What is attenuation in dB?

Attenuation is the reduction in signal strength, measured in decibels (dB). For power: dB = 10 × log10(P_out/P_in). For voltage: dB = 20 × log10(V_out/V_in). Negative dB means loss; positive dB means gain.

How do you convert power ratio to dB?

dB = 10 × log10(P_out / P_in). If output is half the input: 10 × log10(0.5) = −3.01 dB. A −3 dB loss halves the power.

How do you convert voltage ratio to dB?

dB = 20 × log10(V_out / V_in). The factor is 20 (not 10) because power is proportional to voltage squared. A −6 dB voltage drop halves the voltage.

What is free space path loss (FSPL)?

FSPL is the signal loss in a direct line-of-sight radio link with no obstacles: FSPL(dB) = 20×log10(d) + 20×log10(f) + 32.44, where d is distance in km and f is frequency in MHz. It increases with both distance and frequency.

How do you calculate cable attenuation?

Total cable loss = loss_per_metre × cable_length. Cable specifications give loss in dB/m or dB/100ft at specific frequencies. Add connector losses (typically 0.1–0.5 dB each) for the complete path.

What is a link budget?

A link budget sums all gains and losses in a communication path: P_received = P_transmit + G_tx − L_cable_tx − FSPL + G_rx − L_cable_rx. The received power must exceed the receiver sensitivity with adequate margin (typically 10–20 dB).

How do I convert dB back to a ratio?

Power ratio = 10^(dB/10). Voltage ratio = 10^(dB/20). For example, −10 dB power ratio = 10^(−1) = 0.1 (10% of input).

Attenuation Calculator – dB, Power Ratio, Voltage Ratio, FSPL & Cable Loss

A(dB) = 10 log₁₀(P_in / P_out)

= 20 log₁₀(V_in / V_out)

Select Calculation Method

⚡ Input Power (P_in)

0.001100 W

⚡ Output Power (P_out)

0.001100 mW

Attenuation

40.00 dB

Signal loss

Output Power

—

After attenuation

Power Ratio (P_in/P_out)

10000

Linear ratio

Voltage Ratio (V_in/V_out)

100

Linear ratio

Power Remaining

0.01%

% of input power

Attenuation Level

High Loss

Significant signal loss

Signal Loss Level 40.00 dB

ℹ️

Every 3 dB of attenuation halves the power. Every 10 dB reduces power to one-tenth. Use the method selector to switch between power ratio, voltage ratio, dB conversion, free space path loss, or cable loss calculations.

⚙️ Calculation Steps

Identify the input power (P_in) and output power (P_out) in the same units.

Calculate the power ratio: R = P_in / P_out

Apply logarithm: A(dB) = 10 × log₁₀(R)

For voltage ratio: A(dB) = 20 × log₁₀(V_in/V_out)

Calculate power remaining: P_out/P_in × 100% = 10^−A/10 × 100%

📊 Live Calculation

Step 1 — Inputs

P_in = 10 W P_out = 1 mW

Step 2 — Ratio

Ratio = 10 / 0.001 = 10000

Step 3 — Attenuation (dB)

A = 10 × log₁₀(10000) = 40.00 dB

Step 4 — Power Remaining

P_remaining = 10^(-40/10) × 100% = 0.01%

📋 Quick Reference — Attenuation Levels & Common dB Values

Attenuation (dB)	Power Ratio	Voltage Ratio	Power Remaining	Level	Typical Application
0 dB	1 : 1	1 : 1	100%	No loss	Perfect transmission
1 dB	1.26 : 1	1.12 : 1	79.4%	Minimal	Connector loss
3 dB	2 : 1	1.41 : 1	50.0%	Low	3dB splitter, half power
6 dB	4 : 1	2 : 1	25.0%	Low	Cable run, filter edge
10 dB	10 : 1	3.16 : 1	10.0%	Medium	10 dB attenuator pad
20 dB	100 : 1	10 : 1	1.00%	Medium	Long cable, filter stopband
30 dB	1000 : 1	31.6 : 1	0.10%	High	Filter rejection, isolation
40 dB	10000 : 1	100 : 1	0.01%	High	Shield effectiveness
60 dB	10⁶ : 1	1000 : 1	0.0001%	Very High	EMI shielding requirement
100 dB	10¹⁰ : 1	10⁵ : 1	10⁻⁸%	Extreme	Fiber span, deep space link

ℹ️

Key rule: +3 dB doubles power, −3 dB halves it. +10 dB multiplies power by 10, −10 dB divides it by 10. These relationships make dB an extremely convenient unit for cascaded systems where individual losses and gains simply add up.

Attenuation Calculator — Complete Guide to dB, FSPL & Cable Loss

This attenuation calculator computes signal loss in decibels (dB) from power ratio, voltage ratio, free space path loss (FSPL), and cable attenuation — plus the reverse conversion from dB back to linear ratio. Whether you are designing an RF link budget, specifying cable runs, or characterising an amplifier's gain, this tool gives you instant results with a real-time signal visualization.

Key Formulas

Method	Formula	Example
Power ratio → dB	dB = 10 × log₁₀(P_out/P_in)	P_out=5 W, P_in=10 W → −3.01 dB
Voltage ratio → dB	dB = 20 × log₁₀(V_out/V_in)	V_out=0.5 V, V_in=1 V → −6.02 dB
FSPL	20 log₁₀(d) + 20 log₁₀(f) + 32.44	d=1 km, f=2400 MHz → 100.0 dB
Cable loss	Loss = loss/m × length + connectors	0.15 dB/m × 20 m + 0.5 dB = 3.5 dB
dB → power ratio	Ratio = 10^(dB/10)	−10 dB → 0.1 (10%)
dB → voltage ratio	Ratio = 10^(dB/20)	−20 dB → 0.1 (10%)

Common dB Values — Quick Reference

dB	Power Ratio	Voltage Ratio	Meaning
+10	10×	3.16×	10× power amplification
+6	3.98×	2.0×	Voltage doubles
+3	2.0×	1.41×	Power doubles
0	1.0×	1.0×	No change
−3	0.50×	0.707×	Half power (−3 dB point)
−6	0.25×	0.50×	Voltage halves
−10	0.10×	0.316×	10% of input power
−20	0.01×	0.10×	1% of input power

Worked Examples

📡 Example 1 — Wi-Fi Free Space Path Loss (2.4 GHz, 50 m)

GivenFrequency = 2400 MHz, Distance = 0.05 km

FSPL20×log(0.05) + 20×log(2400) + 32.44 = −26.02 + 67.60 + 32.44 = 74.02 dB

CheckTx = 20 dBm − 74 dB = −54 dBm at receiver (well above −85 dBm Wi-Fi sensitivity)

ResultFSPL = 74 dB at 50 m / 2.4 GHz — indoor link is viable

🔌 Example 2 — Cable Run: 30 m RG-58 at 400 MHz + 2 connectors

GivenCable loss = 0.18 dB/m at 400 MHz, Length = 30 m, Connector loss = 0.3 dB × 2

Cable0.18 × 30 = 5.4 dB

Total5.4 + 0.6 = 6.0 dB total path loss

Power10^(−6/10) = 0.25 → only 25% of input power reaches the end

Result6.0 dB loss | 25% power through | Consider LMR-400 (0.07 dB/m) for lower loss

Common Cable Loss Reference (dB/m at 1 GHz)

Cable Type	Loss/m (dB)	Impedance	Typical Use
RG-174	1.05	50 Ω	Short jumpers, test leads
RG-58	0.55	50 Ω	Lab, amateur radio
RG-213	0.27	50 Ω	Base station, marine
LMR-240	0.25	50 Ω	Indoor wireless, WISP
LMR-400	0.10	50 Ω	Tower runs, outdoor AP
LMR-600	0.07	50 Ω	Long tower feedlines
RG-6	0.20	75 Ω	CATV, satellite

Practical Applications

RF Link Budget

Sum all gains (transmitter power, antenna gain) and subtract all losses (cable, connectors, FSPL, fade margin) to predict the received signal level. The result must exceed the receiver's sensitivity threshold.

Audio Systems

dB is the standard unit for audio levels — microphone preamplifiers amplify by +20 to +60 dB, mixing consoles attenuate channels, and speaker cables introduce small losses over long runs.

Fiber Optics

Optical fiber loss is specified in dB/km (typically 0.2–0.5 dB/km for single-mode). Splices add ~0.1 dB and connectors ~0.3 dB each. The total optical budget determines maximum link distance.

Filter and Amplifier Characterisation

A filter's passband ripple and stopband attenuation are measured in dB. An amplifier's gain is its output-to-input ratio in dB. Cascading stages simply adds the dB values.

Frequently Asked Questions

What does −3 dB mean?

−3 dB means the signal power is halved. In voltage terms, it's about 70.7% of the input. The −3 dB frequency is the standard "half-power" or "cutoff" point for filters.

Why use dB instead of plain ratios?

dB compresses huge dynamic ranges into manageable numbers, and cascaded gain/loss stages simply add in dB instead of multiplying. A signal path with +20 dB gain, −6 dB cable loss, and −80 dB FSPL gives −66 dB total — far easier than multiplying 100 × 0.25 × 0.00000001.

What is the difference between dB and dBm?

dB is a relative ratio between two values. dBm is an absolute power level referenced to 1 mW: 0 dBm = 1 mW, +30 dBm = 1 W, −30 dBm = 1 µW.

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