Back last February, it was reported that FEMA/Department of Homeland Security was mysteriously constructing prepackaged AM transmitter buildings at various PEP (Primary Entry Point) transmitter sites across the country as something call “Primary Entry Point Expansion.” These buildings contain a 5 KW Nautel AM transmitter, EAS gear, satellite equipment (the exact equipment list is undisclosed), and a backup generator all in a shielded (Faraday Cage), prefabricated building placed inside a fenced-in compound at the station’s transmitter site. The buildings are being put in place, but not connected to anything in the outside world. They are planning to have about 80 (the number keeps increasing) of these structures in place by when the project is completed in mid-2013.
Why, inquiring minds want to know, would they do that?
The new buildings and equipment are, of course, not provided to the government for free. I would estimate each unit costs at least $200,000 based on the following:
A new solid-state 5 KW AM transmitter costs $50-55K
A new 35 KW generator costs $23K
A new, shielded communications structure costs $70-85K
Those prices are roughly what a private company might pay, the government procurement costs would be higher. Multiply by 80, which equals at least $16M, perhaps double that when project administration is considered. In the distant past, through something called the Broadcast Station Protection Program (BSPP), FEMA did provide generators, fuel tanks, transfer switches, and occasionally a bomb shelter to key EBS stations throughout the country. In the recent past, FEMA and the government, in general, have been reluctant to fund even mandated changes in the EAS system, first in 1997 when EAS was first implemented and again in 2011 when the CAP modifications were required. Why are they now spending at least $16M to provide EMP-hardened facilities forAM radio stations?
The rationale for this current wave of government spending, as reported in several industryperiodicals, is simply a matter of supplying in-depth backup facilities in accordance with Executive Order 13407. The design of the structure and manner of installation seems to indicate the primary concern of FEMA is some type of Electromagnetic Pulse (EMP). If an EMP were to happen and it took out the station’s main transmitters, these could be connected to the existing antenna system and switched on. They would provide emergency programming and interface directly with FEMA’s IPAWS (Integrated Public Alert and Warning System).
The interesting thing about this is that there is a coincidence with the upswing of solar cycle 24. Back in 2008, likely when this project was likely first dreamed up, the predictions were for a great number of sunspots in this cycle. That has not happened and in fact, this cycle is now predicted to be the weakest solar cycle since 1823. Even weak sunspot cycles can create problems, but does that warrant supplying 80 backup transmitters, generators, fuel tanks, and buildings to various AM broadcasting stations throughout the country? Further, solar flares and Coronal Mass Ejections (CME) are fairly slow-moving events, the sun is well monitored; alerts would be issued and precautions are taken.
One other thing to consider: HEMP (High altitude Electromagnetic Pulse from a nuclear air burst). AM transmitters are more robust when it comes to HEMP than FM transmitters. This is because of their modulation type and frequency of operation. A 5 KW AM transmitter can withstand RF voltages six or eight times its nameplate carrier rating. Tube-type transmitters are even more robust than solid state. The FM broadcast band falls right in the middle of the HEMP fast pulse frequency (72-225 MHz), which will likely resonate in the tuning circuits of the transmitter exposed to it and destroy all of the active devices. Not so with AM transmitters.
A HEMP event would cause catastrophic damage to the electrical grid across wide areas of the continent (see also; Starfish Prime). The voltages instantaneously induced on computer circuit boards and power supplies would be so high, they would likely burst into flames if they were close enough to the detonation. The same for almost all other electronic devices with circuit boards. It would set the country back one hundred or more years, technologically, causing massive disruptions in the food supply chain. Such an act would surely be met with massive nuclear retaliation by the US. The military has not only hardened all of its communications and command facilities, but they have also undergone rigorous EMP testing, finding and fixing design flaws. Thus, the US military’s capacity to wage war would continue undiminished after a HEMP event, a fact that all other members of the nuclear club are surely aware of.
By now, you have heard of the great solar flare of 1859 that set telegraph wires afire across the US and Europe. This phenomenon was due to a large electromagnetic pulse from the earth’s magnetosphere interacting with the particles from a Coronal Mass Ejection (CME) caused by the solar flare. The long stretches of wire suspended above the earth acted like a large generator winding and cutting through the magnetic field, which in turn caused voltage (Electromagnetic force or EMF), which in turn, caused the fires.
In 1859 our understanding of electricity and magnetics is not what it is today, thus, the fires were likely caused by overloaded conductors without means to bypass excess electrical energy to ground.
Fast forward to today. We are in the upswing of solar cycle 24, which is expected to peak in May of 2013. NASA predicts that this will be the lowest sunspot peak since 1907. That does not necessarily mean the coast is clear. Over the last 11 years since solar cycle 23 peaked, computers and electronic automation have proliferated exponentially, becoming the norm. Things like GPS not only guide clueless travelers where to turn but also sync up all those cellular telephone transmitters with timing signals. IP networks, SCADA, telephone networks, and so on all run on some form of CPU. Newer Energy Star appliances like toasters and refrigerators also have CPUs. That technology has yet to experience a large electromagnetic pulse (EMP) in the real world. Things could indeed get hairy if a moderate to large class X solar flare generated a CME that was polarized correctly to interact with Earth’s magnetic field and cause damage at ground level.
Solar flares and CME are slow-moving events, with 1-2 days warning before the effects of a CME reach Earth. One can stay apprised of solar flares and other solar activity by subscribing to NOAA Space Weather Prediction Center’s email service.
Of greater concern is other sources of EMP like high altitude nuclear explosions (HEMP). Those types of events, while rare, can happen. The good news is, the defense mechanisms for solar flares, high-altitude nuclear bursts and lightning-induced EMP are the same. They are effective grounding, shielding, filtering and surge suppression.1 Of the three EMP scenarios, high altitude nuclear burst has the tightest design spec, so creating a building that incorporates the ideas in MIL-STD-188-125-1 specifications is a good start.
The question becomes, is all of this really necessary. It depends on how important it is to radio station ownership to remain on the air during such an event. Based on historical information and global geopolitics, the probabilities of such an occurrence are:
Lightning strike – 1:1 Any radio station that has a tall structure, particularly a steel tower, will get struck by lightning, perhaps several times per year depending on the region.
Large Class X solar flare resulting in damaging CME – 1:21 Since 1859, there have been seven solar flare events that have disrupted communications or power systems on Earth. This is a bit misleading since 6 of the 7 events have occurred in the last 22 years, making the real probability more like 1:3.6
High Altitude EMP – 1:30 Based on seventeen high altitude tests carried out by the US and USSR in 1962, the growing nuclear proliferation and a June 2005 Reuters article “Experts warn of substantial risk of WMD attack” in which the author stipulates a thirty percent risk of a nuclear attack of any type in the period of 2010 to 2015.
High altitude nuclear burst EMP has three components; the fast component (20/550 ns pulse) is an electromagnetic shock-wave, the medium-speed component (1.5/5000 μs pulse), and the slow component (0.2/25 s pulse) resulting from the expansion of the explosion’s fireball in the Earth’s magnetic field.1 Compare that to a lightning strike, which typically has a 1.8 µs rise time. That means the first pulse frequency is from about 72 to 200 MHz, the second pulse frequency is from about 800 Hz to 2.5 MHz and the third pulse is basically DC and affects mostly long wires. Thus, any shielding, grounding and suppression needs to consider the highest frequency down to about 10 KHz.2
The rise time to frequency comparison is an important consideration in the design and construction of grounding systems. Grounds need to present the least amount of inductance possible. This means using solid, not stranded wire, keeping bends to a minimum and where required, use long radiuses. Bond all junctions by welding, exothermic welding or soldering. Use a single point ground system. Zone grounding should also be employed. The definition of zone grounding is concentric grounding areas connected to each other by a single low-inductance ground conductor.2 The idea of isolating grounds by use of a ferrite suppressor may seem odd, however, if there is a separate RF ground, tied together with the building ground using wide copper strap laid on the floor, this will minimize ground system “reception” of incoming EMP.2,3
Shielding means surrounding the protected equipment with a conductive material such as copper plate, aluminum plate, copper mesh, aluminum mesh, brass mesh, steel plate or steel mesh.1,2 There are advantages and disadvantages to each. Seams should be welded to prevent “leaks.” Doors need to have finger stock or metal compression gaskets to ensure proper sealing of opening. Other openings for ventilation, cable ingress, etc. need to be “significantly” less than one wavelength. MIL-STD-188-1 gives the allowable opening size of 10 x 10 cm or 3.9 x 3.9 inches. If openings in the shield become greater than approximately wavelength/6 meters (at 250 Mhz, about 8 inches), significant fields can penetrate to the interior.2
Suppression deals with those connections to the areas outside of the facility. These include incoming electrical service, data service and RF transmission lines to and from antennas. Since the fast component of the HEMP falls within the VHF spectrum, FM broadcast installations are particularly vulnerable. Suppression devices for incoming AC power are readily available from commercial sources and are well proven. LEA makes a series surge suppressor that uses a combination of fast acting silicone diodes, MOVs, and an LC filter made up of series inductance and parallel capacitance to ground. The LEA DYNA family series surge protectors have a system response time of less than one nanosecond and are tested to greater than 1,000 operations.4 The response time depends on a good, non-inductive ground connection.
Suppression for incoming RF and data cables is more difficult because the normal operating frequencies fall within the HEMP rise time frequency. Incoming data at a transmitter site usually consists of a DS-1 circuit however, larger capacity circuits are sometimes used. Fiber optic cables are immune from HEMP as they have no metal conductors. Copper data lines must have a data line surge suppressor between the TELCO demark and the CSU.
RF cables must have their shields ground to the zone 0 ground, then go through a ferrite toroid to add inductance to the outer shield and isolate it from the zone 1 ground. After the ferrite toroid, a gas discharge type inline surge suppressor should be used. These come in a variety of configurations, frequency band and power levels. It is best to keep the suppressor rating as close to the peak carrier power as possible, affording the most protection to the transmission equipment.
Design and implementation of EMP hardened facilities
Of the four strategies for mitigating HEMP; Grounding, Suppression, filtering and shielding, shielding is the hardest and most expensive to implement. Good grounding should be included in any good radio station design same as suppression and filtering.
Grounding. Grounding for a transmitter site must include an outside ring ground around the periphery of the building.2 This is bonded to several ground rods installed at regular intervals. The ground is brought into the building and all coax shields, electrical service entrance, TELCO equipment, suppression equipment and safety grounds are connected to it. This forms the zone 0 ground. One conductor then goes to the zone 1 ground which is the transmitters and racks. Any other conductors at any potential that go from zone 0 to zone 1 are bypassed at EMP frequencies by use of ferrite toroids or other high mu ferrous metal filters. Inductors may need to be bonded to ground to prevent saturation. At studio locations, the building electrical safety ground should be evaluated for adequacy. Additional grounding may need to be installed depending on effectiveness of existing ground. Any outside antennas, supporting structures, satellite dishes and generators need to be bonded together and grounded to the building electrical grounding system. Studios and engineering rack rooms need to be bonded to the ground using star topology. Facilities that are not adequately grounded should be retrofitted.
Suppression and filtering. Good surge suppression and filtering should be a part of all transmitter and studio site designs. Hanging a few MOVs off of the service panel is not enough to prevent damage to a facility. All incoming lines from the street; electric, telephone, and cable need to have surge suppression connected and be bonded to a low inductance path to ground. The only exception to this is fiber optic, which is immune to the effects of EMP. Additional layers of filtering for sensitive, mission-critical computer systems such as FERUPS, shielded category wiring that is properly installed, etc. Facilities that do not have adequate suppression can be retrofitted.
Shielding. Shielding is the most expensive, time-consuming, and difficult to install correctly. The High Altitude nuclear test Starfish Prime in July of 1962 produced a field of 5600 V/m in Honolulu, some 1300 KM away from the blast. Building a shielded structure against those intense magnetic and electrical fields is very difficult. Attenuating the field through layers of shielding is the most effective means provided the distance between the shields is wavelength/6 or more (about 27 inches at 72 MHz) to prevent coupling.2 For example, using a concrete structure with steel mesh creates 35-40 dB of attenuation in zone 0, and a well-designed transmitter with good RF shielding in its cabinet design creates a shield for zone 1. Equipment racks can also be used to create shielding zones by using copper or brass mesh with good metal-to-metal contact around the front and back doors. At studio locations, engineering rack rooms should have copper or brass mesh embedded in the wall structure to create a shield. This will create a safe area to locate computer file servers, routers, switches, STL gear, satellite receivers, and the like equipment. Layered shielding with the use of metal, gasketed door will improve shield performance. Retrofitting shielding is more difficult to accomplish than grounding and suppression. It is best done in new construction. There are many different ways to accomplish even moderate shielding, which may serve well for lightning and solar flare-induced EMP.
From personal experience, investing an extra $10-20K in grounding and suppression at a lightning prone transmitter site in Florida solved all of the issues at that site. Prior to installing the ring ground and bonding, the transmitter was knocked off the air several times per year. Since the work was done in 2005, there has not been one lightning related outage at that site.
References: 1. Protection Technology Group, System Approach to EMP Mitigation, February 2011 2. US Army Corps of Engineers, Engineering and Design – Electromagnetic Pulse (EMP) and Tempest Protection for Facilities EP 1110-3-2, December 31, 1990 3. US Department of Defense, HIGH-ALTITUDE ELECTROMAGNETIC PULSE (HEMP) PROTECTION FOR GROUND-BASED C4I FACILITIES PERFORMING CRITICAL,TIME-URGENT MISSIONS, MIL-STD-188-125-1A, February 15, 1994 4. Protection Technology Group, Modular Hybrid Series Connected Surge Protection Device LEA DS21 data sheet, 2010
It is that time of the year again, at least in the northern hemisphere, for thunderstorms. I am a big proponent of grounding everything, there is simply no such thing as too much grounding. I took a course when I was in the military given by Polyphaser in which grounding for lightning protection and EMP was emphasized. It was very interesting in several respects.
One commonly held belief is that when lightning strikes an object, the ground immediately absorbs all of the charge. That is not true in most cases due to ground resistance. Eventually, the ground will absorb the charge but it can take several seconds to do this, especially with a big strike. Equipment is damaged by current flow, therefore, every effort must be made to keep all of the equipment at the same potential, even if that potential is 10KV. That is where a single-point ground bus comes in. Bonding every piece of equipment to a common ground bus ensures that no one device is at a lower potential while the charge dissipation is occurring.
The second misunderstanding about lightning is that it is DC voltage. That is true, however, a lightning strike has an extremely fast rise time, on the order of 30 microseconds. That makes it behave more like an AC voltage of around 10 KHz. Therefore, ground bus wires need to have a minimum inductance. Solid #2 wire is best, keeping it as straight as possible and using long sweeping turns where needed. All bonds should be exothermically welded (CAD weld).
The ground system was installed at WKZY, WHHZ, and WDVH-FM transmitter site in Trenton, Florida. Central Florida is the lightning capital of the US. Prior to doing this work, the Harris FM25K transmitter was knocked off the air at least once a month. Since this was installed in 2005, they have had zero lightning-related damage. The ground rods are 20 feet long, driven down into the water table, spaced 20-30 feet apart.
All coax shields and metal conduits that come into the building should be bonded to the ground system where they leave the tower and where they enter the building. At most tower sites, I install a ground ring around the outside of the building with rods every 20 feet or so. From that ring, 5 to 6 radials outward 40 feet with ground rods every twenty feet works well. I also install 5 to 6 radial out from the tower base with the same configuration. The tower and building grounds are bonded together. This is important because when the tower gets hit, the ground will quickly become electrically saturated. If the building and the equipment inside are at a different potential, current will begin to flow toward the lower potential, thus damaging gear.
All Coax, control, and AC cables in and out of sensitive equipment should have ferrite toroids on them. Transmitter manufacturers normally supply these with new solid-state transmitters, as MOSFETs are particularly sensitive to lightning damage.
This is a Potomac Instruments AM-19 directional antenna monitor. It was damaged by a lightning strike two weeks ago on the WBNR tower in Beacon, NY. The case arced to the rack it was mounted in. This was a large strike, as several components in the phasor control circuit were also damaged. The fact that this arced means that somehow the sample lines are not attached to the single-point ground for this site, which needs to be corrected.
Insulated AM towers present special design problems when it comes to lightning protection. Generally speaking, tower arc gaps should be set so there is side by side and there is no arcing on positive modulation peaks. Depending on power levels, this can be anywhere from 1/2 inch to 2 inches. Tower impedance also plays a role in setting arc gaps. The final link between the ATU and the tower should have several turns in it. The idea is to make that path a higher impedance path for the lightning, causing it to dissipate through the arc gaps. Incoming transmission lines from the towers should be bonded to a copper bus bar at the entrance to the building. All of this grounding needs to be tied to the RF ground at the base of the tower.
Arial phone cables can act like large lightning antennas for strokes several miles away. It is very important that the cable shield and the cable termination device are bonded to the building ground buss. I have seen installations where the TELCO tech pounds in a separate ground rod outside and connects the TELCO equipment to that. That defeats the concept of single-point grounds and should be fixed ASAP.
Electrical services entrances also can act like big lightning antennas. Normally, pole-mounted transformers will filter some of this energy out. Internal electrical distribution systems can also add impedance, and thus act as inadvertent filters for lightning. In most mountaintop transmitter sites, however, some type of power line surge protection is needed.
There are two types, series, and parallel. Parallel types are the least expensive and least intensive to install. They are usually found mounted next to or on the service panel and fed with their own breakers. They usually have some type of MOV or similar device that acts as a crowbar across the AC mains, conducting spikes to the ground. Series types go in between the service entrance and the main panel. They include a large inductor designed to force spikes off into shunts. A series-type protector offers more complete protection than a parallel.