want your grounds to help you with your RFI problems, not just help with lightning protection, don't you?
A quick word about cable dress. There is a good reason to dress your cables neatly, and it is more than just to make it look nice. If you do it right, it can reduce your RFI problems, and sometimes even eliminate them. And the right way to lace cables it is to start at the ends away from the equipment, and dress the RF cables in one harness, the electrical cables in another, and the ground wires or braids and control cables in a third. Each harness should be dressed as far from the other two as possible - plan your harness layout carefully before you begin, especially keeping electrical and RF cables as far from each other as possible as your highest priority. As you work your way towards the equipment, you may have (hopefully will have) some excess that you can dress behind the equipment which you can use for a "service loop" - giving yourself the ability to pull a radio out of the stack and work on it while it is still connected up to power, signal, antenna and ground. Computer cables should be dressed in the ground wire harness, and kept as far from RF cables as possible. If you are getting RF noise from your computer, obtain from your ham radio supplier a whole bunch of those split ferrite cores sold for TVI resolution, and install several of them on each of the cables separately that come from your computer (such as the ethernet cable, the keyboard and printer cables, etc. Pay particular attention to the power cable going to your computer - this is a common source of noise egress, since most computer power supplies are switch-mode supplies that generate copious harmonics and noise on the lower HF bands. You may have to put a half dozen or more cores on this stiff, heavy cable, as close to the computer cabinet as possible, in order to get the noise down to tolerable levels.
Preventing Ground Loops
A common problem seen on the bands is the station that is operating a soundcard mode (such as PSK or SSTV) and there are hum bars coming from that station all across the waterfall as a result of a ground loop between the rig and the computer to which it is connected. The easiest way to deal with this issue is to install an isolating interface (such as the Signalink or Rigblaster or the many others available out there. This works, but it really just masks the problem - which is the fact that the radio's housing is at a different ground potential than the computer's housing. Since there it is simply not possible to get everything to absolute ground potential, an isolating interface is the usual way out. But it is not a perfect solution, as stray coupling (between the primary and secondary windings of an audio transformer, for example) can still cause noise that will appear on air, and will interfere with decoding on receive.
While I recommend an isolating interface just to be sure, it is better still to prevent ground loop problems from occurring in the first place. The way to minimize ground loops is to ensure that all power supplies (except a linear amplifier) are connected to the same "hot" wire in your house's electrical system. This means that you won't have leakage to ground between the power supplies in adjacent pieces of equipment that are at different potentials. Any leakage that occurs will be of the same phase, and that ensures that the bulkhead panel and everything else in the shack will be raised above ground to the same potential - a ground loop can occur only when equipment is at different potentials. You can assure that both are brought to the same potential by running a number 12 ground wire (insulated with green insulation to help you keep your wiring straight) from the computer, and a separate such wire from the radio itself, back to the bulkhead ground buss.
Since this means that all your equipment will be on the same branch circuit from your breaker panel, you will need to ensure that the breaker and the wire going to your shack are all of sufficient capacity to avoid circuit breaker trips and not heat up from the load. Add all the loads to be connected to the outlet and ensure that added together, they do not equal or exceed the rating of the circuit breaker to which they are connected. If they exceed the total, call an electrician and have him install a heavier circuit for you. He can install a 240v. circuit at the same time for your linear, if you need it.
What To Do At The Common Grounding Point
All of your coaxial cables should enter the house through the bulkhead panel. No exceptions, ifs, ands or buts about it. That includes all your VHF and UHF coax lines. If your house is rented or for any other reason you don't wish to knock a huge hole in the wall to install the grounding bulkhead in the wall itself, you can install it outside, away from the house and then bring everything in through a conduit or a specially fitted window. You just need to ensure that you have access to both sides of the panel itself.
Each coaxial cable, rotator cable shield, or electrical cable that goes out to the antenna farm sould go through this bulkead and should be grounded to it. All manufacturers of waveguides and rigid coax ("heliax") make grounding kits that are designed for this purpose, but you can make your own - just strip a short length of the sheath from the coax, and solder a foot or so of ground braid to the shield strands, and then secure the other end of the ground braid to the bulkhead panel. The coax then needs to be resealed against the weather - a layer of electrical tape by itself is not at all sufficient, but a layer of electrical tape, covered by thin layer of roofing mastic and then another layer of electrical tape will do the job nicely. The coax should pass through a hole in the panel and the ground braid then is grounded to the panel where it passes through. An easier way and far more aesthetic is to secure the grounding you need here is to simply terminate the coaxial cable in the panel using a bulkhead SO239 barrel. Or, better by far, is to use a lightning protector (gas discharge cell) as the bulkhead barrel - several manufacturers, such as Alpha-Delta and Polyphaser among them, make gas discharge cells designed as bulkhead barrels specifically for this purpose. If you are going to use lighting protectors (and I strongly recommend that you do), this bulkhead panel is the only location in your shack where they should be installed. If the bulkhead is outdoors, the connectors should be protected from the weather to keep water out of the gas discharge cells and your coax lines.
The bulkhead panel, whether mounted on the inside or outside wall or on a pedestal outside, needs to be grounded at this point. A piece of copper strap, several inches wide, should be soldered or brazed to the bulkhead, and then the other end secured, preferably by brazing or soldering, to the ground "halo" that we will discuss presently. Or, alternatively, if not mounted on the wall, the bulkhead panel can be secured directly to the ground halo and supported with a piece of pipe or angle-iron driven into the ground. This will ensure the best possible connection to ground, and the lowest common-point ground potential in a lightning strike - and that is precisely what you are trying to achieve.
On the back side of this panel, and as well-grounded to it as possible, is where your station's ground bus should be located. It is to this ground bus that all the ground wires, as described in the previous section, should be brought to for their ground connection. This is also where your house electrical panel's main ground should be connected. It is important to do this - during a lightning strike, two or more different grounding points will always be at different potentials, forming a potentially damaging DC ground loop - and this is exactly what you are trying to avoid. This situation can be avoided by simply moving your electrical service ground to this point, even if that means running a long wire from the existing panel to get there. If necessary, call an electrician to do this for you. But it is important that this is where your house's electrical system ground be connected to earth ground. And I assure you, if you do your "halo" ground properly, your house will end up with a far better electrical safety ground system than it has now.
Just What Is This Ground Halo You Keep Talking About?
A ground halo is a very heavy gauge piece of bare copper wire (typically a number six or heavier) that goes all the way around the perimeter of the house, buried about eight or ten inches from the foundation wall, and equally as deep. It runs the entire perimeter, and the ends are connected together to form a big loop. At each corner, a ground rod is driven, which is connected to the halo. Also, a radial is run out directly away from each outside corner, at least 15 feet if possible, and a ground rod is driven at the end of the radial and in the middle, and they are connected to it - three ground rods at each outside corner and one at each inside corner. The reason for the radials and ground rods is to minimize the inductive effects of taking the halo wire around a corner. As the halo wire goes around the corner, the bend radius should be as large as possible - this also minimizes the inductance. The house electrical safety ground, the tower(s), the coax shields and all station technical grounds are connected to this halo only where the bulkhead panel and only where the bulkhead panel also connects to it. A licensed electrician connecting the electrical system ground to the halo may insist on installing a ground rod where the bulkhead panel connects to the halo. That is not a bad idea and should not be opposed.
The purpose of the ground halo is to ensure that in the event of a lightning strike, either to the tower or to the ground nearby, all the ground around the house is brought to the same electrical potential. This ensures that no arcing will occur through the walls, to ground or to the floor, or between the equipment and the above. The numerous ground rods also provide an excellent safety ground for the electrical power distribution system in the house, and will minimize fluctuations of the voltage on 110v. circuits.
You may have heard of "Ufer" grounds and wondered what they are, and whether you can benefit by using one. If you are building a new home or a new communications site, and are planning to do a slab floor, the answer is yes, you can benefit from the installation of a Ufer ground. Named after its inventor, a Ufer ground takes advantage of the fact that concrete absorbs moisture quickly and gives it up very slowly, making it a surprisingly good conductor. As a result, the slab floor can be used as an effective ground, if you can get connected adequately to the concrete.
The way this is done is normally during construction - have your builder install a number six bare copper wire running from where your bulkhead panel is going to be, to the center of the slab mesh, wiring it to the mesh and/or re-bar here and there as if it were just another piece of re-bar. It is not necessary to bond it firmly to the mesh or re-bar, as just getting it close in the electrically leaky concrete is sufficient to provide the connection needed. But the wire must be bare, clean copper to ensure that enough of a connection is picked up to ensure it is at the same potential as the mesh or re-bar. If you own an existing home and are not afraid to chizel into the floor to find the mesh or rebar (as close as you can get to the bulkhead panel), you can braze a ground wire onto any rebar or mesh wire you find and obtain an adequate Ufer ground connection that way. The wire in either case is then connected to the bulkhead panel along with all the other grounds.
The benefit of a Ufer ground is that it ensures that the floor itself is at the same potential as the equipment during a lightning strike. This means that the chances of arcing between equipment and the floor is eliminated. From an RFI perspective, it also acts as a shield in a far more effective manner than if the mesh remains unconnected to the common grounding point.
Now we come to the problem of tower grounds. This is where some of the worst grounding abuses occur, mostly because there is such ignorance of the subject of what actually takes place during a lightning strike.
Most people assume that when lighning strikes a tower, the current flows through the tower, into the base and is dissipated into the ground. Ah, if it were only that simple!
The reality is that much, even most of the energy in a lightning strike is in the form of radio frequency alternating current. Because of the waveform of the lightning impulse, the spectral peak in most strikes happens to occur at about 150 kilohertz and the spectral distribution slowly tails off to about 150 megahertz. The result is that a significant portion of the energy in a lightning strike will travel down the outside of the conductor through the well-known "skin effect" (this is why people who are struck by lightning are far more likely to survive if they are wet when they are hit, than if they are dry - the bulk of the energy passes through the moisture on their skin rather than through their body).
Because of the skin effect, lightning hitting a tower cannot be fully dissipated through the tower foundation's connection with the earth. The earth itself surrounding the tower foundation can only carry so much of the energy before all the charge carriers are mobilized. This leads to a phenomenon known as "charge saturation." So if the tower grounding system is inadequate, the surplus energy now looks for an alternative path, and will probably find it in the RF and rotator cables. And I need go no further to describe what will happen then - we've all heard (or maybe even have experienced) the horror stories. Charge saturation may never occur during a modest strike of, say, a few thousand amperes. And if it doesn't, the tower foundation may be able to adequately dissipate the current and no significant damage, if any, occurs. But if a whopper hits, and charge saturation does occur, the owner will likely find all his prize radios in smouldering puddles of copper and silicon on the blackened walls and floor - and he will be getting out his checkbook. I have encountered many a site owner who thinks that because he has been hit without damage quite a few times in the past, that he doesn't need to worry - his grounding must surely be adequate for his needs. Little do such people understand how the either-or nature of charge saturation effects can dramatically alter the effects of a lightning strike, with just a few percent difference in the amperage of the strike current itself or in the moisture content of the soil at the time a strike occurs.
To prevent charge saturation from occuring in a whopper of a lightning strike, it is necessary to create a broad surface area through which the energy can be dissipated, carrying the charge away in copper until it is spread out enough that the current density will not lead to charge-saturation of the ground. There are two ways to do this, best used together. One is to pour the tower foundation in a galvanized metal culvert, of the kind used on rural roads for drainage. When each leg of the tower is grounded to the culvert, it ensures the greatest soil contact possible (and the lowest ground resistance), which will reduce the DC component's contribution (but only the DC component's contribution) to the charge saturation problem. The second, and most important preventative measure, is to run two or three radials out from each tower leg (number six wire or heavier), radiating out as far from each other as possible, and install ground rods along their length. The distance between ground rods should be twice their length (eight foot rods would be sixteen feet apart). Ground rods on adjacent radials should be staggered, keeping them as far apart as possible. Ideally, the radials should be 30 feet or so long. If they are 15 feet or so, the protection won't be maximized, but it should be adequate for the vast majority of big strikes.
Each tower on the site should have its own ground system as described above, but also be grounded to the grounding common point (the ground halo where it is secured to the bulkhead panel) through a very heavy gauge bare copper conductor. This conductor must be a minimum of a number-two copper wire, since during a lightning strike, it is apt to be carrying a great deal of current. This ensures that the current carried by the feedlines is minimized, and helps equalize the electrical potential to earth from the tower and the shack ground common point. Another possibility is to use 3/8 or 1/2 inch copper tubing, but if tubing is used, great care must be taken to ensure that no moisture can ever possibly get inside - otherwise, in a lightning strike, the tubing will instantly get very hot, and any moisture inside will flash to steam and cause an explosion.
Grounding Antennas And Feedlines
This is a very simple, easy and cheap thing to do, but it can pay enormous dividends. On every communications site you visit (assuming the installers know what they are doing), you will find that feedlines are grounded to the tower at two places - as close to the antenna as possible, and where the coaxial cable leaves the tower to head for the shelter. And you almost never see this done in ham radio installations. Why?
It's because the communications site engineer knows something that the ham doesn't: during a lightning strike, current travelling down the tower towards ground sets up a very powerful, but short-lived magnetic field. This rising and falling magnetic field induces a voltage equal in magnitude but opposite in polarity in the cables as compared to the voltage-drop across the height of the tower during the main lightning strike. And if the voltage drop along the length of the tower is equal to, say, twenty thousand volts, that means that the voltage along the length of an ungrounded coaxial cable is equal and opposite - twenty thousand volts, but in the opposite polarity. Now, let's see when happens when the shield of the coax is grounded electrically to the tower, but only at the antenna. You have now created a 1:1 transformer, with one end of the primary and the secondary connected together. But at the other end of the "windings" - the base of the tower and the coax where it leaves the tower to head for the shack - now you have an equal voltage in the coax, but opposite in polarity - so the voltage drop along the length of the tower is added to the voltage induced in the coax - or in the present example, forty thousand volts between the bottom of the tower and the coax shield and center-conductor. Do you seriously think that the coaxial cable - any coaxial cable - will survive that kind of voltage? That is why so many hams end up replacing their coax cables after lightning strikes, and communications site owners (at least those with properly installed feedlines) almost never do. In fact, I know of sites where expensive rigid coax and waveguides which have been in use for thirty years have been removed from service and installed elsewhere, good as new, in spite of the fact the original mountaintop site was hit by lightning on a weekly basis. The damage is prevented by the simple expedient of grounding the coax to the tower at both ends of the tower run - simply shorting out the inductive effects - and by keeping it away from the lightning by running it down the inside of the tower.
It is important to run the coax down the inside of the tower. There are two reasons for this. First, because of the skin effect - The most damaging energy in the lightning strike is RF, and so that RF will look to travel down the outermost conductor on the tower it can find. You want the tower to carry the current, not your cable. So running the cable inside the tower, as close to the centerline of the tower as possible, will ensure the minimum amount of lightning's RF ends up in your coax. The second reason for doing this is that it will also minimize the chance that lightning will strike the coaxial cable directly, rather than a tower member, which is much more likely to survive the abuse. An easy and cheap way to make this installation neat and tidy is to purchase some one-inch by half-inch perforated, galvanized unistrut from an electrical contractor. Cut it in lengths that are a bit longer than the width of your tower cross-section, and then use muffler clamps to bolt these lengths to the inside of the tower legs, with the perforated face away from the tower legs. You can then run your feedlines and rotator cables down the face of the unistrut, holding them in place with UV-resistant cable ties or short pieces of wire run through the holes in the face of the unistrut. Neat, tidy, professional-looking and easy to do, and the whole project can be done in a day for less than $50.
The Dreaded Drape. One practice that I have seen that sends chills down my spine every time I see it, is draping a coaxial cable off the side of the tower diagonally towards the shack to save on coax. This is a real no-no. There are two reasons why. First, consider again that the tower has an inductive reactance to the flow of lightning current through it. This means that it is essentially like one big, long resistor. So when you drape a feedline away from the tower, you are, in essence, putting a tap on the resistor, like the wiper on a volume control. So in a lightning strike, you are tapping the voltage drop on the tower, and directing it into your shack. You're also creating a larger target for lightning, looking for a path to ground, to find and strike. Do you really want to do that?
The second reason to never drape cables away from a tower is because of the problem of grounding. Remember the example of the 1:1 transformer? Even for a draped cable, that 1:1 transformer, with one end of the primary and secondary tied together at the antenna, still exists, because that magnetic field extends out for hundreds, even thousands of feet from the tower. But because the cable is away from the tower, it can't be grounded to the bottom of the tower as it needs to be - so there is no way to cancel the inductive effects. And because the cable is well away from the tower, none of the induced energy can arc to the tower, with an ungrounded cable acting as a spark gap to protect the radio. So if your tower is hit by lightning with the radio connected, all the induced voltage will head for the radio - it has nowhere else to go. Is this what you want? An extra thirty feet of cable and a bit of effort to properly ground it to the bottom of the tower is mighty cheap insurance to protect a multi-thousand-dollar radio inventory.
These are the reasons why you will never see draped cables on a cell-tower site. Even though the cost of the enormous heliax cables that cellular networks use is huge - upwards of $50 per foot, and a typical site may have a half-dozen or more runs going up the tower (you crunch the numbers) - and the RF losses are huge - several dB per hundred feet at cell-phone freqencies, they never drape cables from a tower over to an equipment shelter. Given those numbers, the reason why is not just for neatness and appearance, I assure you. It is because protecting their huge investment is a high priority for them.
A third reason that applies to sites, like ham radio sites, where HF is in use, is the fact that a draped cable acts unavoidably as an antenna element, one end of which is connected to your radio housing. This will increase the likelihood that you will have RFI problems in your shack, or distort the pattern of an antenna, or both. Running your coax down the inside of your tower all the way close to the base will enable the tower itself to act something like a zero-cost distributed choke balun. And that is a good thing. A very good thing - itself worth the price of admission.
At both ham and commercial radio sites, it is a common practice to see a coil of several turns of coax feedline coiled up near the bulkhead panel. The reasoning behind this is that doing so forms a choke balun that prevents the RF component of lightning from travelling along the feedline and into the shelter. There is merit to this argument, but the problem with this practice is that it is almost impossible to prevent the coil itself from inductively coupling to the magnetic field created by the lightning current travelling down the tower - with the result that a voltage is induced in the coil by the inductive coupling anyway. And that is a bad thing. A very bad thing - just what you are trying to prevent.
So, what is the best way to get the coax from the tower into the shack? Ideally, the best way to run a coaxial cable to a tower-mounted antenna, is to run it through underground metallic conduit or a walker-duct from the bulkhead panel at the shelter or shack all the way to the tower, then come up out of the conduit at the tower foundation, immediately ground the shields to the tower base, and then run it up the inside of the tower to the antenna, where they are grounded to the tower again. In a lightning strike, nearly all the current is carried to ground by the tower and is dissipated in the ground system (assuming it is properly designed), and very little ends up being carried to the radio - only differential voltages, which are relatively small, and they are safely taken to ground by the gas discharge cells installed in the bulkhead panel (you did spring for those, didn't you?). The radios are saved and the single-minded DX enthusiast working that huge pileup on the DXpedition to Armpit Island isn't crispie-crittered right there in his chair. In an ideal world, that is how we would all do our cable and feeder runs out to the tower. In an ideal world.
Dealing With Balanced Feeders
Balanced feeders can be run inside tower, underground through conduit, flat against a metal surface, anything you would like to do that you are already doing with coax - if you know how to do it. The trick is to use parallel runs of coaxial cable of exactly the same length - simply tie the shields together on each end and ground them, and then connect the balanced feeder to the center wires. While in coax, a balanced feeder can be treated mechanically just the same as coaxial cable - the cables in the pair can be separated with impunity.
The problem with doing this is impedance and loss. If your feeders are tuned, this can get you in trouble, as using the technique described above limits you to a balanced feeder that is twice the impedance of the coax used - if you are using 75-ohm coax, the balanced feeder that results will be 150 ohms, not the 600 ohms of the ladder line connecting to it. Losses are correspondingly greater, too. So this is really kind of a desperation measure.
You can bring in balanced feeders into your shack through the bulkhead panel too. You will just have to use two gas discharge cells or two bulkhead adaptors instead of one - a cell for each feeder in the pair, and run each conductor through the center conductor of the gas-discharge cell. Just understand that there will be a small impedance bump as you do so, since the gas cells are designed for 50 ohms. But the impedance bump is so short it should not cause much of a problem.
A Few Words About The "Cone Of Protection" Myth
One often hears that a house or shelter located next to a tower is protected from lightning strikes by the presence of the tower, and a "cone of protection" that it provides. Well, as in many myths, there is a grain of truth in it, but it is only a grain of truth, and not much more. Because of the nature of how lightning strikes occur (the physics of why the following is true are somewhat complex and beyond the scope of this essay), a modest degree of protection does, in fact, exist. But it is neither complete protection, nor is the protected area accurately described as a cone. The reality works like this:
Imagine a tower, say, two hundred feet tall on flat ground. And imagine a giant round ball, 300 feet in diameter placed next to it. Now, imagine rolling the ball about the tower. The protected zone provided by the presence of the tower is that area that would be below the curved surface described by the surface of the ball as it rolls about the tower and against it. A sort-of cone, and a rather thin, emaciated one at that. And the protection provided is modest. It is only about 95 percent at the tower base, and the percent of protection declines rapidly as one approaches the imaginary curved surface, falling to zero at the surface itself. And note that the side of the tower above the 150 foot level is just as likely to be hit as is its top, and increasing the height of the tower above 150 feet does not increase the protection. So your house or shelter is simply not being adequately protected by the presence of the tower unless the house is quite small and immediately adjacent to the tower base - and even then, it is not completely protected. If the tower ground is inadequate, arcing can occur between the tower base and even the house itself - the tower might be more of a problem than a solution. One more reason to get to work on a proper grounding system.
Coping With An Upper Story Shack
What has been written above is all premised on the assumption that the equipment will be located along an outside wall on the ground floor, facing the tower. But not every ham is so fortunate. What do you do if your shack is on a second floor, or in a room inside the house which does not have an outside wall where a bulkhead panel can be located?
There are several problems with an upper-story or inside-room shack that don't exist for a shack in an outside room on a ground floor. The most serious problem is the physical distance from RF ground. A second is the physical distance from a good electrical ground. But a third, often not considered, is that an upper-story shack, being physically above ground, means that the wires between the grounding common point and the shack will act as a single-turn secondary in the transformer described above - the magnetic field caused by a lightning strike will induce a voltage in the ground wires, and this voltage can be substantial and damaging. Additionally, the chances that the shack are hit directly by a lightning strike are substantially increased, especially the chance that the grounding common point itself (which must be located as close to the station equipment as possible) will be hit directly.
The way to cope with this phenomenon, and the problem it creates, is to locate the station's (and the house's) common grounding point near the shack, and then run a large (number 2 or larger) wire down to the halo ground. Better yet, it is useful to take the coax lines to ground level, and if this is done in a large-diameter metallic conduit, the conduit can be grounded to the ground halo at this point. Even this, as good as it is, is a less than ideal situation, as that large wire or conduit is the secondary in our 1:1 transformer example described above. But the overall objective in our ground design is to ensure that as everything is raised above ground potential in a lightning strike, that it all rises together to the same voltage above ground. This is why it is important in this situation to locate the grounding common point close to where the station actually is, to ensure that all the station equipment is raised as close as possible to the same potential above ground. An interior room on a ground floor does not have this problem, because the cable running to the ground common is at right angles to the tower, so the induction by the magnetic pulse is minimized.
There is also the problem of RF ground, which is an entirely different situation from the electrical ground. Since the upper story is a significant fraction of a wavelength above ground, any conductor going to ground is going to act as much or more like an antenna than it will a ground wire. So how do you cope with this issue?
One way to suitably deal with it is to install what is often called an "artificial ground." This takes advantage of the fact that the ground plane of a 1/4-wave ground plane antenna is effectively a sort of ground, at least at the design frequency - the radials make the antenna feedpoint appear to be close to ground potential, at the operating frequency. Each 1/4 wave radial ideally presents a "ground" impedance of 37 ohms - by itself, a pretty lousy ground, but each time the number of radials is doubled, that "ground" impedance is halved - two radials give you an 18 ohm "ground." Four will give you 9 ohms - a suitable "ground," even if it is only at one frequency. So the key to installing an "artifical ground" is to simply install some 1/4-wave ground radials out from your common grounding point - a minimum of two for each band of interest, and keeping them as far apart as possible. In a small upper story apartment, this could be difficult, but it is usually possible to get at least one radial out from the common point for at least 20 meters, maybe 40 in a big apartment, and this can help a great deal if you are having RFI problems. Bear in mind that just like in a ground plane antenna, the radials can be inductively loaded to resonance, or a capacity hat can be used, etc., to get them back to resonance on the frequency of interest, and improve their effectiveness. All of this is far from ideal, but it can help you cope with an RFI problem and possibly even cure it. In my own case, I have even installed artificial grounds by carefully buring tiny radial wires in the pile of a carpet. Keep in mind, however, that your artificial ground will work only so well as you can approach the ideal of the ground plane of a 1/4 wave ground plane antenna - three or four radials running in opposite directions from the common ground point. The less it resembles that ideal, the less effective it will be. Apartment dwellers and renters will almost certainly be forced to deal with this solution at some point.
If you are the owner of the building and have some money to spend on construction, there is another approach that can be helpful in improving lightning protection and can be quite effective in preventing RFI problems later on when you move in. That is to create something approaching a "faraday shield" by hiding wire mesh in the walls behind the dry wall, under the carpet and/or under the stucco in cinder-block construction. The idea behind a faraday shield is that because of the skin effect, RF travels on the outside, rather than the inside of a hollow space covered by an electrically conducting material. Since the RF is confined to the outside, it is not inside causing you problems. A very cheap, thin, tenuous mesh may be used - it need not be the copper-plated window screen often seen in commercial screen rooms. The key to ensuring that it works is to ensure that all seams and overlaps are electrically bonded - if they are not, the entire effect is reduced or lost. Not much can be done about windows and doors, but if the mesh is present in the walls, ceiling and floor, the shielding effect will be substantial and worthwhile. The grounding common point for the shack - the bulkhead panel - should be located inside the mesh and near the operating position in the shack, and the mesh should be bonded to it. If at all possible, a large diameter metallic conduit should then carry the RF and signal cables to ground level, where the conduit itself should be grounded to the ground halo. The conduit can then continue on underground to the antenna farm. This offers a good degree of lightning protection (offering a protection "cage" which can deflect lightning current away from the equipment and the operator, in a direct strike), but it is often very effective in dealing with the RFI problems of upper-story shacks. It also offers the equipment in the shack a degree of protection from the electromagnetic pulse induction effects of a nearby lighting strike or a strike to the tower.
What about a combination of the two? Yes, combining the two techniques, a faraday cage with artificial grounds can generally solve the RFI issue on an upper-story shack outright and provide a good degree of lightning protection. This is done by installing the radials on the outside of the faraday cage, and bonding them to the ground common point at the bulkhead. This will ensure that the cage is brought as close to possible to RF ground, and the cage itself will reduce signal ingress in station wiring from fields generated by the antenna system. If you are fortunate enough to be able to implement such a solution, you should have few, if any, RFI problems in your shack. While you won't be as safe from lightning as a ground-floor shack, it will certainly be an improvement over nothing.
All This Sounds Like A Lot Of Money. Is It Really Worth It?
Besides the obvious case of your radio ending up in a puddle of smouldering metal among the ashes of the burned-out house, and the operator ending up in the hospital (or worse), a lightning strike, even if it is not direct, but only nearby, can do damage that you may not even become aware of for years to come. Modern solid-state radios depend on integrated circuits that contain extremely tiny structures that are electrically and mechanically fragile, and in some cases, can deteriorate over time. So when a radio is in a facility that has been struck by lightning, it can often continue to operate - with no apparent, obvious damage - for years to come, only to fail suddenly and without warning when a device inside which had been brought to the brink of failure by a lightning strike months or years earlier, finally deteriorates later on sufficiently to fail. A high percentage - probably most - solid state device in-service failures occur as a result of electrically-induced damage after the equipment was put into service, and a significant percentage of those cases are the result of spikes caused by lightning. Having been in the electronic repair business myself, I have seen first hand what lightning can do - even years after the lightning strike occurs. What this means is that even if you think you have been getting away with sloppy grounding practices all this time, doesn't mean that you really have. You may have sustained a significant cost in lightning damage without ever realizing that lightning was the true cause of the equipment failures you have experienced and have been paying for.
So should you commit to doing it? Add up the costs. Plan the system out, creating a rational design based on the design criteria discussed above. Do a spreadsheet, and then divide the number by the liklihood that your site will get hit by lightning, or that you will be unable to conquer those pesky RFI problems that a proper grounding system can go a long way towards resolving, and you can make a rational decision. Maybe, if you live in the Alaskan wilderness, or the Canadian arctic and haven't seen a thunderstorm in thirty years, this doesn't really much matter. But if you live in Kansas, or central Florida, or in the tropics where Intertropical Convergence Zone thunderstorms, with their monster 100,000-amp lighting strikes are a fact of everyday life, it's a whole different story. The fact that you have read this far means that you probably consider yourself vulnerable. Only you can decide, but you should make this decision rationally, not on the basis of emotional appeals by vain hubris - "Haven't had a problem yet, and I have been hit several times! Obviously, my system must be adequate!" Keep in mind the problem of charge saturation - just because your grounds have been able to handle the strikes you have sustained so far without charge saturation occurring, doesn't mean that with the bigger strikes that are surely to come with global warming, that you are good to go into the future with no worries.
But if you are willing to drop a cool ten thousand bucks or so on an inventory of fancy high-class radios, towers and antennas for your shack, what is another thousand to make all your radios last twenty years instead of two? And you can continue to pursue working those Armpit Island DXpeditions, even during a thunderstorm?
Curing RFI And Noise Problems In The Shack
Assuming you have a decent grounding system in place as described above, what do you do if you still have RFI problems in the shack? Well, there is plenty you can do. Fortunately, three of the items needed to cure some of the most intractible RFI problems are found in most ham radio stores: double (or better yet, foil) shielded coax, baluns and split ferrite cores.
If you are experiencing RFI problems when using antennas fed by coax (such as dipoles, quads, etc.), and you have not yet installed baluns on those antennas, that should be your first line of attack. Not only will installing a balun likely improve your RFI situation if not cure it outright, but it will also improve the performance of the antenna. If RF standing waves develop on the outside of a coaxial cable, they distort the pattern of the antenna, reducing its gain and degrading the front-to-rear ratio. But the worst effect is that the coaxial cable's shield can act as a single-wire transmission line and carry that RF back into the house. Offset-fed or end-fed antennas, such as random wires, windoms, etc., are the worst offenders, since the feedpoint is unbalanced and the RF is looking for a way to balance the current coming out of the inside of the coax. There are two possible cures for this: first, use a 1:1 or 1:4 ferrite balun at the feedpoint, or take a piece of coax shield braid from an old piece of coax from which the sheath has been removed (you have kept your old junk RG8 coax around for this purpose, haven't you?), and create a 1/4-wave balun on the coax by slipping a piece 1/4-wave long over the sheath of your feedline coax, and then bond the end of the braid that is away from the antenna feedpoint to the feedline's coax braid (be sure to re-seal it very well against the weather). This should be done as close to the antenna feedpoint as possible. This is called a "sleeve" balun. It can also be fabricated from a piece of EMT conduit - if your coax is going up a tower, where the conduit can enjoy mechanical support, just running the coax inside the 1/4-wave section of 1/2 or 3/4 inch EMT and bonding the coax braid at the bottom, makes for an inexpensive solution. The EMT must, however, be electrically isolated from the tower to function effectively as a sleeve balun. Sleeve baluns are effective only at one frequency, however, and if you have problems on several bands, they may not be the answer.
Another way to do a balun easily and inexpensively, and a solution that works well on several bands, is to simply wind some of the feedline coax around a 4 inch piece of plastic drain pipe to form a coil. About 20 turns will suffice for 10 or 15 meters, 40 or more may be required for 80 meters - this is called a "choke balun." The problem with this solution is that with large coax, such as RG8, it puts a lot of stress on the inner conductor of the coax, and over a period of years, will cause it to "migrate" through the dielectric, dramatically increasing the losses in the balun.
The best solution is the W2AU or similar baluns that are available at your local friendly ham radio store. Or, if you are ambitious, you can wind your own using the directions in the ARRL Antenna Book or elsewhere. The beauty of a ferrite balun is that the losses are low, if properly designed, they are electrically rugged, and they are very effective at decoupling parallel currents.
Now that you have proper and adequate baluns on all your antennas, and you are still having trouble, what else can you do? If you are still having trouble, I can almost guarantee that it is a wire or cable in the shack that is resonant at the offending frequency. Measure the length of all your coax jumpers, power cables, signal cables, etc., and see if any of them are close to a 1/4 wave or an odd multiple of 1/4 wave at the frequency at which you are having trouble. If so, the easiest thing to do is simply slip some RFI ferrite cores (several may be required, especially if the cable is too large to be coiled around the core) over one end of the cable. If it makes the problem worse, move the cores to the other end, or try the middle. I have seen it take as many as seven cores to fix the problem, but it can be an investment that is well worth it in terms of frustration and impaired performance. This is a quick and dirty fix, but it can be quite effective.
The same approach can be taken to curing noise sources in the shack. The worst offenders for generating unwanted noise are computer power supplies, monitors and keyboards, and after that, the second worst offenders are switch-mode power supplies and third come florescent lamps. All can be treated with ferrite cores wrapped around their cables, though it is important to keep the cores as close to the offending device as possible (to prevent the cable from acting as even a short antenna), and don't be afraid to use as many as necessary - they are not cheap, but they can be quite effective. Don't forget to treat your radio's power supply cord in the same manner - you don't want noise traveling up your rig's power cord and into the radio, do you?
By far the best approach to dealing with noise problems is to deal with the source. If you can cure it, you will cure it for all your radios, and probably on all or most bands, not just one radio and one band. But sometimes, when the noise source is on a neighbor's property, or She Who Must Be Obeyed says no, then you will have to deal with the problem from a standpoint of signal ingress into your own equipment.
The first approach to solving noise ingress problems is to use good quality foil shielded coaxial cable for all, not just some, of your inside station wiring (even for radios on which you are not having a problem), and also outside on antennas that are prone to pickup noise from local sources (such as ground-mounted verticals, for example). This is another argument for running your coax out to the antenna farm through buried conduit - not only does it help protect your shack from lighting strikes (the conduit acts as a low-pass filter, shaving most of the lightning RF energy off of the outer shield during a lightning strike), but it also ensures that there is no noise ingress (pickup) at all between the shack and the antenna. A buried conduit also acts as a balun, preventing standing waves from developing on the outside of your coax and bringing RF into your shack. If the cable between your radio and the end of the conduit is entirely foil shielded, you will (theoretically) not have problems with noise ingress at all - you're as fully protected as you can get. The only noise you will hear is noise that is picked up by the antenna itself, or is picked up by a poorly-shielded receiver front-end in your rig. You can't do much about the former (other than move the antenna), but the latter can be addressed through more aggressive shielding techniques.
To determine if your noise problem is antenna noise pickup or is a shielding problem in the receiver front-end, and you have already installed foil shielded coax and/or metal conduit all the way to your antenna and have already decoupled your power and signal cables with ferrite cores, you can determine whether the problem is antenna noise or front-end ingress by shorting the feedpoint of the antenna. If the noise that is troubling you is not attenuated, or is attenuated only modestly (or possibly even increased), the problem is inadequate shielding of the receiver front-end. Often times this can also be diagnosed by grasping, close to the radio, the coax cables, signal cables and power cables going to your radio, or simply placing your hand on the radio. If you hear the noise level change (up or down), the problem is front-end shielding. Note that this test means nothing if you have not already taken the measures described above. Most early ham rigs had poorly shielded receivers, but with the advent of a lot of noise sources in the shack, such as computers, switch-mode power supplies and the like, the big manufacturers of ham rigs nowadays realize that it is in their best interest to design receivers that are heavily shielded in all stages ahead of the first mixer. This ensures the maximum user satisfaction that the manufacturer can provide, and hence, most modern radios nowadays are heavily shielded in the front ends. So if all your cables, including the coax, that are going to the radio are adequately decoupled with ferrite cores, and you still have problems with noise ingress, and you are sure the problem is not noise pickup at the antenna, your only option is to mount the radio in a cabinet that is designed as a faraday shield. That means wire mesh in the top, sides, bottom and back, and the radio itself is mounted in metal rack panels, which are themselves bonded to the mesh. This is a truly desperate measure, but I doubt very much if you do the other things I have previously recommended, that you will have to take things this far.
I hope this has been of benefit to you. If you have tried all these things, and are still plagued with RFI or are concerned about lightning, please feel free to contact me by email at the link to the left. I am happy to offer the benefit of my professional experience to anyone who is willing to follow my recommendations.
© 2008, Scott Bidstrup, all rights reserved. Todos derechos reservados.