TOOL 03 / THE TRANSMISSION PATH
Feeder & Cable Loss
Follow the power from cable input to delivered output.
Cable & path
POSITIVE LOSS IN dBUse dot decimals or scientific notation. Changing a unit reinterprets its value.
Single-frequency data requires matching frequencies. The √f option assumes loss ∝ √frequency; it omits dielectric loss and is not a manufacturer prediction.
Length & additional losses
Enter losses applicable at the operating frequency: zero, or 1e-30–300 dB. Count each specified interface once. The 0.1 dB default is an illustrative assumption. Exclude losses already included in an assembly specification.
Input power
Average real power entering this path. Zero is allowed in linear units.
Loss budget
INPUT → OUTPUT- Cable attenuation
- dB / m
- Equivalent length
- m
- Cable loss
- dB
- Connector interfaces
- dB
- Other insertion losses
- dB
Input & delivered power
Results update as you type.
Display & examples
dB uses 1 / 3 / 6 decimal places. All calculations use unrounded values; displayed percentages can round to 0 or 100.
Assumes a matched passive path. Cable estimates are typical at +25 °C. Connector and other losses are your assumptions. Temperature, ageing, installation effects and power ratings are not modelled.
FREQUENCY COMPARISON
Same path, different frequency
Cable model
unrounded calculations| MHz | dB / 100 m | Total dB | Delivered W |
|---|
POWER RATIO ANCHORS
Every 10 dB divides power by ten
0 dB → 100% delivered
3 dB → 50.1187% delivered
6 dB → 25.1189% delivered
10 dB → 10% delivered
20 dB → 1% delivered
A positive loss subtracts from the input dBm level. For example, 10 W = 40 dBm; after 10 dB loss, 1 W = 30 dBm remains.
The comparison uses the selected cable equation, not an interpolation of rounded datasheet tables. A row above the 300 dB limit displays a dash. Single-frequency custom data produces one row at its specified frequency.
Equations & assumptions
SHOW THE WORKLoss along the path
A₁₀₀ft = a √f_MHz + b f_MHz
α [dB/m] = A₁₀₀ft / 30.48
L_cable = α × length [m]
L_total = L_cable + N × L_interface + L_other
LMR-240: a = 0.242080, b = 0.000330. LMR-400: a = 0.122290, b = 0.000260. These are the manufacturer's typical equations, with attenuation in dB per 100 ft. This tool limits both presets to the tabulated 30–8000 MHz range. Coefficients and rounded table entries can differ slightly.
Custom √f estimate: α(f) = α_ref √(f / f_ref)
Single-frequency custom mode applies only at f_ref. The optional √f estimate models conductor-dominated loss and omits dielectric loss; use only where that assumption is justified. A single data point cannot determine a cable's actual frequency response.
Delivered power
L [dB] = 10 log₁₀(P_in / P_out)
P_out = P_in × 10^(−L_total / 10)
P_out [dBm] = P_in [dBm] − L_total
Removed fraction = 1 − 10^(−L_total / 10)
1 ft = 0.3048 m exactly · 100 ft = 30.48 m
The power ratio uses 10 log, with a positive loss. Power removed is the aggregate reduction in forward delivered power, not a component heat or temperature calculation. Assume matched terminations and additive insertion losses; exclude duplicate losses. No reflection or impedance calculations are performed.
At zero input power, both powers are zero and logarithmic levels display −∞. The delivery percentage remains the model's transmission factor. Power and frequency validation reuse Tools 01 and 02.
Numerical limits, rounding & verification
Custom frequency and reference frequency: 1 Hz–1 THz. Presets: 30 MHz–8 GHz. Length: zero, or 1e-9–100000 m. Custom loss specification: zero, or 1e-12–10000 dB/m before frequency scaling. Connector count: integer 0–1000. Per-interface, other and total losses: zero, or 1e-30–300 dB. All limits are inclusive, numerical limits rather than equipment ratings. Positive lower bounds prevent numerical underflow.
Input power: zero, or 1e-30–1e30 W, using Tool 01's units and limits. Output power may be as low as 1e-60 W (−600 dBW), without reapplying Tool 01's input limit. Single-frequency equality allows two machine-epsilon rounding units for equivalent SI entries.
Invalid inputs clear dependent results. Invalid power leaves the loss budget usable; invalid path data leaves valid input power visible. Inactive custom inputs are ignored. Connector loss is validated even with count zero. Calculator inputs are not saved or uploaded by this app. Scientific notation follows the existing workbench formatting.
Calculation checks cover fixed reference values, units, invalid inputs and numerical boundaries, with separate high-precision numerical comparisons. No hardware characterisation or independent-person review is claimed.
Primary sources (checked 24 October 2026): Times Microwave LMR-240 datasheet, PDF p. 2 · LMR-400 datasheet, PDF p. 2 · NIST international foot · Cisco power ratio / dBm definitions. References require internet; calculations run in your browser after the page loads.