NTIA Report 01-382

Vulnerability of Wireline and Cellular

Telecommunications Networks to High

Power Radio Frequency Fields

Perry Wilson

Eldon Haakinson

Roger Dalke

U.S. DEPARTMENT OF COMMERCE

Norman Y. Mineta, Secretary

Gregory L. Rohde, Assistant Secretary

for Communications and Information

November 2000
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iii

CONTENTS

Page

EXECUTIVE SUMMARY ............................................................................................. v

LIST OF FIGURES ....................................................................................................... vi

LIST OF TABLES........................................................................................................ vii

ABSTRACT ................................................................................................................... 1

1. INTRODUCTION..................................................................................................... 1

2. NETWORK LEVEL VULNERABILITY ................................................................. 3

2.1 Telecommunication System Architectures.................................................... 3

2.1.1 Public Switched Telephone Network.............................................. 4

2.1.1.1 Switching System ............................................................ 6

2.1.1.2 Signaling System ............................................................. 8

2.1.2 Internet..... ..................................................................................... 9

2.1.3 Cellular Mobile Radio Telephone ................................................. 10

2.1.4 Private Radio Networks................................................................ 11

2.2 Network Level Vulnerability Summary...................................................... 12

3. NODAL LEVEL VULNERABILITY..................................................................... 12

3.1 PSTN Switching and Signaling Stations..................................................... 13

3.2 Cellular Sites............................................................................................ 14

3.2.1 Base Stations................................................................................ 14

3.2.1.1 Antennas......................................................................... 15

3.2.1.2 Cables............................................................................. 16

3.2.1.3 RF Equipment................................................................. 17

3.2.2 Mobile Telephone Switching Offices............................................ 19

3.3 Private Radio Networks Dispatch Stations ................................................. 19

3.4 Nodal Level Vulnerability Summary.......................................................... 19

4. CONCLUSIONS..................................................................................................... 19

5. REFERENCES........................................................................................................ 21

APPENDIX A: PLANE WAVE PENETRATION OF REINFORCED CONCRETE.... 23

APPENDIX B: CABLE COUPLING............................................................................ 27
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v

EXECUTIVE SUMMARY

High level electromagnetic fields can upset and damage electronics, as well as disrupt or 

disable computer software. Thus, high power radio frequency (RF) fields pose a threat to 

electronics and software dependent systems. In particular, critical infrastructures such as 

telecommunications could be targeted. This report examines the general vulnerability of 

public and emergency telecommunications networks to high power RF fields. In 

particular, the following primary questions are addressed.

- Can the loss of a node (e.g., a switching station or a wireless base station), or nodes, 

cascade through the telecommunications network, causing a large-scale system 

blackout or crash?

- Can a high power RF device disrupt or disable a node? What vulnerable equipment is 

located at various nodal types?

The first question addresses network level vulnerability and requires an examination of 

the system architecture and how redundancy and robustness are used to compensate for 

nodal loss or overload. The second question addresses nodal level vulnerability and 

involves estimating the coupling of high level RF fields to sensitive equipment located at 

a node. In particular, switching stations (the buildings that house switching equipment) 

and cellular base stations (antenna towers and associated equipment sheds) are 

considered. Because of the size and complexity of the various telecommunications 

networks, as well as the multiple providers involved, detailed evaluations of specific 

systems and nodes are beyond the scope of this examination. Nonetheless, some general 

conclusions are possible.

- Network level vulnerability is low. The overall public telecommunications network 

has sufficient redundancy and capacity to withstand the loss of even multiple nodes. 

A full system collapse is not envisioned as a possible scenario. The loss of certain key 

system nodes could, however, lead to large local blackouts.

- Nodal level vulnerability is high. Neither public network switching stations nor 

cellular base stations are intentionally hardened against high level RF fields. Typical 

steel reinforced concrete walls provide little shielding for frequencies above a few 

hundred MHz. A switching station could be disrupted (10 to 100 thousand users), 

although the probability of disruption is difficult to assess due to the variety and 

complexity of building layouts. Wireless base stations are highly vulnerable to high 

power devices operating within the base station frequency. The loss of a base station 

would largely black out the associated cell.
vi

LIST OF FIGURES

Page

Figure 1. Tiered switching centers. . ........................................................................... 5

Figure 2. Bell system 5-tier heirarchical structure. ..................................................... 5

Figure 3. Tiered hierarchy plus high usage trunks. ..................................................... 7

Figure 4. Dynamic nonhierarchical routing. ............................................................... 7

Figure 5. The DNHR switching and signalling hierarchy. .......................................... 8

Figure 6. The cellular mobile telephone service network. ......................................... 10

Figure 7. Typical mobile cellular radio telephone base station. ................................ 15

Figure A1. Reinforced concrete wall and FDTD computational volume. ...................... 23

Figure A2. Transmission coefficients for a concrete wall with a 2 dimensional 

re-bar lattice: P = 15.24 cm, D = 1.91 cm, and W = 20.32 cm. ................... 25

Figure A3. Transmission coefficients for a concrete wall with a 2 dimensional 

re-bar lattice: P = 7.62 cm, D = 1.91 cm, and W = 20.32 cm. .................... 26

Figure A4. Transmission coefficients for a concrete wall with a 2 dimensional 

re-bar lattice: P = 15.24 cm, D = 5.08 cm, and W = 20.32 cm. ................... 26

Figure B1. Magnitude of the normalized transfer impedance t Rdc Z (s) / versus gd 

for a tubular cable. ...................................................................................... 28

Figure B2. A braided shield (a) and its surface (b). ..................................................... 29
vii

LIST OF TABLES

Page

Table 1. A Comparison of Cellular Mobile Radio Systems.......................................... 10

Table 2. A Comparison of Dispatch Systems............................................................... 11

Table 3. Base Station Received Power versus Frequency, Transmitter Power, 

and Transmitter Distance ............................................................................... 18

Table B1.The Function [(1 ) , ]

3/ 2

g − K y for Some Typical Values................................ 30

Table B2.The Function [(1 ) , ]

3/ 2

h − K f

d y for Some Typical Values ............................ 30
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___________________

1

 The author is with the National Institute of Standards and Technology, U.S. Department of 

Commerce, Boulder, CO 80305. The author was with the Institute for Telecommunication 

Sciences when the work was done.

2

 The authors are with the Institute for Telecommunication Sciences, National 

Telecommunications and Information Administration, U.S. Department of Commerce, Boulder, 

CO 80305.

VULNERABILITY OF WIRELINE AND CELLULAR 

TELECOMMUNICATIONS NETWORKS TO HIGH POWER RADIO 

FREQUENCY FIELDS

Perry Wilson1, Eldon Haakinson2, Roger Dalke2

High level electromagnetic fields can upset and damage electronics, as well as 

disrupt or disable computer software. Thus, high power radio frequency (RF) 

fields pose a threat to critical infrastructures such as telecommunications. This 

report examines the vulnerability of public and emergency

telecommunications networks to high power RF fields, at the network and

nodal levels. The assessment of network level vulnerability requires an

examination of the system architecture and how redundancy and robustness

are used to compensate for nodal loss or overload. The overall public

telecommunications network has sufficient redundancy and capacity to

withstand the loss of even multiple nodes. A full system collapse is not

envisioned as a possible scenario. However, large local blackouts are possible. 

The assessment of nodal level vulnerability involves estimating the coupling 

of high level RF fields to sensitive equipment located at a node. Neither public 

network switching stations nor cellular base stations are intentionally

hardened against high level RF fields. Typical steel reinforced concrete walls 

provide little shielding for frequencies above a few hundred MHz. Thus, a

switching station could be disrupted (10 to 100 thousand users) by a high

power RF device in this frequency range, although the probability of

disruption is difficult to assess due to the variety and complexity of building 

layouts. Wireless base stations are highly vulnerable to high power fields

coupling via the antennas.

Key words: critical infrastructure protection, high power RF fields, public switched

telephone network, shielding, vulnerability

1. INTRODUCTION

High level electromagnetic fields can upset and damage electronics, as well as disrupt or 

disable computer software (e.g., [1]). Thus, high power radio frequency (RF) fields pose 

a threat to electronics and software dependent systems. In particular, critical 

infrastructures such as public and private telecommunications networks could be 

affected. The objective of this study is to briefly assess whether high power RF fields 

could significantly disrupt public and emergency telecommunications systems.
2

A telecommunications network may be viewed as a set of nodes (e.g., collection and 

routing points) connected by links (e.g., wires, fibers, wireless radio paths). The 

following questions will be addressed.

- Can the loss of a node (or nodes) cascade through the telecommunications network 

causing a large-scale system blackout or crash?

- Can high power fields disrupt or disable a node (such as a switching station or a 

wireless base station)? 

The first question addresses network vulnerability and requires an examination of how 

redundancy and robustness within a network are used to compensate for nodal loss or 

overload. This report will primarily address the public switched telephone network. Other 

networks such as cellular networks or the Internet are either interfaced to, or piggybacked 

on the switched network. Emergency networks are usually stand-alone and will be only 

briefly discussed. The second question addresses nodal vulnerability and involves 

estimating the coupling of high level RF fields to sensitive equipment located at a nodal 

location. In particular, switching stations (i.e., buildings that house switching equipment) 

and cellular base stations (i.e., antenna towers and associated equipment sheds) are 

considered.

The high power RF devices assumed here are characterized by powers in the MW to GW 

range at frequencies between approximately 800 MHz and 30 GHz. The 800 MHz 

practical lower limit is due primarily to two factors. First, high gain antennas become too 

large for easy concealment and mobility much below 1 GHz. Second, RF power becomes 

significantly more expensive at lower frequencies.

It needs to be emphasized that the results presented in this report are preliminary. The 

telecommunication network architectures are considered at a very general level only. The 

penetration of RF fields into buildings is simplified to a two-dimensional model in order 

to show basic trends. The coupling to a more realistic three-dimensional model is not 

considered. The complex problem of field coupling to a complex layout of equipment and 

interconnections is also not considered. Nonetheless, some initial conclusions may be 

drawn as to the potential vulnerability of wireline and cellular telecommunication 

networks. We hope that this report will serve as a basis for more detailed future studies. 

The report is organized as follows. Section 2 reviews basic telecommunications networks 

and their system architectures. Nodal types within the system are identified, their 

functions briefly described, and the impact of their loss evaluated. Section 3 examines the 

physical location of system nodes and assesses their vulnerability to high power RF 

fields. Section 4 summarizes results. Appendices A and B give details as to the 

calculation of field penetration through reinforced concrete and shielded cables.
3

2. NETWORK LEVEL VULNERABILITY

Telecommunications, broadly defined, includes all forms of electronic voice and data 

communications systems: public, private, emergency, government, and military; wire, 

fiber, and wireless; overhead and buried; ground-based and satellite. Telecommunications 

systems considered here are:

- Public Switched Telephone Network

- Internet

- Cellular

- Private Radio Networks (Specialized Mobile Radio, Public Safety, Law 

Enforcement)

The Public Switched Telephone Network (PSTN) is the oldest telecommunications 

network to span the U.S. and is still in existence today. Once the call is set up, the circuit 

remains “connected” between the caller and called party until one of them hangs up. The 

system architecture has evolved to make it very robust and able to handle varying loads 

and system outages.

The Internet spans the U.S. using much of the PSTN as its backbone along with its own 

partial network. The Internet sends all data from source to destination by breaking the 

original message into packets with each packet containing the addresses of the destination 

as well as the source. It uses a connectionless scheme to transmit the information 

allowing each packet to find its own route through the network. 

Cellular Mobile Telephone Service and Personal Communication Service (PCS) systems 

use wireless means to link mobile subscribers to each other, to the PSTN, or to the 

Internet. Once a call is set up, the subscriber can move from cell to cell within the 

network while maintaining access.

Private Radio Network (e.g., Specialized Mobile Radio, Public Safety, Transportation, 

Public Utility) systems provide mobile users access to other mobile users of the same 

network or to local area networks (LANs) on the network. These are typically stand-alone 

networks. Thus, they do not always tie the mobile user to the PSTN or the Internet, 

although such a link is possible. The systems can be small to serve a community, or large 

to serve a city or a state, or can span the U.S. to serve a particular industry.

This report will mainly focus on the PSTN and cellular systems, since these affect the 

largest set of users. The Internet, which piggybacks the PSTN network, and private and 

emergency networks will be only briefly discussed.

2.1 Telecommunication System Architectures

Telecommunication systems have traditionally been either stand-alone wireline or standalone wireless systems. The typical stand-alone wireline system is a switched telephone 
4

circuit that connects one telephone user with another. The typical stand-alone wireless 

telecommunication system allows mobile radio users to communicate with each other or 

with a base station radio and dispatcher. The stand-alone wireline system is independent 

of the wireless system (although it might have microwave relay (wireless) links to 

connect portions of the network). Conversely, the stand-alone wireless system is 

independent of the wireline system and its features. 

From these traditional concepts, the separate wireless and wireline systems have evolved 

into systems that require both wireline and wireless components and networks in order to 

provide the features that users demand. The seamless integration of the networks also 

provides connectivity to other users who are not normally accessible on either the 

wireline or wireless networks. 

This high level of interconnectivity means that breaks in one system (e.g. wireless), due 

to a software failure for example, could propagate into another system (e.g. wireline). 

Interconnectivity, however, also leads to robustness by providing alternate links to 

compensate for any single lost link. Indeed, rapidly expanding interconnectivity and 

capacity is likely to decrease the vulnerability of the overall telecommunications network 

to the failure of any particular subset of the system. 

2.1.1 Public Switched Telephone Network

During the late 1940s and early 1950s, the PSTN underwent an architectural evolution in 

order to offer direct distance dialing (DDD). The network was structured to be 

hierarchical so that a call originating at one end office (alternately termed a local or 

central office) would be routed up, across, and down the structure to the destination end 

office. Figure 1 shows the concept of tiers consisting of switching centers for routing 

telephone calls. Business and residential phones are connected by wireline to an end 

office switch. If the calling and called telephones are connected to the same switch, the 

call is routed directly within the switch. If the called telephone is not connected to the 

switch, the call needs to be routed up in the tier to the switch that is common to both the 

calling and called telephones. Figure 2 shows the 5-tier hierarchical structure used for 

many years by the Bell System and uses the nomenclature of the Bell System to identify 

the switching centers in the hierarchy. 

Up to 10,000 lines could be connected to one end office switch handling the prefix NNN 

to give the numbering sequence NNN-XXXX. Additional prefixes (NNA, NNB etc.) can 

be handled within a single end office. Thus an end office can have anywhere from 10,000 

to 100,000 subscriber lines. At this point, there is normally no redundancy built into the 

system from the residence or business to the end office switch. That is, if the end office 

switch is unable to handle the call due to either heavy use or a physical disruption, then 

the telephone user will be without service until the load eases or the physical problem is 

repaired.
5

The above simple hierarchical system is now replaced by a dynamic routing system. This 

dynamic system consists of two basic parts; the switching system and the signaling 

system. 

Figure 1. Tiered switching centers.

Figure 2. Bell system 5-tier hierarchical structure.
6

The switching system (hardware) creates the physical wireline connection between users. 

The signaling system (software) checks for the availability of connections and routes, 

monitors traffic, controls system logic, and collects billing data. Breaks in these two 

systems will affect the overall system differently. These two systems, switching and 

signaling, are discussed in more detail in the next two sections.

2.1.1.1 Switching System

The switching system creates the physical wireline connection between users. If the call’s 

destination is at another end office, then the call is routed up the hierarchy until the 

appropriate crossover point is reached for routing the call to the destination end office. 

The structure ties several end offices to a toll center, several toll centers to a primary 

center, etc. Finally, the call might reach the regional center for routing to another regional 

center. For example, there were 10 regional centers in this hierarchy within the U.S. and 

the centers represented the last possible route for the telephone call.

Although this hierarchy works for completing calls, it forces each call through many 

intermediate-switching centers that could be avoided if additional trunks are made 

available. These trunks (called High Usage circuits) were added between switching 

centers where usage statistics showed they were economically valuable. Figure 3 shows 

routes in place between switching centers available for high usage links. The switches are 

programmed to route calls along the various route choices depending upon loading at the 

individual switches. Routing algorithms attempt to limit the number of switching centers 

that the call must go through.

In the mid 1980s, the Bell System began using fourth generation Electronic Switching 

System (ESS) switches which allow high-speed dialing throughout the network and use 

computer-controlled intelligence to select the best routes for the calls. The system uses 

Dynamic Nonhierarchical Routing (DNHR) which eliminates the 5-tier hierarchy and 

allows pre-planned routes between switches to change dynamically throughout the day. 

Figure 4 shows the structure of DNHR with first choice routes for call setup but also 

many alternate routes as well. The objective is to maximize the use of the existing 

network to route calls efficiently and quickly without adding more switches and switch 

centers to the existing network. The result also provides reliability to the network: if a 

switch center is overloaded, telephone traffic can be routed around the busy switch. But 

equally important, if a switch center has suffered a loss of capacity or capability due to a 

natural disaster or equipment failure, the network will be virtually unaffected except for 

some loss of redundancy and capacity. Calls will be routed around the impaired switch 

center until the problem is corrected. 
7

Figure 3. Tiered hierarchy plus high usage trunks.

Figure 4. Dynamic nonhierarchical routing.
8

As with the hierarchical structure, the lowest level is an end office (EO) switch that 

connects to the end users, as indicated in Figure 5. An EO typically serves 20,000 to 

40,000 users with 10,000 to 100,000 users possible, as mentioned above. The loss of an 

EO would mean that those users connected to that EO would be blacked out (phone line, 

911, Internet). The rest of the network would be unaffected, however. The software used 

in an EO is local. Thus, it is also not expected that a software failure at an EO would 

propagate further through the system.

Figure 5. The DNHR switching and signalling hierarchy.

The next level in the DNHR switching network is a tandem switch (TS), which connects 

local EOs and forms the long distance links (see Fig. 5). The loss of a tandem switch 

could sever long distance connections (assuming no alternate TS exists); however, it is 

more likely that another path would be found. With a TS out of service, traffic overflow 

could cause temporary blackouts. Local connections would remain largely intact due to 

direct connections between EOs. This could also mean continuation of 911 service.

2.1.1.2 Signaling System

The signaling layer checks for the availability of connections and routes, monitors traffic 

loads, and controls the system logic (Fig. 5 - dotted lines). At the lowest level is the 

EO

SSP

STP TS

SCP

EO = End Office

TS = Tandem Switch

SSP = Service Switch Point

STP = Signaling Transfer Point

SCP = Signaling Control Point

EO

SSP

EO

SSP
9

service switch point (SSP). If a service switch point (SSP) goes down, local customers 

could again be affected. One scenario is simply the loss of dial tone. Although EO 

switching equipment might be intact, the end user will not be able to complete a call if 

the dial tone is not initiated. If significant numbers of customers load the system trying to 

obtain a dial tone, the signaling system may experience blocking even in undamaged 

areas.

The next level is the signaling transfer point (STP). If an STP is disabled, then the status 

of the end user (line open, line busy) will not be clear and a call may not be completed. 

The worst scenario is if the final level, a signaling control point (SCP), is out. In this case 

a call could not be completed. The signaling net has dual redundancy. Dual STPs (and 

SCPs) are typically physically separated by at least 15 miles. Each dual system is 

designed to operate at 40% capacity under normal conditions. Thus, if one system goes 

down, the remaining system is designed to handle the full load at an 80% capacity level. 

However, the load may exceed capacity if sufficient extra users try to access the system 

(due to a station loss or a perceived emergency). 

Additional potential problems are regionally handled services such as credit card calling, 

800 numbers, Internet, and FAA and government circuits. These are usually handled at 

the SCP level and could be lost if the SCP went down.

In summary, the only portion of the route that does not have alternative connections is 

from the residence or business to the EO switch. If an EO were to suffer a disaster, the 

telephones from residences and businesses that are connected to this office’s switches 

would be disabled from the network. However, the loss of an EO (although disastrous to 

the customers connected to the office directly) does not result in a disaster for the overall 

PSTN. The worst case would be the loss of one or both STPs (dual redundancy) as these 

serve a very large number of users. A single STP is designed to have sufficient capacity 

to handle normal traffic should the other STP be disabled. But abnormal traffic levels or 

the loss of the second STP could black out the area served (for example a large metro 

area). It is not expected that the PSTN system would be affected beyond this level.

2.1.2 Internet

The Internet is structured much like the network shown in Figure 4, except that Internet 

messages are partitioned into a number of smaller packets before transmission. These 

packets are then reassembled at the destination to form the message. The packets of data 

do not rely on a “call setup” to construct a single route for the data to flow along from 

data source to data destination. Instead the packets can all travel different routes from 

source to destination. With many nodes able to correctly send the data to the proper 

destination, individual nodes can be removed from the network and the data flow will not 

be affected except for a loss of redundancy and capacity. The weak link is from the 

residence or business to the Internet, because as in the PSTN case there is typically no 

redundancy provided. Thus if the center where the lines to the Internet converge is 

affected by a disaster, these customers of the Internet will lose their access.
10

2.1.3 Cellular Mobile Radio Telephone

Cellular mobile telephone service derives its name from the many cells used to partition a 

large area to be covered. The partitioning of an area into cells allows for channel re-use 

via frequency, time, or code multiplexing, as well as for lower mobile transmitter power. 

Present cellular mobile radio systems are summarized in Table 1 [2]. Cellular service 

provides both PSTN and Internet features to mobile users. The important factor is that the 

frequencies of operation largely overlap the frequency range (0.8-30 GHz) of the RF 

devices assumed here. 

Table 1. A Comparison of Cellular Mobile Radio Systems

Parameter

AMPS MCS-L1

MCS-L2

NMT C450 TACS GSM PCN IS-54

TX Freq., MHz

 Base

 Mobile

869-894

824-849

870-885

925-940

935-960

890-915

461-466

451-456

935-960

890-915

890-915

935-960

1710-1785

1805-1880

869-894

824-849

Multiple Access FDMA FDMA FDMA FDMA FDMA TDMA TDMA TDMA

Duplex Method FDD FDD FDD FDD FDD FDD FDD FDD

Channel BW, kHz 30.0 25.0

12.5

12.5 20.0

10.0

25.0 200.0 200.0 30.0

Traffic Channels 

per RF Channel 1 1 1 1 1 8 16 3

Modulation Type

 Voice

 Data

PM

PSK

PM

PSK

PM

PPSK

PM

PSK

PM

PSK

GMSK

GMSK

GMSK

GMSK

π/4

π/4

Figure 6. The cellular mobile telephone service network.
11

Figure 6 shows cells of the system providing wireless signal coverage to mobile users. 

Control channels at the cell base stations transmit and receive signals from the mobile 

radios. When the mobile moves beyond the coverage of the serving cell, control channels 

of adjacent cells measure the received signal strength of the mobile transmitter and 

determine which cell should take over the call. Although there is some overlap of the 

signal coverage from adjacent cells, if the base station of a cell were disabled then 

adjacent cells could not provide complete redundant coverage, nor could they provide 

sufficient capacity to replace the disabled base station. Figure 6 also shows connections 

from the base stations of a group of cells to a mobile telephone switching office (MTSO). 

The number of cells that belong to a particular MTSO depends on the provider, the 

geographic area being served, and the MTSO manufacturer. Typically, an MTSO serves 

50 to 100 cell base stations. If an MTSO were disabled, then the service to all mobile 

users served by those cells would be terminated. Thus an individual cell’s base station or 

an MTSO would be the vulnerable points in the Cellular Mobile Telephone Service.

The above discussion in regards to the typical operation of a Cellular Mobile Telephone 

Service also relates to the operation of Personal Communications Service (PCS). PCS 

systems provide mobile user access to the PSTN and Internet but the cells are much 

smaller in size. This requires more cell base stations to cover the same geographical area 

as a cellular service, but the control and functions are similar between the two services.

2.1.4 Private Radio Networks (SMR, Public Safety, Law Enforcement)

Private radio networks are built and administered by the organizations that use them: 

such as public safety agencies (e.g., fire, police, etc.), land transportation industries (e.g., 

railroad, trucking, taxi, etc.), industrial services groups (e.g., business, power, petroleum, 

forest products, etc.), and specialized mobile radio (SMR) providers. The SMR providers 

build private radio networks for other users and charge a subscription for use. Table 2 

summarizes common dispatch systems worldwide [2]. Table 2 is not complete; however, 

like Table 1, it indicates that many dispatch systems operate within the 0.8 to 30 GHz 

frequency range considered critical here.

Table 2. A Comparison of Dispatch Systems

Parameter

US Sweden Japan Australia

TX Freq., MHz

 Base

 

 Mobile

935-940

851-866

824-849

806-821

76.0-77.5

81.0-82.5

850-860

905-915

865.00-870.00

415.55-418.05

820.00-825.00

406.10-408.60

Duplex Method FDD/semi, full FDD/semi FDD/semi FDD/semi, full

Channel BW, kHz 12.5

25.0

25.0 12.5 25.0

12.5

Modulation Type

 Voice

 Data

PM

PSK

PM

MSK-PM

PM

MSK-PM

PM

PSK
12

Private Radio Networks can be local to serve a community, they can cover a county or a 

state, or they can cover the entire U.S. (e.g., the railroad’s radio network). These 

networks are used for internal communications, control, and administration. They can but 

usually do not allow the mobile users to gain access to the PSTN or the Internet. 

Private radio networks share few common characteristics. There is no typical network 

design for the systems. The equipment is usually housed in buildings designed to protect 

the radio components from the elements but not extraordinary electromagnetic signals. 

The designs do not usually include redundant components, links, or signal coverage. In 

many cases, disrupting operations at one location could disable a local or county network. 

An added concern is that some of the national networks run all of their control and 

dispatch operations from one location.

2.2 Network Level Vulnerability Summary

As discussed above, the public telecommunications networks have built-in redundancy to 

mitigate transmission failures due to congestion, load, and equipment failures. If the only 

means of attacking the various networks is through physical or RF devices and not cyber 

methods, then the networks will suffer from drops in capacity and reliability but not in 

overall operation. If a single node is disrupted, then the network will continue to operate. 

The cellular networks have less redundancy in their designs than do the wireline 

networks. The private radio networks are the most vulnerable, as they are usually 

designed to provide a service or tool to the owning agency and the network is not the 

agency’s primary business or concern.

Almost all networks have a portion that is connected by a wireline or wireless link to the 

subscriber or user. These links are not redundant, so that if the link is disrupted or if the 

EO is disrupted, individual subscribers and users will lose service. Beyond this lowest 

level that directly connects the user with the network, most networks are robust and 

designed with high levels of redundancy.

3. NODAL LEVEL VULNERABILITY

The telecommunications networks discussed above all feature nodes where signals are 

collected or routed. For the PSTN and Internet networks the nodes are the various 

switching and signaling stations. For the cellular network the nodes are the base stations 

and MTSO stations. For the private radio and emergency networks the nodes are typically 

dispatch stations. This section will examine the vulnerability of the various node types to 

high power RF fields.
13

3.1 PSTN Switching and Signaling Stations

Switching and signaling stations are located within buildings of sufficient size to house 

the large amount of equipment involved. The susceptibility of telecommunications 

systems within buildings or other structures to external RF radiation is a function of the 

level of RF shielding provided by the building, the layout of the system within the 

building, and the components comprising the system. The latter two factors are highly 

complex and beyond the scope of this report. If we assume there exists some estimate of 

the field level inside a building necessary to cause disruption or damage to a particular 

system, then the RF shielding of the building can be used as part of a link analysis to 

determine the distance at which a given source poses a threat. This type of link analysis 

will not be done here as the RF source and antenna characteristics are not specified. This 

section seeks to make clear that in such a link analysis the RF shielding contribution of 

the building is minimal for the frequency range of interest. 

In general, electromagnetic energy can penetrate walls constructed of common building 

materials such as masonry, concrete, wood, and composites. Penetration is possible over 

a broad range of RF frequencies. RF radiation can also penetrate metal structures at 

seams, and at apertures such as windows, doors, and ventilation panels. Unless deliberate 

design and construction efforts are utilized to assure RF shielding over the threat 

frequency range, it must be assumed that the telecommunication equipment inside of a 

building is not protected by a particular structure, as discussed in Appendix A.

Strict RF shielding would mean that all or part of the volume housing sensitive 

equipment would be in a Faraday cage (i.e., a metallic enclosure). It is our understanding 

from discussions with representatives of telecommunications companies that RF 

shielding is not generally incorporated into the design of structures used to house 

telecommunications equipment. Building construction may range from wood in older 

facilities, to pre-cast concrete, to steel frame structures with non-load bearing walls. In 

addition to the electrical properties of the construction material, the overall shielding of a 

particular building will also depend on its geometry (shape, size compared to the incident 

wavelength) and location (ground parameters, surrounding terrain). Because these factors 

may vary widely it is difficult to construct a simple, yet general model for building 

penetration. However, the key component is the shielding effectiveness of building walls. 

The shielding properties of a given wall are difficult to generalize because of its finite 

size, the variability of its material composition, and the variability of the incident RF 

field. A useful approximation is to idealize the wall as a slab of infinite extent and given 

thickness, and the incident field as being a plane wave perpendicular to the wall. These 

idealizations reasonably approximate direct-path RF fields incident on a building and 

create a worst case scenario where the greatest RF field transmission through the wall 

occurs.

As an example of the shielding provided by a typical construction type, plane wave 

penetration of reinforced concrete (a re-bar mesh) is considered in Appendix A. The 

diameter and spacing of the re-bar is varied to highlight basic trends. The simulations 
14

show that reinforced concrete is a good shield (> 20 dB transmission loss) as long as the 

re-bar spacing is small compared to a wavelength. Once the re-bar spacing is on the order 

of a wavelength (or larger) the reinforced concrete provides little intrinsic shielding. For 

typical re-bar spacing this should occur above a few hundred MHz. Eventually, at very 

high frequencies (> 30 GHz) the attenuation through the concrete will again provide 

reasonable shielding (> 20 dB). The conclusion is that buildings made of reinforced 

concrete do not provide significant shielding from potential RF fields, particularly in the 

critical 1-5 GHz range. Buildings constructed from other materials will likely show 

similar results unless intentionally shielded. Thus, nodal sites housed in standard 

buildings are vulnerable to high power RF fields.

3.2 Cellular Sites

An overview of cellular mobile radio systems is shown in Table 1 [2]. Base station 

frequencies may be grouped into three bands: 461-466 MHz (C450), 869-960 MHz 

(AMPS, MCS, NMT, TACS, GSM, IS-54), and 1710-1785 MHz (PCN). The first band is 

well below the expected frequency range (0.8-30 GHz) for the assumed RF devices. The 

two higher bands may be subject to in-band excitation, however. The vulnerability of 

base stations and mobile switching centers is discussed in Sections 3.2.1 and 3.2.2. 

3.2.1 Base Stations

A base station consists of an antenna (or an array of antennas), an elevated mount (tower, 

rooftop, hilltop), an equipment enclosure (box, shed, building), a cable connection 

between the antenna and equipment enclosure, and either a cable, fiber, or microwave 

link to the MTSO (see Figure 6). An example of a cellular base station is shown in Figure 

7. 

The main points of entry are expected to be the antenna, the connecting cable, and the 

equipment enclosure. Direct antenna or front door coupling will depend on the in-band 

and out-of-band response of the antenna. Cable coupling is a function of the cable 

transfer impedance, the field polarization, and the cable location with respect to the 

mount. Equipment enclosure coupling will depend on the shielding properties of the 

enclosure, geometry and size, ventilation apertures, access panel gasketing, and any other 

breaks in the shield integrity. 

As most base station antennas are elevated with respect to the surrounding terrain, they 

will have some form of lightning protection. Typically, lightning protection consists of a 

low impedance ground path that shunts transient currents away from antennas, cables, 

and RF equipment. Grounding of exposed components and the use of surge arrestors in 

cable links are the primary protection measures. The base station grounding system 

should provide some protection against currents induced by high power RF fields. The 

key base station elements, antennas, cables, and RF equipment, are considered next.
15

3.2.1.1 Antennas

Antennas are intentional RF energy transceivers. Cellular base station antenna horizontal 

plane radiation patterns are either omni-directional (i.e., uniform 360 degree radiation 

pattern) or sectoral (e.g., covering a 120 degree pie shaped sector). In the vertical plane 

the beam is narrower and directed along the ground. A typical configuration is three 

antennas: a single transmit antenna and two receive antennas. The two receive antennas 

allow for diversity gain, which helps to compensate for the relatively weak mobile 

transmitter signal.

Figure 7. Typical mobile cellular radio telephone base station.
16

Outdoor antennas consist of metallic elements, linear or planar, often covered with some 

non-metallic weather protection. The antenna elements themselves are expected to be 

unaffected by high incident field levels. The antenna usually features a balun (balanced to 

unbalanced line transformer) or a matching circuit to transition from the antenna 

impedance (geometry dependent) to its load (typically 50 ohm RF equipment). Proper 

matching is necessary to achieve maximum power transfer. Most matching circuits are 

passive devices (pi-networks, transmatch circuits, matching stubs, quarter wave sections, 

etc.) and should be immune to high levels of RF energy. Thus it is not expected that high 

power RF fields can directly damage a cellular antenna.

The power received at the antenna pr may be analyzed using

2

2

(4 r)

g g

p

p r t

t

r

p

l

= , (1)

where pt is the transmitted power, gr and gt are the transmit and receive antenna gains 

compared to an isotropic antenna, r is the antenna separation in meters, and l is the free 

space wavelength in meters. In terms of dB (denoted by capital letters) this may be 

written

P (dBm) ≈ P (dBm) + G (dBi) + G (dBi) − 20log( f (MHz)) − 20log(r(m)) + 28 r t r t. (2)

These expressions will be of interest in Section 3.2.1.3 where we consider the power 

delivered by an antenna to an RF amplifier.

3.2.1.2 Cables

Coaxial cables (typically 50 ohm) are used to connect transmit and receive antennas to 

the RF equipment. Shielded cable is used to provide both interference and weather 

protection. Both lightning and RF fields can induce currents on the exterior of a coaxial 

cable. Cellular base stations often feature long runs of cable. The currents induced on the 

interior of the cable will depend on the basic shielding properties of the cable, cable 

interruptions (e.g., connections) and possible shield flaws (e.g., cracks or holes). The 

voltage induced in the interior of a shielded cable due to an external current is detailed in 

Appendix B. 

Two types of shielded cable are considered in Appendix B: rigid (i.e., tubular) and 

flexible (e.g., braided). Tubular coaxial cable is much like a pipe with a rigid outer 

conductor. It is often used because it can be pressurized to combat environmental 

contamination such as moisture. Flexible coaxial cable is more common. The outer and 

inner conductors are no longer rigid so that the cable can be bent, facilitating routing and 

connecting. The outer conductor is typically a metallic weave or a spiral wrap.
17

The analysis in Appendix B shows that, even for long runs of cable, the induced voltages 

will be insignificant if the shield integrity is intact. A more accurate estimate of cable 

coupling would involve accounting for the geometry of a specific station, the cable 

parameters, and the specifics of the incident fields. In addition, any breeches in the cable 

shield integrity and imperfect connections need to be considered. Based on the transfer 

quantities for tubular and braided cable, however, it is expected that good quality cable 

should not allow significant voltages to appear internally.

3.2.1.3 RF Equipment

The RF equipment at a base station can be affected by direct or indirect RF devicegenerated fields. Direct coupling will be through the front-door antennas used to transmit 

and receive mobile links. Indirect coupling will be via the antenna-to-RF-equipment 

connecting cables and via coupling through the enclosure housing the RF equipment. 

Cable coupling has been briefly discussed above and is not expected to be significant if 

good quality shielded cable is used. Enclosure coupling is more difficult to assess 

because of the variety of possible enclosures. However, the standard practice is to use a 

metal shed or box with metal access panels. Enclosures are not intentionally designed to 

be RF tight; thus, gasketing is typically non-conductive rubber or equivalent. Nonconductive gasketing performs well as a weather seal but will not ensure a good RF seal. 

Nonetheless, a metallic enclosure should act as a good shield. 

In addition most RF components are themselves housed in metallic cases. Most 

components will have been tested to FCC (Federal Communications Commission) 

emission standards (Part 15) and possibly to IEC (International Electrotechnical 

Commission) immunity standards (61000 series). These ensure that the RF equipment 

should not act as efficient antennas, either radiating or receiving. Thus, enclosure 

coupling will pose only a minor threat to the base station RF equipment. 

The main threat to base stations is direct in-band coupling (near the frequency of 

operation) via transmit and receive antennas. The signals received from mobile sources 

are often very weak. Thus, RF amplifiers are placed close to the antennas within the 

receiver circuit to boost these weak signals. Normally the amplifiers will be located in a 

shed along with the other RF equipment. However, as indicated in Figure 7, amplifiers 

may be mounted up on a tower to help compensate for long cable runs. RF amplifiers can 

be damaged if overloaded, that is, if the input signal is too large. Under normal conditions 

this is avoided by limiting the power output of the mobile user. Narrow band RF filters 

are also placed in the antenna-to-RF equipment link. These filters limit the out-of-band 

energy received by the antennas from unintended sources. Thus, the main problem is the 

in-band response of low noise amplifiers. Typical low noise amplifiers have overload 

thresholds on the order of 10-20 dBm.
18

As an example, let

1000 .

1 10

10

1 90

3

r m

f GHz MHz

G G dBi

P MW dBm

r t

t

=

= =

= =

= =

Then equation (2) yields

P (dBm) = 90 +10 +10 − 60 − 60 + 28 = 18 r .

As mentioned, the nominal input damage level for a typical low noise receiver is on the 

order of 10-20 dBm. Thus, the above example is near the damage threshold. For transmit 

powers significantly exceeding 1 MW, damage at ranges less than 1 km will be likely. 

The above example assumes only free space path loss. For areas where vegetation or 

structures block line of sight, the path losses will be higher. Nonetheless, the elevated 

location for most base station antennas means that a direct clear path will often be 

obtainable. Table 3 summarizes the above estimate (equation 2) for 900 MHz and 1800 

MHz systems. Transmit (Tx) and receive (Rx) antenna gains of 10 dBi are assumed. A 

variety of transmit powers and separations are considered. The table clearly shows that 

overloading base station amplifiers can easily be achieved by high power RF fields. It 

should also be noted that base stations can be easily jammed even at much lower power 

levels by transmitting above the power allowed for mobile sources. In summary, base 

stations are highly vulnerable to RF fields.

Table 3. Base Station Received Power versus Frequency, Transmitter Power, 

and Transmitter Distance

Frequency (MHz) 900 1800

Transmit Power (MW) 1 10 100 1000 1 10 100 1000

Tx/Rx Distance (m) Received Power (dBm)

100 37 47 57 67 31 41 51 61

300 27 37 47 57 21 31 41 51

500 23 33 43 53 17 27 37 47

1000 17 27 37 47 11 21 31 41

2000 11 22 32 42 5 15 25 35

3000 7 17 27 37 1 11 21 31
19

3.2.2 Mobile Telephone Switching Offices

MTSOs are similar to the PSTN switching and signaling stations. Thus, the same 

discussion applies (see Section 3.1), namely, it should be assumed that building 

construction does not provide shielding from potential RF fields.

3.3 Private Radio Networks Dispatch Stations

Dispatch systems occupy numerous bands in the spectrum. For example, police radio 

bands exist in the bands 30-50 MHz, 150-174 MHz, 406-470 MHz, and 764-940 MHz [3-

5]. Table 2 gives a summary of some dispatch systems worldwide.

The discussion of cellular base station vulnerability (Section 3.2.1.3) also applies to 

dispatch base stations as these are configured in a very similar manner. Thus, it is 

expected that the primary threat is the in-band response of the system via direct antenna 

coupling and the resulting potential to overload RF amplifiers. Assuming that high power 

RF fields have a nominal 800 MHz lower frequency limit, then most dispatch systems 

operating below 500 MHz will be safe. Dispatch systems in bands near to 1 GHz will be 

highly vulnerable, however. 

3.4 Nodal Level Vulnerability Summary

Neither PSTN switching stations nor cellular base stations are intentionally hardened 

against high level RF fields. As a result, both are vulnerable to high-power RF fields. 

PSTN switching stations are complex installations and the probability of a disruption is 

hard to predict. Because typical re-bar and concrete construction provides negligible 

shielding above a few hundred MHz, it can be assumed that fields will penetrate such 

buildings with the potential to couple to sensitive equipment. Cellular and dispatch base 

stations are highly vulnerable to high power devices operating in their pass band.

4. CONCLUSIONS

This report has briefly examined the vulnerability of public and private 

telecommunications systems to be upset and/or damaged by high power RF fields. The 

basic conclusions reached are:

- Network vulnerability: 

ß The overall public telecommunications network has sufficient redundancy and 

capacity to withstand the loss of even multiple nodes. A full nationwide system 

collapse is not envisioned as a possible scenario.
20

ß Loss of equipment or the failure of software at a node within the public switched 

telephone network is not likely to cascade further within the system. Nonetheless, 

the loss of certain key system nodes could lead to large local blackouts, such as a 

metro region (greater Denver for example).

ß The loss of a base station would largely black out the associated cell. However, 

adjacent cells should still function.

ß The loss of a mobile telephone switching center could black out up to 100 base 

stations; however, it is not expected that the upset will propagate further within 

the system.

- Nodal vulnerability: 

ß Neither public network switching stations nor cellular base stations are 

intentionally hardened against high level RF fields. 

ß The shielding provided by typical concrete and re-bar construction is negligible 

for wavelengths on the order of, or smaller than, the re-bar spacing.

ß A switching station could be disrupted, although the probability of disruption is 

difficult to assess due to the variety and complexity of building layouts. The loss 

of a switching station would black out the end users connected to that station 

(typically 10-100 thousand). 

ß Wireless base stations are highly vulnerable to high power devices operating 

within the base station pass band. This conclusion is based on free space link 

budget calculations. These should be valid approximations for raised base 

stations. Direct coupling into the system via transmit and receive antennas could 

easily damage RF amplifiers. The loss of a base station would largely black out 

the associated cell. However, adjacent cells will still function.

ß Dispatch system base stations are highly vulnerable to high power devices 

operating within the pass band. To the extent that dispatch systems are stand alone 

they will not affect other telecommunications systems.

These conclusions are based on a review of telecommunications networks, discussions 

with providers, and basic estimates of the coupling of RF energy into systems. However, 

to more accurately assess the threat posed by high power RF fields, more detailed 

modeling and measurements are needed. In particular, statistics on the transfer function 

relating a source external to a building and equipment internal to a building need to be 

developed. This would more accurately define the threat to computer processors and 

other equipment located within buildings.
21

5. REFERENCES

[1] M. Wik, R. Gardner and W. Radasky, “Electromagnetic terrorism and adverse 

effects of high power electromagnetic environments,” in Proc. 13th Intl. Zurich 

Symp. and Techn. Exhb. on EMC, Zurich, Switzerland, 1999, pp. 181-185.

[2] J. Gibson, Ed., The Mobile Communications Handbook, Boca Raton, FL: CRC 

Press, 1996, Ch. 15.

[3] G. Leon and L. Sands, Dial 911: Modern Emergency Communications Networks, 

Rochelle Park, NJ: Hayden Book Company, 1975.

[4] Code of Federal Regulations, Title 47-Telecommunication, Part 90, Private Land 

Mobile Radio Services, Oct. 1996 (Revision).

[5] NTIA, Manual of Regulations and Procedures for Federal Radio Frequency 

Management, Sept. 1995 (Edition).

[6] K. Yee, “Numerical solution of initial boundary value problems involving 

Maxwell's equations in isotropic media,” IEEE Transactions on Antennas and 

Propagation, AP-14, No. 3, pp. 302-307, May 1966.

[7] A. Taflove, Computational Electrodynamics: The Finite-Difference Time-Domain 

Method, Boston, MA: Artech House, 1995.

[8] K. Lee, Ed., EMP Interaction: Principles, Techniques, and Reference Data, 

Washington, DC: Hemisphere Publishing, 1986, Ch. 2.
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This Page Intentionally Left Blank
23

APPENDIX A: PLANE WAVE PENETRATION OF REINFORCED CONCRETE

The shielding effectiveness of reinforced concrete is examined in this Appendix. 

Reinforced concrete is a typical wall composition and thus serves as a representative 

example of the level of shielding that can be expected from many buildings housing 

sensitive equipment. To estimate the inherent shielding due to a reinforced concrete wall, 

the finite-difference time-domain (FDTD) technique can be used to calculate reflection 

and transmission coefficients for reinforced concrete walls as a function of re-bar 

spacing, diameter, wall thickness, and frequency. An idealized reinforced concrete wall is 

characterized by its thickness W, the period of the re-bar lattice P, the re-bar diameter D, 

and the electrical properties of the concrete, as indicated in Figure A1. The results of this 

analysis are given in terms of the reflection and transmission coefficients for a normally 

incident plane wave. The transmission coefficient T is defined by Et = TE0 where E0 is 

the incident electric field and Et is the transmitted electric field. The reflection coefficient 

R is defined by Er = RE0 where Er is the reflected electric field.

Figure A1. Reinforced concrete wall and FDTD computational volume.
24

In general, the FDTD technique requires the volume of the computational space (i.e., the 

reinforced concrete wall and the air region on either side of the wall) to be subdivided 

into unit rectangular parallelopiped cells. The algorithm is implemented using a staggered 

grid in both space and time [6-7]. For this calculation, the scattering object was included 

by setting the conductivity to 1.95 mS/m and the relative permittivity to 6 at cell locations 

occupied by the concrete wall and setting the tangential electric fields on the boundaries 

of the re-bar equal to zero. These electrical parameters are typical for concrete at 1 GHz.

A normally incident plane wave is introduced into the computational volume by using a 

numerical implementation of the electromagnetic equivalence principle. The equivalent 

electric and magnetic currents on a Huygens surface outside of the scattering object are 

set so as to produce the desired plane wave fields inside the surface (in the absence of the 

concrete wall) and zero field outside the surface. Thus when the scatterer is introduced, 

the numerical code produces the total field (incident + scattered) inside the surface and 

the scattered field outside of the surface. The electric field is aligned with one of the rebar axes for these simulations. A time-domain pulse with a useful bandwidth that 

exceeded 6 GHz was used as the forcing function for the Huygens surface.

The size of the spatial increment is governed by two requirements. First, the finite 

difference grid needs to resolve the highest frequency of interest. This is usually 

accomplished by using at least 10 cells per wavelength at this frequency. Secondly, the 

cells need to be small enough to resolve all scattering objects in the computational 

volume (re-bar in the present case). For this calculation, sizing the finite difference cells 

to resolve the re-bar was the more restrictive condition.

For the cell sizes required to resolve the re-bar (on the order of millimeters), the 

computational volume must be kept as small as possible so as not to exceed computer 

resources. This is accomplished by exploiting the periodic structure of the concrete wall 

and using a normally incident plane wave polarized parallel to the re-bar. Under these 

conditions, symmetric boundary conditions are applied in two spatial directions and 

hence, two dimensions of the computational volume were reduced to half a period of the 

structure. Absorbing boundary conditions are applied on boundaries that are not planes of 

symmetry.

Figure A2 gives the transmission coefficient for a re-bar structure having a period of 

15.24 cm (6”), re-bar diameter of 1.91 cm (3/4”), and wall thickness of 20.32 cm (8”). 

Below 250 MHz very little electric field is transmitted. For frequencies above 500 MHz 

there is a complicated resonance pattern; however, the average electric field attenuation is 

only on the order of 3-5 dB. 

Two effects influence shielding: re-bar reflection and concrete wall transmission. First, 

lower frequency fields are reflected by the re-bar structure. If the wavelength is very 

large compared to the re-bar spacing, then the re-bar grid acts much like a solid shield 

(Faraday cage) allowing very little field penetration. As the wavelength decreases 

(frequency increases) and nears the dimension of the re-bar spacing, the field penetration 

increases. Full transmission occurs when the re-bar spacing is on the order of a 
25

wavelength in the concrete, lconcrete = p-D, where p and D are defined in Figure A1. For 

the parameters here this occurs around 890 MHz. Second, a lossless slab in air will have 

zero reflection when its thickness equals half the wavelength in the material. Thus, 

although the concrete slab will have some attenuation due to its conductivity, a 

transmission peak is expected to occur when lconcrete/2 = W. For the parameters here this 

occurs around 300 MHz. Thus, the first resonance near 300 MHz is due to the concrete 

slab thickness. The resonance is somewhat attenuated due to the re-bar shielding. The 

next resonance near 600 MHz is the second slab resonance. Above 900 MHz, the two 

effects combine in a complicated manner; however, the re-bar no longer provides 

significant shielding. The slight decrease in average transmission coefficient above 2 

GHz is due to attenuation loss in the concrete. 

-50

-40

-30

-20

-10

0

0 2000 4000 6000

Frequency (MHz)

Amplitude (dB)

Figure A2. Transmission coefficients for a concrete wall with a 2 dimensional re-bar

lattice: P = 15.24 cm, D = 1.91 cm, and W = 20.32 cm.

If the re-bar spacing is reduced, then the re-bar pass frequency will increase. Figure A3 

shows results for the parameters as in Figure A2 but with the re-bar spacing halved to 

7.62 cm (3”). The re-bar pass frequency will increase to approximately 2140 MHz 

(lconcrete = p-D). The first local transmission peak in Figure A3 still occurs near the 

concrete wall pass frequency (300 MHz) but is still significantly attenuated by re-bar 

reflection (<10 dB). Further slab resonances occur near 800 MHz and 1300 MHz.

Increasing the re-bar thickness will improve shielding somewhat. Figure A4 shows the 

transmission coefficient for the Figure A2 parameters with the re-bar diameter increased 

to 5.08 cm (2”). This reduces the spacing to 10.16 cm (vs. 13.33 cm) and increases the rebar pass frequency to approximately 1200 MHz (vs. 890 MHz). However, using thinner 

re-bar at a closer spacing is the more viable method to improve shielding.

The shielding effectiveness of reinforced concrete walls depends on the bandwidth and 

frequency of the potential RF fields, the particular wall structure, and electrical 
26

properties. Changes in these variables can significantly alter the reflection and 

transmission properties of the structure. Hence, reflection and transmission properties 

based on single frequency measurements or calculations for a particular wall structure 

cannot be reliably extrapolated to predict the characteristics for similar structures with 

different physical dimensions or frequencies. The conclusion here is that the shielding 

effectiveness of a reinforced concrete structure is quite variable and cannot be reliably 

determined without a priori knowledge of the RF frequency and specific construction 

methods. In general, it should be assumed that building construction does not provide 

shielding from potential RF fields, particularly in the range 1-5 GHz.

-50

-40

-30

-20

-10

0

0 2000 4000 6000

Frequency (MHz)

Amplitude (dB)

Figure A3. Transmission coefficients for a concrete wall with a 2 dimensional re-bar

lattice: P = 7.62 cm, D = 1.91 cm, and W = 20.32 cm.

-50

-40

-30

-20

-10

0

0 2000 4000 6000

Frequency (MHz)

Amplitude (dB)

Figure A4. Transmission coefficients for a concrete wall with a 2 dimensional re-bar

lattice: P = 15.24 cm, D = 5.08 cm, and W = 20.32 cm.
27

APPENDIX B: CABLE COUPLING

Electromagnetic fields incident on a shielded cable will excite currents on the cable. The 

current excitation will depend on the polarization of the fields with respect to the cable 

and the strength of the fields. These external currents will then excite voltages internal to 

the cable. These internal voltages will appear at sensitive electronics as an undesired 

signal. Damage is possible if the voltage strength is large.

Cable coupling is most easily analyzed in the spectral domain. The time domain response 

can be found by multiplying the spectral domain response of the cable with the spectral 

domain representation of the excitation field (the pulsed RF) and then transforming back 

to the time domain. In the spectral domain (s-domain, s = jw) the per-unit-length (along 

the cable direction – here defined as the z-axis) voltages (V) and currents (I) induced 

inside the cable shield are related to the total (exterior plus interior) current (IT) and perunit-length total charge (QT) on the cables by

( , ) 0

( , )

( , ) ( ) ( , )

( , ) ( ) ( , )

+ =

= Ω

=

sQ z s

dz

dI z s

I z s s Q s z

V z s Z s I s z

T

T

t T

t T

(B1)

where Zt is the transfer impedance per-unit-length and Wt is the charge transfer frequency 

[8]. The transfer impedance relates the current induced on the cable exterior to the 

voltage induced within the cable and thus is a key parameter. Low transfer impedance 

indicates a well shielded cable. The currents and charges induced on the cable exterior 

will depend on the cable geometry, the presence of ground planes and other nearby 

conductors (the cable may be routed along a tower leg), and the polarization of the 

incident field. The time domain response can be found by multiplying the spectral 

domain response of the cable with the spectral domain representation of the excitation 

field (the pulsed RF) and then transforming back to the time domain. Two typical cable 

types, tubular and braided, will be considered next.

B1. Tubular Cable

For a tubular cable shield with conductivity s, and inner and outer radii b and c

respectively (shield thickness d = c-b), assuming d/b << 1 and d/c << 1 (thin wall), the 

transfer impedance and charge transfer frequency are given by

g m s

ps

g

g

0

2

1

sinh

( )

s

d bc

R

d

d

Z s R

dc

t dc

=

=

≈

 , (B2)
28

where Rdc is the dc shield resistance, Ω = 0 t, and m0 is the free space permeability. As the 

shield material approaches that of a perfect conductor (s → ∞ ) the transfer impedance 

goes to zero, → 0 Zt, implying zero internal induced voltage. Thus a good metallic 

shield will protect against high frequency RF coupling.

10

-2

10

-1

10

0

10

1

10

2

10

-2

10

-1

10

0

|gamma*d|

|Zt/Rdc|

Figure B1. Magnitude of the normalized transfer impedance t Rdc Z (s) / versus gd 

(gamma*d) for a tubular cable.

Figure B1 shows the transfer impedance normalized to its dc value for a range of skin 

depth to wall thickness ratios (equivalent to gd). As an example, assume a metal tube with 

conductivity 7 s = 10 (S/m) and wall thickness d = 1 (mm), and let the frequency f = 1 

GHz. These values yield 2

gd ≈ 2.8(10) which is well above the range shown in the 

graph and yields a normalized transfer impedance on the order of (10)-83. Thus, tubular 

coaxial cables should provide high levels of shielding for RF device frequencies. It is not 

expected that a solid tubular shield itself will allow any significant voltages to appear 

internally. However, coupling could still occur at connection points.

B2. Braided Cable

A braided cable is depicted in Figure B2. Useful approximations for the braided cable 

transfer quantities are given by [8] as

[(1 ) , ]

ln( / )

( )

[(1 ) , ]

2 sinh 2

1

( )

3/ 2

0 3/ 2

y

p

y

m

g

g

ps

t d

t

h K f

N b a

s

s

g K

N

s

d

d

bdK

Z s

−

−

Ω ≈

≈ + −

(3)
29

d j

d

d

f

e e

e

+

=

2

where K is the optical coverage of the braid, N is the number of carriers (bands of shield 

wires) in the braid, a and b are the inner and outer radii of the cable, d is the braid 

thickness typically taken as the diameter of the braid wire, y is the pitch angle of the 

woven braid (see Fig. B2), g and h are functions of the braid geometry, and ed and ej are 

the relative dielectric constants of the internal cable dielectric and the jacket respectively. 

Tables B1 and B2 give g and h for some typical parameters. 

Figure B2. A braided shield (a) and its surface (b).

As an example consider standard RG8 cable with the following parameters: a = 1 mm, b 

= 5 mm, d = 0.73 mm, ed = 3, ej = 6, 7 s = 10 (S/m), K = 0.95, y = 45°, and N = 8. If as 

above we assume the frequency f = 1 GHz, then Zt(s) ≈ 0 + j4.9 and 6(s) j7(10) Ωt ≈ − .

Zt has a positive imaginary part indicating that coupling for this braided cable will be 

primarily inductive. Although the impedance is low, long runs of cable could mean that 

some voltage would appear at the cable end. The charge transfer frequency is well below 

the operating frequency and will not contribute significantly.
30

An accurate estimate of cable coupling would involve accounting for the geometry of a 

specific station (length of cable, nearby metallic structures), the cable parameters, the 

specifics of the incident field, and any breeches in the cable shield integrity or imperfect 

connections. However, based on the transfer quantities for tubular and braided cable it is 

expected that a good quality cable should not allow significant voltages to appear 

internally.

Table B1. The Function [(1 ) , ]

3/ 2

g − K y for Some Typical Values

(1-K)3/2

y 0.01 0.02 0.03 0.04 0.05

(x 10-2)

5° 0.539 1.091 1.657 2.236 2.829

10° 0.555 1.117 1.686 2.261 2.844

15° 0.584 1.173 1.767 2.366 2.969

20° 0.621 1.246 1.874 2.507 3.143

25° 0.667 1.337 2.010 2.687 3.368

30° 0.727 1.457 2.190 2.928 3.668

35° 0.807 1.618 2.433 3.253 4.076

40° 0.910 1.825 2.745 3.671 4.603

45° 1.045 1.098 3.158 4.226 5.303

Table B2. The Function [(1 ) , ]

3/ 2

h − K f

d y for Some Typical Values

(1-K)3/2fd

y 0.01 0.02 0.03 0.04 0.05

(x 10-2)

5° 0.521 1.036 1.545 2.049 2.547

10° 0.502 1.001 1.496 1.987 2.475

15° 0.491 0.987 1.463 1.944 2.423

20° 0.480 0.956 1.431 1.902 2.371

25° 0.471 0.939 1.404 1.866 2.326

30° 0.463 0.922 1.379 1.834 2.285

35° 0.458 0.912 1.364 1.813 2.259

40° 0.455 0.906 1.355 1.801 2.244

45° 0.454 0.904 1.352 1.796 2.238