NIOSH Firefighter Radio Communications 

TriData Corporation 29 September 2003 

CHAPTER IV: STRUCTURE COMMUNICATIONS ISSUES 

Buildings and other structures pose difficult problems for wireless (radio) 

communications. Whether communication is via hand-held radio or personal cellular 

phone, communications to, from, and within structures can degrade depending on a 

variety of factors. These factors include multipath effects, reflection from coated exterior 

glass, non-line-of-sight path loss, and signal absorption in the building construction 

materials, among others. The communications problems may be compounded by lack of a 

repeater to amplify and retransmit the signal or by poor placement of the repeater. RF 

propagation in structures can be so poor that there may be areas where the signal is 

virtually nonexistent, rendering radio communication impossible. Those who design and 

select firefighter communications systems cannot dictate what building materials or 

methods are used in structures, but they can conduct research and select the radio system 

designs and deployments that provide significantly improved radio communications in 

this extremely difficult environment.4

Communication Problems Inherent in Structures 

MULTIPATH

Multipath fading and noise is a major cause of poor radio performance. Multipath is a 

phenomenon that results from the fact that a transmitted signal does not arrive at the 

receiver solely from a single straight line-of-sight path. Because there are obstacles in the 

path of a transmitted radio signal, the signal may be reflected multiple times and in 

multiple paths, and arrive at the receiver from various directions along various paths, 

with various signal strengths per path. In fact, a radio signal received by a firefighter 

within a building is rarely a signal that traveled directly by line of sight from the 

transmitter. In general, the radio signal received by a firefighter’s radio is the 

combination of a number of radio signals from different directions. This phenomenon can 

have the effect of nulling or canceling the signal of interest and result in a temporary loss 

of the radio link. The sources of these various component radio signals are: 

• Diffraction – occurs when radio signals hit the edge of an object causing the radio 

waves to bend around the object. 

• Reflections – occurs when radio signals hit an object that acts as a conductor, 

such as a metal post, or a dielectric; a perfect conductor reflects all RF signals 

while a perfect insulator reflects none.5

• Scattering – occurs when radio signals hit nearby small objects such as lighting 

fixtures or even bumps on a rough wall. 

The individual signals caused by reflection, diffraction, and scattering that arrive at the 

firefighters radio are collectively known as multipath signals. Multipath is a set of 

 

4

 It is, however, not inconceivable that building features affecting emergency communications might be a 

consideration in building design or even codes in the future. 

5

 Neither perfect conductors nor perfect insulators exist. In the real world, most materials act as dielectrics. 
NIOSH Firefighter Radio Communications Chapter IV 

TriData Corporation September 2003 30

reflections and direct waves, with each of these multipath signals traveling a different 

path to the radio. These paths will have different lengths and different number of 

reflections and are characterized by varying signal strengths caused by passing through 

materials that reflect (and absorb) differing amounts of energy. The results are 

unpredictable delays in transmission and increased attenuation and time dispersion, or a 

“smearing” or spreading out of the signal 

When these various signals combine in the receiving radio, the randomly phased signals 

add together (Figure 1).6 Depending on the exact phases the resulting radio signal can 

increase or essentially disappear. This effect is called multipath fading. Even in cases in 

which there is a strong signal and a direct line of sight to the receiver, it may be 

impossible to use a radio due to multipath fading. For example, radio wave propagation 

inside a building with materials that reflect RF energy efficiently (e.g., metallic materials) 

can have areas where the signal essentially disappears because the reflections are large 

enough to effectively cancel the direct radio signal. 

FIGURE 1: MULTIPATH REFLECTIONS COMBINE AT THE RECEIVER

Source: IEEE Internet Computing Online. 

http://www.computer.org/internet/v6n1/fig3.htm. 

Multipath signals (that is, reflection, scattering, and diffraction) also cause radio signal 

distortion (noise) as well as additional signal propagation losses (attenuation). Figure 2 

shows two key properties of multipath fading, amplitude and period. The fading 

amplitude is the amount of signal amplification or reduction and is a function of position. 

Fading period, the time between fades, is a function of speed (both the firefighters’ 

movement and the speed of moving objects, if any, from which multipath signals are 

reflecting). Because fading level varies by physical position, frequently a firefighter 

needs only move a few inches to get a 20-dB change in signal, for better or worse. [243] 

Advanced automatic gain control (AGC) techniques have been developed to compensate 

for this effect, thereby increasing the operational dynamic range of the receiver. Even 

 

6

 Radio waves are characterized as sine waves. When the signal is reflected off of a surface, it travels a 

longer path to its destination. As a result, the reflected radio wave may arrive at the receiver at a different 

point in the sine cycle. This cycle offset is referred to as phase. 
NIOSH Firefighter Radio Communications Chapter IV 

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these systems, however, usually operate with fixed time constants that cannot fully 

compensate for the effects when the signals vary quickly. 

On the other hand, because the firefighter is moving slowly and the objects around him 

remain stationary, the fading period can be long. In other words, a slowly moving 

firefighter can remain in a fade area. In digital communications, this situation can lead to 

periods of time where the error rates are too high to be corrected by error-correcting 

algorithms and result in unintelligible voices or lost communications. In analog systems, 

this fading phenomenon is exhibited in noise and static. The human brain acts as the 

correction algorithm in this case. But the brain tries to synthetically “fill in the gaps” 

based on contextual factors and, while it performs this error correction at a remarkable 

level, the listener may misconstrue messages. 

FIGURE 2: FADING RATE AND AMPLITUDE

Source: Georgia Institute of Technology, Smart Antenna Research Laboratory 

http://users.ece.gatech.edu/~mai/tutorial_multipath.htm 

It is impossible to find a building that does not have multipath reflections, diffraction 

around sharp edges, or scattering from wall, window, ceiling, or floor surfaces. As noted 

earlier, a perfect conductor reflects 100 percent of RF energy while a perfect insulator 

reflects none. In practice, most materials act like dielectrics that partially reflect (with a 

180-degree phase shift) and partially refract (transmits through the material and then 

bends) signals incident upon them. Metallic walls are the best reflectors in buildings, but 

in practice even insulating materials cause significant reflections. An all-wood or allfiberglass structure is better for radio propagation than the typical building, but it too will 

still have reflections, multipath, and other radio propagation disturbances that make 

communication worse than outdoors. 

Propagation effects also become worse in large buildings, such as high rises, or in other 

large, complex structures. As the number of obstacles the radio signal passes through 

increases, such as the successive floors of a high-rise building, the signal degrades from 

multipath reflection, diffraction, and scattering. The signal strength will vary 

considerably from position to position on a particular floor as a result of the changing 

phases and signal strengths of the various multipath signals from within the building and 
NIOSH Firefighter Radio Communications Chapter IV 

TriData Corporation September 2003 32

from nearby buildings. [290] Consider the transmission geometries as a signal propagates 

from ground level to a receiver located on the opposite side of a high-rise building. In this 

case, the signal strength will degrade significantly. Houses and other smaller residential 

structures are typically less problematical as they are constructed largely of insulators that 

reflect less efficiently, such as wood, and have fewer reflective surfaces (less floors, 

furniture, and walls). Unfortunately, it is presently impossible to effectively reduce 

multipath through building design. 

In general, multipath fading is a major contributor to inadequate radio operation inside 

buildings. 

NON-LINE-OF-SIGHT PATH LOSS

One of the largest causes of signal degradation is non-line-of-site (NLOS) path loss. As 

discussed above, it is common for communications in structures that no direct path exists 

from the transmitting radio to the receiving radio. Signals radiating from the transmitting 

antenna reach the receiving radio only by hitting obstacles and reflecting. In a simple 

structure such as a residential building, it may only take one or two reflections to reach 

the receiving radio. In a more complex building, the reflections can number orders of 

magnitude higher. The result of the reflections is an increase in distance that the signal 

travels between transmitter and receiver. In outdoors situations, the path losses are 

inversely proportional to the distance traveled raised to the fourth. Indoor path loss is 

higher.If the path traveled is doubled by reflections, the loss will be at least 16 times 

greater than if the direct path had existed. The NLOS path distance could easily be much 

more than twice the direct path in a high rise. 

ABSORPTION

Another source of poor radio performance can be signal absorption by building materials. 

NIST has performed extensive studies of electromagnetic signal attenuation in 

construction material from 0.5 to 8.0 GHz. [78,79,80] The results have shown that the 

majority of building materials are relatively transparent to electromagnetic signals for the 

application of using RF to survey through walls. For example, a 1-milliwatt (mW) 

broadband pulse (0.5 to 2.0 GHz) will penetrate slightly less than 2.5 feet of reinforced 

concrete and still be above the noise floor of a typical receiver. Wood or concrete block, 

with its hollow cores, absorbs much less signal than poured concrete and, therefore is 

easier to penetrate and causes less problems to radio communications. In general, signal 

attenuation from absorption increases with increasing frequency. Thus, lower frequencies 

such as very low frequency (VLF) and medium frequency (MF) bands will penetrate 

building material and even earth to a considerable distance, while 800-MHz ultra high 

frequencies (UHF) will not. Unfortunately, due to historical factors and spectrum use 

regulated by the FCC, firefighter radio communications generally operate at frequencies 

between 100 and 900 MHz. With these frequencies, cumulative signal attenuation can be 

a serious problem. 
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BUILDING CONSTRUCTION

Modern construction methods and materials for large commercial buildings have changed 

quite substantially and can be a source of poor radio performance. Curtain-wall 

construction (today's approach for large high-rise buildings) means that the walls are 

“hung” from the structure (typically steel). The walls are generally glass, frequently 

reflective to radio signals. The glass material used in the windows themselves is RF 

friendly, but the windows usually contain tinting using a metallic-based composite 

embedded in the glass which can significantly shield and reflect radio signals. Even when 

pre-cast concrete panels are used as the “curtain,” there will be lots of “ribbon” window 

area. Steel or some other metal will be the superstructure. The steel may become a 

problem at frequencies where wavelengths are twice the distance between the structural 

beams or longer. The reflective coatings on the large window surfaces, usually metallic, 

are also culprits in poor radio performance. 

The large number of stories that modern construction methods allow also affects radio 

performance. Building height contributes several factors that degrade communication in 

structures: NLOS propagation to and from the exterior ground level; and propagation 

through more building materials (floors, walls, etc.) which increases the absorption, 

reflection, and refraction of the radio signals off and through these materials. These 

factors increase distances traveled by signals and exacerbate multipath fading conditions 

and create the need for repeaters (see below). 

By contrast, even though older buildings are usually thick-walled construction they often 

are more easily penetrated by radio waves because of the different materials used in their 

construction. Support for upper stories may also include a substructure separate from the 

walled construction, but not to the same extent as current high-rise construction. Many 

older buildings have thicker walls, because the walls are the primary support. However, 

buildings constructed in this manner usually are not tall, perhaps four or five stories. 

Older buildings with thick masonry walls, especially of concrete, can cause radio 

reception problems from absorption. But because their construction limits the number of 

stories, radio signals need only pass through a few floors to reach the firefighters within. 

On the surface, it would appear that transmission in older concrete-based buildings 

should not be a problem; however, many older buildings indeed are not conducive to 

radio communications. While each building is unique and requires careful modeling to 

fully understand the propagation characteristics, several common factors can impede 

radio transmission within these structures, including: 

• Many older buildings have replaced windows with newer tinted products. 

• Fiberglass insulation, even in wood structures, often has a metallic backing. 

• Older lead-based paints used on interior walls may still exist in the under-layers 

of paint. 

• Metallic furniture is still in common use today. This furniture includes desks, 

shelving, and cabinets—virtually all office file cabinets are constructed from 

metal. 
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TriData Corporation September 2003 34

• In addition to rebar, concrete reinforcing wire mesh may be used in older 

buildings to provide increased strength and to mitigate building damage from 

environmental hazards such as earthquakes. While the mesh in general may 

somewhat impede radio transmission, in some cases it may be worse than others 

depending on the size and geometry of the mesh patterns in relation to the 

wavelength of the radio transmission. 

• The majority of internal wall construction today in office buildings today use 

drywall-over-steel framing material. Internal office build-out in even the oldest 

buildings frequently changes with new tenants or new requirements. 

So even in structures where the inherent building properties may appear to be more RF 

friendly, multiple factors can affect transmission 

Studies have found that placement of repeaters that amplify and retransmit radio signals 

can have a large impact on coverage within a building. [244,290] For example, a repeater 

positioned such that the radio waves strike the building façade directly at a 90-degree 

angle results in very little energy reflecting from the surface. Except for the absorption 

characteristic of the material, the radio signal passes through to the building interior. 

Signals that graze the surface of a building have greater reflection off the façade and 

fewer signals entering the building. The result can be poor radio coverage even on a floor 

at the same height as the repeater. Experience has shown that placing a repeater on the 

roof of a building can provide the best coverage in many cases as the signal enters the 

building at a near 90-degree angle and avoids the typical coated, reflective glass on 

today’s modern office buildings. [244,290] 

RADIO SYSTEM CONSIDERATIONS

The problems inherent in radio communications into, from, and within structures should 

be addressed on several fronts whenever possible. Some of the options are: 

• Radio system design – Provide redundant paths for each receiver if at all possible. 

[308] 

• Antenna system design – Accommodate dual-diversity antennas used at each 

receiver at a minimum. Provide sufficient repeater antenna gain for hand-held 

radio transmissions to be received by the incident commander. Place repeater 

antennas properly. [308] 

• Signal/waveform design – Incorporate techniques that mitigate multipath, such as 

spread spectrum radio design with high signal-to-channel bandwidth ratio. 

Example spread spectrum waveforms currently in use to combat multipath are 

CDMA (common in cellular systems), ultra wideband (UWB) in development for 

robust in-building communications and “see-through walls” radar products, 

frequency hopping (available in emergency personnel evacuation paging 

systems), and OFDM (used for television broadcast applications and in-building 

radio location systems currently being developed). Other options include using 

narrowband multipath reduction digital signal processing algorithms, such as 
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equalization. These algorithms essentially function as AGC circuits that are 

designed and implemented in digital signal processors. These algorithms (and 

circuits) can remove the deep amplitude notches that otherwise occur from 

fading.7

[308] 

• Building/environment design – Firefighters cannot dictate building methods or 

materials, but they may be able to influence modifications for their use. For 

example, leaky feeder cable could be built into structures for firefighters, HVAC 

systems could be specially modified to act as waveguides [164] for firefighter 

communications, or a method to communicate by coupling RF signals onto the ac 

power lines in a structure could be provided. [308] 

Signal absorption can be addressed directly if it is possible to select a radio frequency 

that has minimal absorption in construction materials or has other characteristics to allow 

penetration of structures. Medium frequency waves penetrate building materials more 

readily and can parasitically couple to building conductors such as telephone lines or ac 

power lines, both of which can act as a distributed antenna within the structure. [115, 

116,128,129,130,175,198,211,288] Another way to address signal absorption is to begin 

with more radio transmit power and higher gain antennas. A mobile repeater properly 

sited at a fire scene will help get signal into a building, but because the firefighters’ handheld radios have limited transmit power, the repeater needs to have a high-gain antennas 

to enhance the reception of communications from the firefighters within the structure. 

In general, the radio systems used by firefighters do not have the required features for 

operation in difficult indoor areas. 

Risk to Firefighters 

As a result of the radio communications problems inherent in structures, high-rise 

structures, as well as warehouses and other non-residential structures, pose the greatest 

challenges to firefighter radio communication. These buildings tend to be those where 

communication problems are of most concern. While fires are fewer in number in these 

properties than in typical one- and two-family dwellings, firefighters are at greatest risk 

in these buildings. As illustrated in Figure 3, basic industry, manufacturing, vacant 

buildings and construction sites, retail stores and commercial offices, storage facilities, 

and apartment buildings rank higher in risk of injury to firefighters than the typical home. 

 

7

 The AGC design and associated time constants need to be carefully selected as to not impair the signal-tonoise ratio of the signal or saturate components in the receiver chain. 
NIOSH Firefighter Radio Communications Chapter IV 

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FIGURE 3: FIREFIGHTER INJURY RATE IN STRUCTURES

Source: Fire in the United States 1989-1998, United States Fire Administration 

Radio Spectrum Considerations 

The radio spectrum is the lowest end of the larger electromagnetic spectrum. This energy 

spectrum ranges from radio waves to gamma rays. Radio waves, visible light, X-rays, and 

all the other parts of the electromagnetic spectrum are fundamentally the same thing. 

They are all electromagnetic radiation that travels and spreads out as it goes (called 

propagation) and can be expressed in terms of energy, wavelength, or frequency. 

RADIO FREQUENCY BANDS

The radio spectrum is segregated into frequency bands, each with propagation 

characteristics and regulated uses. The use of these frequencies bands in the U.S. is 

licensed and regulated by the Federal Communications Commission (FCC). The use of 

all radio frequencies (and equipment) is determined by the rules and regulations issued by 

the FCC in conjunction with the National Telecommunications and Information 

Administration (NTIA). All radio users must abide by them. Much of the current 

assignment of frequencies is founded on historical use and not necessarily on the 

optimum assignment of the spectrum, based on the physical properties of the spectrum, to 

specific applications. 
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FIGURE 4: RADIO SPECTRUM

Source: Adapted from Allocation of Radio Spectrum in the United States, 

http://www.jneuhaus.com/fccindex/spectrum.html#table_of_contents 

Lower frequencies propagate further than higher frequencies, both through the air and 

into the ground. For propagation through the air, the spectrum can be divided into three 

sub-spectra by the type of propagation exhibited: 

• Surface: The longest electromagnetic waves, below 500 kHz, tend to follow the 

curvature of the Earth, guided between the Earth and the ionized layers of the 

upper atmosphere (i.e., the ionosphere). 

• Sky: High frequency waves, 3–30 MHz, are reflected by the ionosphere and are 

capable of propagation around the Earth. 

• Line-of-sight: Above 1 GHz, electromagnetic waves behave much like light 

propagating through a clear, reasonably uniform atmosphere. When originating 

from a point source, they propagate in all directions. 

The intermediate frequencies from 500 kHz to 3 MHz and 30 MHz to 1 GHz are 

transition bands where the propagation characteristics in the atmosphere are more 

complex. 

Despite the propagation advantages, a disadvantage of the lower frequency bands, 

however, is the limited amount of available spectrum. At very low frequency (VLF) the 

spectrum available for public safety radio is only 27 kHz. This is barely enough to pass a 

single analog voice channel. As a result, this band is primarily used for applications that 

only require low-speed data transmission. By contrast, the super high frequency (SHF) 

band has 27 GHz of spectrum, a million times the available spectrum at VLF, but it 

suffers from poor propagation characteristics. Also, radio equipment, particularly 

antennas, is frequently larger at lower frequencies. As a result, the equipment to support 

the use of these frequencies may be too cumbersome to be practical on the fireground. 

Table 6 shows some of the basic uses of the frequency bands and some of the more 

relevant propagation characteristics. 

ELF

 30 

km 

| 3 

km 

| 300 

m 

| 30

m

| 3

m

| 30

cm

| 3

cm

| 3 

mm 

| 

VLF LF MF HF VHF UHF SHF EHF Infrared Visible

Light

 | 

10 

kHz | 

100 

kHz | 

1 

MHz | 

10

MHz |

100

MHz |

1

GHz |

10

GHz | 

100 

GHz 

 Surface Waves Sky Waves Line-of-Sight Waves 
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TABLE 6: RADIO FREQUENCY BANDS

Band 

Frequency 

Range Wavelength General Applications 

Major Propagation 

Characteristics 

ELF 300–3000 Hz 100–1000 km 

Navigation, long distance 

communication with 

submarines 

Wave tube between 

Earth surface and the 

ionosphere 

VLF 3–30 kHz 10–100 km 

Navigation, long distance 

communication, emergency 

paging 

Ground propagation; 

stable 

LF 30–300 kHz 1–10 km Navigation, long distance 

communication with ships 

Ground propagation; 

stable 

MF 300–3000 kHz 100–1000 m 

AM broadcasting, radio 

navigation, powerline 

communications, mine 

voice communications 

Ground-wave, sky-wave 

propagation; fading 

HF 3–30 MHz 10–100 m 

Radio broadcasting, fixed 

point-to-point (around the 

world) 

Large perturbation, 

reflection in ionosphere 

VHF 30–300 MHz 1–10 m Radio & TV broadcasting, 

mobile services Diffraction 

UHF 300–3000 MHz 10–100 cm 

Cellular telephony (GSM, 

NMT, AMPS), fixed pointto-point, satellite, radar 

Shadowing by mountains 

and building 

SHF 3–30 GHz 1–10 cm 

Broadband indoor systems, 

microwave links, satellite 

communications 

Attenuation due to rain, 

snow, and fog 

EHF 30–300 GHz 1–10 mm LOS communication (short 

distance or satellite) 

Attenuation due to rain, 

snow, and gases 

Source: Adapted from http://www.s3.kth.se/radio/COURSES/RKBASIC_2E1511_2001/lecture_1.pdf. 

Satellites operate at ultra high frequency (UHF), super high frequency (SHF), and 

extremely high frequency (EHF). Table 7 shows the various frequencies and examples of 

some of the major satellites that operate in these bands. The frequencies represent typical 

uplink and downlink transmissions. 

TABLE 7: SATELLITE FREQUENCY BANDS

Band Frequency 

Range Bandwidth User Satellites 

 200 – 400 MHz 160 Hz Military FLTSAT, LEASAT UHF 

L 1.5 – 1.6 GHz 47 MHz Commercial MARISAT, INMARSAT 

C 6/4 GHz 800 MHz Commercial INTELSAT, DOMSATS, 

ANIK E 

X 8/7 GHz 500 MHz Military DSCS, SKYNET, NATO 

Ku 14/12 GHz 500 MHz Commercial INTELSAT, DOMSATS, 

ANIK E 

2500 MHz Commercial JCS 

SHF 

Ka 30/20 GHz 

1000 MHz Military DSCS IV 
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Band Frequency 

Range Bandwidth User Satellites 

Q 44/20 GHz 3500 MHz Military/DOT MILSTAR EHF 

V 64/59 GHz 5000 MHz Military CROSSLINKS 

The applications and potential applications of these frequencies are undergoing a rapid 

transformation with the growth of technological developments occurring in modern 

communications. Recent developments in digital signal processing algorithms and 

powerful, inexpensive, small hardware allow communication systems to be built that can 

use the spectrum in new ways. Systems can be built that use the narrow bands at the 

lower frequency end of the spectrum to transfer much more data or many more voice 

channels than previously possible. Another new technology, ultra wideband 

communications, uses broad swaths of the spectrum across more than one band. These 

new technologies are forcing the FCC to consider changes in spectrum allocations to 

accommodate the new spectrum users. In spite of the apparent abundance of spectrum for 

many applications, the lack of sufficient and appropriate spectrum remains a major issue. 

In June 1995, Congress empowered the Public Safety Wireless Advisory Committee 

(PSWAC) to specifically address issues relating to spectrum needs and uses for the public 

safety community. The PSWAC determined a short-term need for an additional 2.5 MHz 

of spectrum, and long-term need for an additional 25 MHz of spectrum for public safety 

use. The Balanced Budget Act of 1997 mandated the reallocation of 24 MHz of broadcast 

television spectrum by December 31, 2006, for public safety uses. However, this will not 

solve the frequency congestion issues in the near term as the Act includes provisions for 

extensions beyond 2006 if the penetration of digital television service in a given market is 

less than 85 percent. 

As a result, the uncertainty of when and if this spectrum will become available severely 

limits planning by the public safety sector and causes reluctance in manufacturers to 

invest in products and technologies for this spectrum. 

The current bands and primary allocated users are outlined below: 

Extremely Low Frequency (ELF) – The ELF band is primarily used by the Navy to 

contact submarines deep under water, depths that normal radio waves cannot penetrate. 

(Water absorbs high frequencies.) The low frequency, however, cannot broadcast much 

useful information. As such, the Navy only uses the ELF system to signal their 

submarines to come closer to the surface, and then broadcasts the actual message by other 

means. There has been some controversy about the human health effects of ELF. This is a 

relatively new band designation. 

Very Low Frequency (VLF) – The VLF band is very narrow, only 27 kHz, and currently 

allocated for uses such as low-data-rate power line communications by a power utility for 

general supervision of the power system and cable locating equipment. Radio waves in 

this band can propagate by surface waves over long distances in the air and ground waves 

penetrating long distances through the earth. At one time, these frequencies were used by 

commercial interests to transmit and receive the majority of “wireless” message traffic in 
NIOSH Firefighter Radio Communications Chapter IV 

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a large network of stations, but today only a handful of military and government stations 

remain on the air on a regular basis. The narrowness of this band has limited the use of 

the VLF band for fireground communications to applications such as a one-way 

emergency evacuation paging system. 

Low Frequency (LF) – The LF band covers 270 kHz from 30 to 300 kHz and is currently 

used for applications such as marine and aviation navigation beacons, low-data-rate 

power line communications, and cable locating equipment. This band is also used by 

European broadcast radio for its ability to “hug the earth,” eliminating shadowing by 

mountains. 

Medium Frequency (MF) – The MF band is from 300 kHz to 3 MHz. This band currently 

includes AM radio (535 to 1605 kHz or almost half of the band), radio navigation, and 

various others. Land mobile public safety (fire, police, etc.) agencies are allocated several 

hundred kilohertz in this band. MF signals are useful in that they can propagate over 

fairly substantial distances (e.g., two mobile units could easily talk to one another with 

tens of miles of separation). However, the signals may have problems propagating 

through steel-reinforced structures. MF RF has been shown to have an ability to 

inductively or parasitically couple to conductors such as telephone and ac power lines or 

the steel tracks used for underground mining vehicles. [115,116,128,129,130,175,198, 

211,288] This type of coupling has been used to communicate for thousands of feet in 

underground mines. Miners and cave explorers have used MF for communications 

because of its ability to travel through the earth and solid rock. 

High Frequency (HF) – The HF band is used for long-distance communications because 

of its sky-wave propagation mode. The band is allocated in large part to maritime 

radiotelephone, amateur radio, and flight test stations. The new broadband power line 

communications products for in-house LAN use (e.g., HomePlug 1.0) and backhaul to the 

Internet, access power line communications (PLC), over the medium-voltage power grid 

operate in this band as well. Organizations such as the American Radio Relay League 

(ARRL) are fighting access PLC use of the HF band with the FCC. 

Very High Frequency (VHF) – Radio astronomy, cordless telephones, amateur radio, FM 

radio, television, aviation, and public/private land mobile make up a large portion of this 

band. 

Ultra High Frequency (UHF) – Radio location, new 700-MHz public safety band, 

television, satellite communications, military and other government communications, 

maritime communications, and public and private land mobile are some of the users of 

this band. 

Super High Frequency (SHF) – Radio location, radio navigation, point-to-point 

microwave, and satellites are users of this band. This band has unlicensed frequencies. 

Extremely High Frequency (EHF) – Specialized satellite. Much of this band is proposed 

for services but is not currently allocated. 
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PROPAGATION

For propagation through earth, building material, and structures, the picture is complex 

and not easily categorized. As noted under the discussion of absorption, a significant 

amount of work has been done by NIST on RF transmission through building materials 

for its program on RF surveying for construction automation. [78,79,80] In addition to 

work on signal attenuation through construction materials, the NIST study also 

characterized power attenuation in relation to material thickness, electrical permittivity, 

and dielectric constant as a function of frequency. These construction materials included 

various brick configurations, various mixtures and thicknesses of concrete, interior 

finishing materials, glass, and reinforced concrete. Measurements of the frequency 

dependency of the material dielectric and conductivity constants were curtailed due to 

funding cuts. A sample of the data taken is shown in Figure 5 for the transmission 

coefficient8 of a 102-mm-thick section of concrete vs. frequency. 

FIGURE 5: TRANSMISSION COEFFICIENT FOR CONCRETE MIXTURE #5 

Source: Adapted from NIST Construction Automation Program Report No. 3: Electromagnetic Signal 

Attenuation in Construction Materials, National Institute of Standards and Technology, 

Gaithersburg, MD. October 1997. 

The frequency dependency of RF transmissions in building material does not necessarily 

mean that the same is true for penetration into structures. Conductive screens with 

 

8

 The transmission coefficient is a measure of absorption. The higher the transmission coefficient, the lower 

the absorption in the material. 
NIOSH Firefighter Radio Communications Chapter IV 

TriData Corporation September 2003 42

openings of approximately one-half the RF wavelength or smaller attenuate the RF waves 

heavily. Typical examples of potential barriers to RF signals would be rebar grids in 

reinforced concrete or the steel support structures used in modern curtain wall buildings. 

An example of this effect is shown in Figure 6 for transmission through two different 

rebar grid patterns. [79] Both rebar grids are square patterns. Grid 1 had 140-mm spacing 

and grid 2 had 70-mm spacing. Looking at one frequency, 1.5 GHz, grid 1 is a little 

larger than a half wavelength (1/2 x 200 mm), but grid 2 is smaller. The figure shows that 

grid 1 is relatively transparent to this frequency but grid 2 attenuates it heavily, over 10 

dB more than the bigger grid. The figure also shows that the attenuation increases with 

frequency as expected. 

FIGURE 6: RF ATTENUATION THROUGH REBAR GRID

Source: Adapted from NIST Construction Automation Program Report No. 3: Electromagnetic Signal 

Attenuation in Construction Materials, National Institute of Standards and Technology, 

Gaithersburg, MD. October 1997.