Decibels and Link Budgets

Why Decibels Matter in RF Work

Radio-frequency (RF) systems deal with signal levels that range from hundreds of watts at a transmitter output down to millionths of a watt at a receiver input. Multiplying and dividing those numbers by hand is error-prone. The decibel (dB) solves this by converting multiplication and division into addition and subtraction. Every RF technician—and every candidate for the ETA CET, ETA FOI/FOT, and BICSI Installer credentials—must be fluent with decibel arithmetic.

The Decibel Defined

A decibel is one-tenth of a bel, a logarithmic unit named after Alexander Graham Bell. There are two forms depending on whether you are comparing power or voltage.

Power ratio:

> dB = 10 × log₁₀(P₂ / P₁)

Voltage ratio (measured across equal impedances):

> dB = 20 × log₁₀(V₂ / V₁)

The factor doubles from 10 to 20 for voltage because power is proportional to the square of voltage (P = V²/R), and log(x²) = 2 log(x).

Key reference points to memorize:

• Doubling power = +3 dB; halving power = −3 dB
• Ten times the power = +10 dB; one-tenth = −10 dB
• Doubling voltage = +6 dB

A positive dB value means a gain (output larger than input). A negative dB value means a loss (output smaller than input).

Absolute Reference Units: dBm

Plain dB is a ratio between two quantities. When you need an absolute power level, you reference a fixed standard. The most common in RF work is dBm, where the reference is 1 milliwatt (1 mW).

> dBm = 10 × log₁₀(P_mW / 1 mW)

Examples:

• 1 mW = 0 dBm
• 10 mW = +10 dBm
• 0.1 mW = −10 dBm
• 1 W (1000 mW) = +30 dBm

Using dBm, gains and losses along a signal path are simply added or subtracted in sequence.

Antenna Gain Units: dBi and dBd

Antennas concentrate energy in preferred directions. Gain is always relative to a reference antenna:

• dBi — gain compared to a theoretical isotropic radiator (a point source that radiates equally in all directions). This is the most common reference in specifications.
• dBd — gain compared to a half-wave dipole antenna in free space.

Because a dipole itself has approximately 2.15 dB of gain over an isotropic source:

> dBi = dBd + 2.15

So a 5 dBd antenna is equivalent to 7.15 dBi. When comparing antennas from different manufacturers, always convert to the same reference before ranking them. A specification stated in dBd will appear numerically smaller than the same antenna described in dBi—this is not a difference in performance, only a difference in reference.

Building an RF Link Budget

A link budget is a running tally of every gain and loss between transmitter output and receiver input. It answers the critical question: does the received signal level (RSL) meet or exceed the receiver's minimum threshold?

A simple four-row link budget follows this structure:

| Row | Item | Value (dBm or dB) | Running Total (dBm) |

|-----|------|--------------------|----------------------|

| 1 | Transmitter output power | e.g., +30 dBm | +30 dBm |

| 2 | Feedline / cable loss | e.g., −3 dB | +27 dBm |

| 3 | Antenna gain | e.g., +9 dBi | +36 dBm |

| 4 | Free-space path loss (FSPL) | e.g., −110 dB | −74 dBm |

The final running total is the received signal level (RSL). If RSL ≥ receiver threshold, the link closes (it works). If RSL < threshold, you must improve something—more transmit power, lower-loss cable, higher-gain antenna, shorter path, or a combination.

Free-Space Path Loss

Free-space path loss (FSPL) is the signal attenuation due to the natural spreading of a radio wave through open space. A widely used approximation for distances in miles and frequency in MHz is:

> FSPL (dB) ≈ 36.6 + 20 × log₁₀(f_MHz) + 20 × log₁₀(d_miles)

This formula applies only to unobstructed line-of-sight paths. Real-world paths add additional losses for terrain, foliage, buildings, and atmospheric effects, but FSPL is the starting baseline for any link budget.

Safety Notes for RF Work

RF signals carry energy that can cause burns and tissue damage at sufficiently high power levels. Before working on any energized RF system:

• Follow your facility's lockout/tagout (LOTO) procedure whenever you must work on transmitters or feedlines that could be energized unexpectedly.
• Observe manufacturer and regulatory maximum permissible exposure (MPE) limits—never stand directly in front of a high-power antenna while the transmitter is on.
• When climbing towers or working at height to install antennas, wear a properly rated fall-arrest harness and follow all applicable fall-protection requirements.
• Coaxial connectors carry DC voltages in systems that use power-over-coax—verify circuits are de-energized before cutting or terminating cable.
• Use RF power meters and spectrum analyzers rather than your hands to verify whether a system is active.