What is the difference between dBm and dBi?

dBm is a unit of absolute power referenced to 1 milliwatt — it tells you how strong a signal actually is (for example, -95 dBm is a common minimum usable signal threshold for public safety radio). dBi is a unit of relative antenna gain referenced to a theoretical isotropic radiator — it tells you how much an antenna concentrates energy in a given direction compared to radiating equally in all directions. You cannot add dBm and dBi as if they were the same unit conceptually, but in a link budget calculation you do add them arithmetically: transmitter power (dBm) plus antenna gain (dBi) minus cable loss (dB) gives you EIRP, still expressed in dBm.

What signal strength is required for an in-building public safety radio system?

Most jurisdictions adopting IFC Chapter 51 or NFPA 1225 require a minimum of -95 dBm signal strength in at least 95% of the general building area and 99% of critical areas (fire command center, elevator lobbies, exit stairs, fire pump rooms), measured with a delivered audio quality (DAQ) of 3.0 or better. The exact thresholds, testing methodology, and percentage requirements are set by the locally adopted code edition and the authority having jurisdiction (AHJ), so always confirm the specific requirements with the local fire marshal before design.

What is the difference between a BDA and a DAS?

A BDA (bidirectional amplifier) is the active electronic component that amplifies the radio signal in both directions. A DAS (distributed antenna system) is the passive or hybrid network of cabling, splitters, combiners, and antennas that distributes the amplified signal throughout the building. In practice, a complete in-building radio system almost always consists of a BDA feeding a DAS — the two terms describe different parts of the same overall system, not competing alternatives.

What is the FCC 100 dB gain limit for public safety BDAs?

FCC rules (and IFC provisions that reference them) limit the maximum gain of a Class A BDA used in a public safety system to prevent it from radiating enough power to interfere with the outdoor radio network the system is meant to supplement. The specific numeric limit and how it is measured (composite gain versus per-channel gain) depends on the FCC Part 90 rules in effect and the BDA type; designers size donor antenna signal strength and amplifier gain together so the system stays within the allowed limit while still meeting the -95 dBm in-building coverage target.

What is the difference between P25 and DMR?

Both are digital land mobile radio standards, but they were developed by different bodies for different markets. P25 (Project 25/APCO-25) was developed specifically for North American public safety agencies and is the dominant standard for U.S. police, fire, and EMS trunked radio systems. DMR (Digital Mobile Radio) is a European Telecommunications Standards Institute (ETSI) standard more common in commercial, industrial, and international markets, using TDMA to fit two voice channels into the bandwidth of one analog channel. The two standards are not natively interoperable without a gateway device.

Why does building material matter so much for in-building radio coverage?

Different construction materials impose very different building penetration loss (BPL) values on an RF signal. Wood-frame and older masonry buildings might impose only 10–15 dB of loss, while modern low-E coated glass, metal stud/gypsum assemblies, and reinforced concrete tilt-up construction can impose 20–35+ dB of loss per wall. Because the link budget is a straight subtraction of losses from available signal, a building with high-BPL construction may need a full BDA/DAS system even in an area with otherwise strong outdoor coverage, while a low-BPL building in the same location might pass with only a donor antenna and no amplification at all.

Electrical·13 min read·August 29, 2026

📡 Radio Communications Terminology Glossary: 55 Essential Terms Explained

Plain-language definitions of 55 essential radio and wireless communications terms — dBm, EIRP, BDA, DAS, ERRCS, link budget, path loss, P25, spectrum, and more, with standard and code references.

📖Radio Communications — Illustrated Guide (Full Access)

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Why These Terms Matter

Radio and wireless communications engineering blends RF physics, code compliance, and telecommunications jargon that rarely overlaps with other engineering disciplines. A fire alarm designer knows NFPA 72; a structural engineer knows ASCE 7 — but in-building public safety radio systems, distributed antenna systems (DAS), and land mobile radio (LMR) networks draw on their own vocabulary defined across FCC rules, IFC/NFPA codes, and telecommunications standards like TIA/EIA. Misreading a term like "path loss" as "attenuation" only, or confusing dBm with dBi, leads to undersized amplifiers, failed acceptance tests, and coverage that does not meet the 95%/99% signal strength thresholds required for life-safety systems.

This glossary covers 55 of the most frequently used terms across public safety in-building radio, distributed antenna systems, land mobile radio, and general RF engineering — organized alphabetically with the code, standard, or industry reference where each term is formally defined or commonly applied.

A

Amplifier (Bidirectional Amplifier, BDA) — IFC Chapter 51 / NFPA 1225: An active RF device that receives a weak signal, amplifies it, and retransmits it in both directions — from the donor antenna/signal source to the building (downlink) and from the building back out (uplink). BDAs are the core component of an in-building public safety radio system, boosting the outdoor donor signal so first responders get usable coverage inside a building. Class A BDAs channelize the specific frequencies used by the local public safety agency; Class B BDAs amplify a broader frequency band.
Antenna Gain — expressed in dBi or dBd: The degree to which an antenna concentrates radio energy in a particular direction compared to a reference antenna, expressed in decibels. dBi compares to a theoretical isotropic radiator (radiating equally in all directions); dBd compares to a half-wave dipole (dBi = dBd + 2.15). Higher gain antennas focus energy more narrowly, which is useful for donor antennas aimed at a distant tower but undesirable for indoor coverage antennas that need to radiate broadly.
ARRL: American Radio Relay League — the national association for amateur ("ham") radio in the United States. Not a regulatory body (the FCC regulates spectrum), but the leading advocacy and technical resource organization for licensed amateur radio operators.

B

Bandwidth: The range of frequencies a signal, channel, or system occupies, measured in Hz, kHz, or MHz. Narrower bandwidth channels (12.5 kHz, common in P25 narrowband systems) pack more channels into a given spectrum allocation but carry less data per channel than wider bandwidth channels (25 kHz analog or wideband LTE channels).
Building Penetration Loss (BPL): The reduction in signal strength (measured in dB) that occurs when an RF signal passes through a building's exterior walls, windows, and roof. Low-E glass, metal studs, concrete tilt-up walls, and EIFS stucco systems can each impose 15–30+ dB of loss. BPL is a critical input to the link budget calculation that determines whether a donor antenna alone can meet in-building coverage requirements or whether a full DAS/BDA system is needed.

C

Class A / Class B (BDA classification) — 47 CFR Part 90 / IFC 510: FCC classifications for signal boosters. Class A devices are channelized — they amplify only specific, narrow frequency bands assigned to a particular licensee (typically public safety). Class B devices amplify a wider "broadband" swath of spectrum. Most jurisdictions require Class A BDAs for public safety in-building systems because they reduce the risk of interference to adjacent channels and other licensees.
Coaxial Cable: A shielded transmission line consisting of a center conductor, dielectric insulator, outer conductive shield, and protective jacket, used to carry RF signals between radios, amplifiers, and antennas with minimal signal leakage or external interference. Common types in DAS work include LMR-400, 1/2-inch and 7/8-inch heliax (corrugated copper) cable — larger diameter cable has lower loss per foot but is more expensive and harder to route.
Combiner: A passive RF device that merges multiple signal sources (for example, several frequency-specific amplifier outputs) onto a single antenna feed line without those signals interfering with each other, typically at the cost of a few dB of insertion loss per combined path.
Coupler (Directional Coupler): A passive RF component that samples a small, known fraction of the power flowing through a transmission line — used for test points, signal monitoring, and tapping a small amount of signal off a trunk line to feed a branch without significantly degrading the main signal path.

D

DAS (Distributed Antenna System): A network of spatially separated antenna elements connected to a common signal source via coaxial cable or fiber, designed to provide radio coverage throughout a building or campus that a single antenna could not reach. DAS can carry public safety radio, cellular carrier signals, or both (a "combined" or "neutral host" DAS). Passive DAS uses only cable and splitters; active DAS uses fiber and remote radio units to overcome cable loss over large distances.
dBd: Decibels relative to a half-wave dipole antenna — see Antenna Gain.
dBi: Decibels relative to an isotropic radiator — see Antenna Gain.
dBm: A logarithmic unit of power referenced to one milliwatt (0 dBm = 1 mW). Used to express received and transmitted signal strength in radio systems because RF power spans many orders of magnitude — 0 dBm is 1 mW, -30 dBm is 1 microwatt, and -95 dBm (a common minimum coverage threshold for public safety radio) is an extremely small fraction of a milliwatt. dBm values are additive with dB gain/loss figures, which is what makes link budget math tractable.
Donor Antenna: The outdoor antenna that captures the usable outdoor radio signal (from a public safety tower or cellular macro site) and feeds it to the BDA for amplification and distribution inside the building. Donor antenna placement and orientation — aimed at the strongest, cleanest available signal — is one of the first design decisions in any in-building radio system.
Duplexer: A passive device that allows a single antenna to be used simultaneously for both transmitting and receiving on different frequencies, by filtering and isolating the transmit and receive paths so the much stronger transmit signal does not desensitize or damage the receiver.

E

EIRP (Effective Isotropic Radiated Power): The total power that would need to be radiated by a theoretical isotropic antenna to produce the same signal strength observed in the direction of an antenna's peak gain. EIRP = transmitter power + antenna gain − cable/connector losses (all in dB terms, or multiplied in linear terms). The FCC and local code officials cap EIRP for in-building BDA systems (commonly 100 dB gain limits per IFC and specific EIRP ceilings per antenna) to prevent interference with the outdoor radio network.
ERRCS (Emergency Responder Radio Communication System) — IFC Chapter 51 / NFPA 1225: The formal code term for an in-building system (typically DAS + BDA) installed to ensure fire department and other first-responder radios work reliably throughout a building, including areas below grade, in stairwells, and in areas with heavy RF-blocking construction. ERRCS requirements are triggered by building size, occupancy type, and measured signal strength deficiencies, and are enforced by the local fire code official (AHJ), not the FCC.

F

Fade Margin: The extra signal strength margin, above the minimum usable level, designed into a radio link to accommodate temporary signal degradation from weather, multipath fading, foliage growth, or equipment aging. A link budget with no fade margin may pass acceptance testing on a clear day and fail during rain fade or seasonal foliage changes.
FCC (Federal Communications Commission): The U.S. federal agency that regulates interstate and international radio, television, wire, satellite, and cable communications, including allocating and licensing radio spectrum. Part 90 governs private land mobile radio; Part 90.219 specifically governs in-building signal boosters (BDAs) used for public safety and commercial systems.
Frequency Coordination: The process of registering a radio system's operating frequencies with a regional or national coordinating body (or directly with the FCC) to minimize interference with other licensed users operating on the same or adjacent frequencies in the same geographic area.

G

Ground Plane: A conductive surface (metal panel, vehicle roof, or the earth itself) that an antenna uses as an electrical reference/mirror to complete its radiation pattern. Many antenna designs (such as a quarter-wave monopole) require an adequate ground plane to perform as specified; without one, gain and radiation pattern suffer significantly.

I

Insertion Loss: The reduction in signal power that occurs when a signal passes through a passive component (cable, splitter, coupler, combiner, connector) compared to a direct connection, expressed in dB. Every passive component in a DAS signal path contributes insertion loss that must be tallied and subtracted in the link budget.
Intermodulation Distortion (IMD/PIM): Unwanted signals generated when two or more RF signals mix in a nonlinear component (an amplifier, a corroded connector, or even a loose metal joint), producing spurious frequencies that were not present in the original signals. Passive intermodulation (PIM) from poor connector torque or corrosion is a leading cause of DAS performance problems and acceptance test failures.
ISM Band: Industrial, Scientific, and Medical radio bands (900 MHz, 2.4 GHz, 5.8 GHz in the U.S.) that the FCC allocates for unlicensed use, subject to power limits — the bands used by Wi-Fi, Bluetooth, and many IoT devices, distinct from the licensed spectrum used by public safety and cellular carriers.

L

Link Budget: A systematic accounting of every gain and loss in a radio path — transmitter power, cable loss, connector loss, splitter/combiner loss, antenna gain, building penetration loss, and free-space path loss — used to predict the received signal strength at a given point and confirm it exceeds the minimum required threshold (commonly -95 dBm for public safety DAS design). The link budget is the fundamental calculation underlying every DAS and BDA system design.
LMR (Land Mobile Radio): Two-way radio systems used by public safety agencies, utilities, and businesses for mobile and portable voice (and increasingly data) communication, as distinct from cellular/commercial wireless networks. P25 and DMR are the two dominant digital LMR standards in North America.

M

MHz / GHz: Megahertz (millions of cycles per second) and gigahertz (billions of cycles per second) — units of radio frequency. Public safety land mobile radio in the U.S. commonly operates in VHF (150–174 MHz), UHF (450–512 MHz), 700 MHz, and 800 MHz bands; cellular and Wi-Fi systems commonly use 600 MHz–6 GHz.
Multipath: The phenomenon where a radio signal reaches a receiver via multiple paths (direct plus reflections off walls, floors, and structures) with slightly different arrival times and phases, causing constructive or destructive interference that produces signal fading and distortion — especially significant indoors and in dense urban environments.

N

NFPA 1225: The National Fire Protection Association standard (which absorbed and superseded NFPA 1221 provisions relevant to in-building coverage) covering emergency communications systems, including in-building public safety radio coverage requirements, testing, and documentation — frequently referenced alongside or in place of IFC Chapter 51 depending on jurisdiction adoption.
Noise Floor: The aggregate level of background RF noise (thermal noise, electronic interference, and unintended signals) present in a given frequency band at a given location, which sets the practical lower limit on how weak a signal can be and still be reliably received above the noise. A rising noise floor from RF-noisy equipment (LED drivers, switching power supplies, poorly shielded electronics) can degrade radio performance even when the desired signal strength itself is unchanged.

O

Oscillation (RF Oscillation): An unstable, self-sustaining feedback condition in a BDA/DAS system that occurs when too much of the amplified downlink signal leaks back into the uplink path (or vice versa) with enough gain to sustain itself, similar to audio feedback from a microphone too close to a speaker. Oscillation can degrade system performance, generate interference, and in severe cases damage amplifier components; proper isolation between donor and server antennas prevents it.

P

P25 (Project 25 / APCO-25): A suite of digital radio communications standards developed by public safety agencies in North America (through APCO, the Association of Public-Safety Communications Officials) to ensure interoperability between digital LMR equipment from different manufacturers. P25 Phase 1 uses 12.5 kHz FDMA channels; P25 Phase 2 uses 6.25 kHz-equivalent TDMA channels, doubling capacity within the same spectrum.
Path Loss: The reduction in signal power as an RF wave travels from transmitter to receiver through free space and any intervening obstructions, combining free-space path loss (which increases with distance and frequency per the inverse-square law) with additional losses from terrain, foliage, and building materials. Path loss is the largest single term in most link budget calculations.
Propagation Model: A mathematical or empirical model (such as Okumura-Hata, COST-231, or ray-tracing software) used to predict how radio signals will propagate through a given environment, accounting for frequency, distance, terrain, and clutter — used in the design phase to predict coverage before a system is built and tested.

R

Repeater: A radio device that receives a signal on one frequency and simultaneously retransmits it on another (or the same) frequency at higher power, typically from an elevated location, to extend the effective range of a radio system — conceptually similar to a BDA but typically a standalone outdoor system rather than an in-building distribution system.
RF (Radio Frequency): The general term for electromagnetic energy in the frequency range roughly 3 kHz to 300 GHz used for wireless communication, as distinct from lower-frequency electrical power or higher-frequency infrared/visible light.
RSSI (Received Signal Strength Indicator): A relative measurement, usually expressed in dBm, of the power level of a received radio signal, used by radios, spectrum analyzers, and DAS monitoring systems to quantify how strong a signal is at a given test point.

S

Simulcast: A radio system architecture where multiple transmitter sites broadcast the same signal simultaneously on the same frequency, precisely time-synchronized, to provide seamless wide-area coverage without requiring the user's radio to switch channels between sites.
Spectrum: The full range of electromagnetic frequencies available for radio communication, allocated by the FCC (in the U.S.) into bands licensed for specific uses — public safety, broadcast, cellular, satellite, amateur radio, and unlicensed ISM bands — to prevent interference between unrelated users.
Splitter: A passive RF device that divides an input signal among two or more output ports, each carrying a fraction of the input power (a 2-way splitter imposes roughly 3.5 dB of insertion loss per leg). Splitters are used throughout passive DAS designs to distribute signal from a BDA to multiple antenna branches.
Squelch: A circuit function in a radio receiver that mutes audio output when the received signal falls below a set threshold, preventing the user from hearing constant background noise when no valid signal is present.
Standing Wave Ratio (VSWR): A measure of how well an antenna or load is impedance-matched to its transmission line, expressed as a ratio (ideal is 1:1). A poor VSWR (caused by a damaged cable, bad connector, or mismatched antenna) reflects power back toward the transmitter instead of radiating it, wasting power and potentially damaging amplifier output stages.

T

TIA/EIA-603: A Telecommunications Industry Association standard defining minimum performance requirements for land mobile radio equipment, commonly referenced in procurement specifications for LMR transceivers.
Trunked Radio System: A radio system architecture where a pool of frequencies is shared dynamically among many user groups (rather than each group having a permanently dedicated channel), with a computer controller automatically assigning an available channel to each call — dramatically increasing the effective capacity of a limited number of frequencies. Most modern county and regional public safety radio systems are trunked P25 systems.

U

UHF (Ultra High Frequency): The radio frequency band from 300 MHz to 3 GHz. In U.S. public safety use, "UHF" commonly refers specifically to the 450–512 MHz land mobile radio band, distinct from the higher 700/800 MHz public safety bands and the lower VHF band.

V

VHF (Very High Frequency): The radio frequency band from 30 MHz to 300 MHz. U.S. public safety VHF operations typically occupy the 150–174 MHz range; VHF signals generally propagate farther over open terrain than UHF but penetrate buildings less effectively.

Topics covered

radio communications terminologyradio engineering glossaryRF terms explainedDAS terminologyBDA glossaryERRCS termslink budget definitiondBm vs dBiEIRP definitionpath loss explainedIFC Chapter 51 termsNFPA 1225 terminologyP25 radio glossaryFCC spectrum termsin-building wireless glossarypublic safety radio termsantenna terminologycoaxial cable RF terms