Transmission Line Loss Calculator
Calculate transmission line loss, output power, efficiency and cable performance.
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Math
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Enter parameters and click Calculate to view results
Formula & Theory
Pout = Pin × 10^(-Loss/10)This formula is used to calculate antenna parameters for transmission line loss calculator.
Overview
The Transmission Line Loss Calculator determines how much RF power is lost in a feedline before it reaches the antenna, and how much actually makes it through. Enter the operating frequency in MHz, the input power from your transmitter in watts, the physical cable length in meters, and the cable's loss rating in dB per meter (found on the manufacturer's datasheet), and the calculator returns the total cable loss in dB, output power delivered to the antenna, power lost as heat, overall efficiency, and voltage/power ratios. This is essential for radio operators, RF engineers, and antenna installers evaluating whether a given coax run — RG-58, RG-8, LMR-400, hardline, or any other cable type — is suitable for a specific frequency, run length, and power level, or whether a lower-loss cable is needed to avoid wasting significant transmit power.
Input Guide
Enter Frequency exactly as shown on the calculator. Confirm every unit, selected option, and decimal position before calculating so the transmission line loss result matches the intended waveguide & feedline design case.
Output Guide
The output section reports Operating Frequency, Total Cable Loss, Output Power, Power Lost, Power Efficiency, Loss Percentage, Voltage Ratio, Power Ratio, Cable Performance for the values you entered. Use these values as design targets, then compare them with available space, component limits, feed system behavior, installation environment, and measured performance before finalizing the design.
How This Calculator Works
The Transmission Line Loss applies Pout = Pin × 10^(-Loss/10) to the entered values. Calculate transmission line loss, output power, efficiency and cable performance. Use the result as a first-pass waveguide & feedline target, then validate it against losses, tolerances, mounting, nearby conductors, feed-line effects, and measurement conditions.
Design Notes
Transmission line loss follows an exponential (logarithmic) relationship rather than a linear one, calculated as Pout = Pin × 10^(-Loss/10), where total loss in dB is simply the cable's per-meter loss rating multiplied by run length. This means loss compounds multiplicatively with distance, not additively in terms of power — a cable with 3 dB of total loss delivers only 50% of input power to the antenna, while 10 dB of loss delivers just 10%, regardless of how much power was originally transmitted. Cable loss also increases with frequency for any given cable type, since dielectric and conductor losses both grow at higher frequencies, which is why a cable perfectly adequate at HF can become significantly lossy at VHF/UHF or microwave frequencies — always use the manufacturer's loss specification for the actual operating frequency, not a generic or lower-frequency figure.
Build and Tuning Notes
When planning a feedline run, use this calculator during the design stage to compare cable types before purchasing — a cable with a lower dB/m rating costs more per meter but can meaningfully increase delivered power and reduce transmitter strain, especially on long runs or higher frequencies. After installation, if measured antenna performance or received signal reports are weaker than expected, revisit this calculation using the actual installed cable length (including any extra length from routing, connectors, and slack) rather than the straight-line distance, since real-world runs are almost always longer than planned. Keep in mind that connectors, adapters, lightning arrestors, and any splices in the line add additional loss beyond the cable's rated per-meter figure, so treat the calculator's output as a best-case estimate and budget extra margin for these components, particularly on critical high-power or weak-signal-receive systems.
Inputs used by this calculator
- Frequency — use MHz.
- Input Power — use W.
- Cable Length — use m.
- Cable Loss — use dB/m.
Frequently Asked Questions
How do I calculate transmission line loss?
Total cable loss in dB is calculated by multiplying the cable's loss rating (in dB per meter, from the manufacturer's datasheet) by the physical cable length. Output power is then found using Pout = Pin × 10^(-Loss/10), which this calculator computes automatically from your frequency, power, cable length, and loss-per-meter inputs.
Why does cable loss increase with frequency?
Both dielectric loss and conductor (skin effect) loss in coaxial cable increase as frequency rises, so the same cable that performs well at HF frequencies can lose significantly more power at VHF, UHF, or microwave frequencies. Always check the cable's loss specification at your actual operating frequency rather than assuming a flat loss figure.
What is considered acceptable transmission line loss?
Many installers aim to keep feedline loss under 1–3 dB for typical amateur and commercial installations, since 3 dB represents a 50% power loss. Losses above 3 dB are still usable but increasingly wasteful, and losses above 6–10 dB (75–90% power lost) generally indicate the cable is undersized for the frequency and run length.
How does cable length affect power loss?
Loss increases proportionally with cable length in dB terms (a 20 m run has twice the dB loss of a 10 m run of the same cable), but because dB loss translates to power loss exponentially, doubling the cable length loses considerably more than double the actual power delivered to the antenna.
Does transmission line loss affect receive performance too?
Yes. Transmission line loss is reciprocal, meaning a lossy feedline weakens received signals by the same dB amount it weakens transmitted power, which can be especially significant for weak-signal work like satellite reception, DXing, or receive-only antenna systems.
What's the difference between loss percentage and dB loss?
Decibel (dB) loss is a logarithmic measure of the input-to-output power ratio, while loss percentage expresses how much of the original power was lost in linear terms. A small dB figure can represent a large percentage loss — for example, 3 dB of loss corresponds to a 50% power loss — which is why this calculator provides both figures for clarity.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.