Figure 1: A signal path consists of gains (amplifiers, antennas) and losses (cables, splitters, path loss). The link budget adds them in dB to find the power arriving at the receiver.
Table of Contents
What Is Signal Attenuation?
Signal attenuation is the reduction in signal strength as it travels through any medium — cables, connectors, free space, or electronic components. Every metre of cable, every connector junction, and every passive device removes some energy from the signal. Attenuation is measured in decibels (dB), where a higher dB number means more loss. The Decibel Calculator converts between dB and voltage/power ratios if you need to see what that loss means in linear terms.
The calculator above has three modes. Mode 1 finds the dB difference between two power levels. Mode 2 calculates loss for a specific cable type at a given frequency and length, using a built-in database of 10 common cables. Mode 3 builds a complete link budget by cascading gains and losses through up to 10 stages.
What Causes Attenuation?
Conductor loss (I²R): Current flowing through the cable’s resistance converts signal energy into heat. This is the dominant loss mechanism in copper cables at lower frequencies.
Dielectric loss: The insulating material between conductors absorbs energy as the electric field reverses with each cycle. Higher frequencies reverse faster, increasing dielectric loss.
Skin effect: At higher frequencies, current concentrates in a thin layer on the conductor surface. The effective cross-section shrinks, resistance increases, and loss rises. Cable loss roughly scales with the square root of frequency because of this effect.
Radiation: Poorly shielded cables radiate signal energy. This is minor in well-constructed coax but significant in unshielded twisted pair at high frequencies.
Connector loss: Each connector adds 0.1–0.5 dB of loss due to impedance mismatches, contact resistance, and small air gaps. Over many connectors, this adds up.
The Formulas
Attenuation (dB) = Pin − Pout (both in dBm)From cable spec:
Loss (dB) = α × length where α = loss per metreFrequency scaling:
α(f) = αref × √(f / fref) for coax cablesPower remaining:
% = 10(−dB/10) × 100Voltage ratio:
Vout/Vin = 10(−dB/20)
Mode 1 — dB Level Difference
Enter input and output power levels in dBm, dBW, or dBV. The calculator subtracts them to find the attenuation, then shows the equivalent voltage ratio, power ratio, percentage remaining, percentage lost, and nepers. A signal strength bar visualises how much of the original signal survived.
Example: Measuring Cable Loss
Attenuation = 10 − (−20) = 30 dB
Power remaining = 10−30/10 = 0.001 = 0.1%
Voltage ratio = 10−30/20 = 0.0316×
99.9% of the signal power was lost. This is typical for a long coax run at UHF frequencies.
Example: Amplifier Output Check
Attenuation = 20 − 14 = 6 dB
Power remaining = 25% | Voltage ratio = 0.5×
6 dB loss means the voltage halved and power dropped to one quarter. The Gain Calculator can then design an amplifier stage to recover that loss.
Mode 2 — Cable Loss Calculator
Select a cable type from the built-in database, enter the length and operating frequency, and the calculator computes total loss using manufacturer specifications. For coaxial cables, loss is scaled with the square root of frequency to account for skin effect. For fibre optics, loss is frequency-independent.
The signal-vs-distance visualisation shows how the signal decays along the cable at 25%, 50%, 75%, and 100% of the total length. A cable specification table summarises the selected cable’s impedance, reference loss, and your calculated loss.
Example: RG-58 at 100 MHz Over 50 Metres
Reference loss: 16.4 dB/100m at 100 MHz
Loss rate at 100 MHz: 16.4 / 100 = 0.164 dB/m
Total loss: 0.164 × 50 = 8.2 dB
Signal remaining: 10−8.2/10 × 100 = 15.1%
At 25 m (halfway): 4.1 dB loss, 38.9% remaining. Signal weakens progressively — it does not drop suddenly at the end.
Example: Single-Mode Fibre Over 10 km
Loss rate: 0.2 dB/km (frequency-independent)
Total loss: 0.2 × 10 = 2 dB
Signal remaining: 63.1%
Fibre loses only 2 dB over 10 km. The same distance in RG-58 at 100 MHz would lose 1,640 dB — effectively zero signal. This is why long-haul communications use fibre.
Mode 3 — Link Budget
A link budget lists every gain and every loss in a signal path from source to destination, summed in dB. If the received power exceeds the receiver’s minimum sensitivity, the link works. Mode 3 lets you enter up to 10 named stages with positive dB for gains (amplifiers, antennas) and negative dB for losses (cables, splitters, free-space path loss).
Example: Wireless Link Budget
Stage 1: Tx Power +10 dBm → running: +10
Stage 2: Tx Cable −6 dB → running: +4
Stage 3: Tx Antenna +12 dBi → running: +16
Stage 4: Path Loss −90 dB → running: −74
Stage 5: Rx Antenna +8 dBi → running: −66
Stage 6: Rx Cable −3 dB → running: −69 dBm
If the receiver sensitivity is −85 dBm, the link margin is −69 − (−85) = 16 dB margin. That is comfortable — the signal arrives 16 dB above the minimum.
Example: CATV Distribution
Stage 1: Head-end amplifier +20 dB
Stage 2: Trunk cable 200 m −12 dB
Stage 3: 4-way splitter −7 dB
Stage 4: Drop cable 30 m −3 dB
Net: −2 dB from head-end output to subscriber tap.
If the head-end output is +40 dBmV, the subscriber receives +38 dBmV — well within the +5 to +40 dBmV range most set-top boxes require.
Cable Loss Comparison Table
| Cable | Type | Z (Ω) | Loss at 100 MHz | Best For |
|---|---|---|---|---|
| RG-6 | Coax | 75 | 6.6 dB/100m | CATV, satellite, CCTV |
| RG-58 | Coax | 50 | 16.4 dB/100m | General RF, test leads |
| RG-59 | Coax | 75 | 11.5 dB/100m | Analogue video, CCTV |
| RG-213 | Coax | 50 | 8.0 dB/100m | HF/VHF amateur radio |
| LMR-400 | Coax | 50 | 3.9 dB/100m | Low-loss RF, base stations |
| Cat5e | Twisted pair | 100 | 22 dB/100m | Ethernet to 1 Gbps |
| Cat6 | Twisted pair | 100 | 19.8 dB/100m | Ethernet to 1 Gbps (longer) |
| Cat6a | Twisted pair | 100 | 18 dB/100m | 10GbE Ethernet |
| SM Fibre (OS2) | Fibre optic | — | 0.02 dB/100m | Long-haul, data centres |
| MM Fibre (OM3) | Fibre optic | — | 0.35 dB/100m | Short-haul, campus links |
The calculator includes all 10 cable types above plus a custom entry for any cable where you know the dB/m loss rate.
Why Loss Increases with Frequency
At higher frequencies, the skin effect forces current into a thinner layer on the conductor surface. Less conductor cross-section means higher resistance, which means more I²R loss per metre. Dielectric losses also rise because the insulator’s electric field reverses more times per second.
For coaxial cables, total loss scales approximately with √f. A cable rated at 6.6 dB/100m at 100 MHz will lose roughly 6.6 × √(400/100) = 13.2 dB/100m at 400 MHz. The calculator applies this √f scaling automatically when you change the frequency. For fibre optics, loss is set by wavelength-dependent absorption and scattering, not skin effect, so it remains constant across signal frequencies.
The Bandwidth Calculator relates to this because higher bandwidth means higher frequencies, which means more cable loss — system bandwidth and cable loss are fundamentally linked.
System Margin and Acceptable Loss
Every link needs a safety margin above the minimum received signal level. Without margin, the link works on paper but fails in practice when temperatures shift, connectors age, or cables flex.
Acceptable total loss depends on the system: Wi-Fi tolerates about 100 dB path loss. CATV requires signal between +5 and +40 dBmV at the receiver. Fibre links typically allow 10–30 dB end-to-end. Coaxial TV distribution allows 20–40 dB from amplifier to tap.
How to Reduce Attenuation
Use lower-loss cable. Upgrading from RG-58 (16.4 dB/100m) to LMR-400 (3.9 dB/100m) cuts loss by 76% for the same run length. For distances over a few hundred metres, switching to fibre optic eliminates cable loss as a practical concern.
Shorten the cable run. Loss is proportional to length. Halving the distance halves the dB loss. Route cables directly rather than around obstacles wherever possible.
Use fewer connectors. Each connector adds 0.1–0.5 dB. Eliminating unnecessary patch panels and adapters can save several dB in a long chain.
Add amplifiers. An inline amplifier or line extender recovers lost signal. However, amplifiers also add noise, so the Noise Figure Calculator should be used to check that overall system noise remains acceptable.
Lower the operating frequency. If the application allows it, operating at a lower frequency reduces skin-effect losses. This is why HF amateur radio uses thick coax rated for lower frequencies, while microwave links use waveguides or very short cable runs.
Frequently Asked Questions
What causes signal attenuation?
Why does cable loss increase with frequency?
What cable should I use?
What is a link budget?
How much loss is acceptable?
How does the frequency scaling work?
Can I enter a custom cable type?
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