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06 Numbers and Powers

Microwave Cable Loss Calculator: Estimate RF Attenuation

Card No. 06·61081 · Updated · Standard formula, shown below

Cable loss 1.50 dB
Power delivered
70.79 %
Result
Cable attenuation
3 dB/100 ft
You entered
Cable length
50 ft
You entered
You enter
Cable attenuation, Cable length
You get
Cable loss, Power delivered
Rounding
2 dp

A microwave cable loss calculator estimates how much radio-frequency signal power a coaxial cable loses over a chosen length, using the cable’s attenuation per unit length at the operating frequency. Enter attenuation and cable length in compatible units to get total loss in decibels; the calculator also converts that loss to the fraction of input power that reaches the far end.

Formula

Total cable loss is attenuation per unit length multiplied by cable length, after both quantities are expressed on the same length basis. For this calculator, the input attenuation is in decibels per 100 feet and length is in feet:

L = A × D / 100

  • L is the cable’s total insertion loss in dB.
  • A is the cable attenuation in dB per 100 ft, at the frequency of interest.
  • D is cable length in ft.

The power fraction is calculated from the decibel loss:

F = 100 × 10−L/10

F is the percentage of input power delivered by an ideal matched cable, ignoring connector, adapter, and mismatch losses. Decibels are logarithmic: a loss of 3 dB is approximately half the input power, not a 3% reduction.

Worked Example

With the calculator’s default values, attenuation is 3 dB per 100 ft and cable length is 50 ft.

  1. Total cable loss: 3 × 50 / 100 = 1.50 dB.
  2. Power delivered: 100 × 10−1.50/10 = 100 × 0.70795 = 70.79%.

That means an ideal 1-watt input would become about 0.708 watts at the cable end, before accounting for other losses.

What the Calculator Tells You

Microwave cable loss is the reduction in signal power as a radio-frequency signal travels through a cable. The calculator applies a simple length scaling to a manufacturer’s attenuation figure. It is useful for an initial link budget, comparing cable lengths, or understanding how much signal a run may consume.

The answer depends on the attenuation value you enter. Cable loss changes with frequency and cable construction, so there is no single loss value that applies to every coaxial cable. Look up the cable’s attenuation specification or a reliable data sheet at the actual operating frequency. If a table gives values in dB per 100 m, convert the length basis before using this calculator: 100 m is about 328.084 ft. Alternatively, convert the stated attenuation to dB per 100 ft.

For metric measurements, the same formula works with attenuation in dB per metre and length in metres: total loss in dB equals attenuation in dB/m multiplied by length in m. Keep the units consistent; do not enter a dB/m figure as though it were dB/100 ft.

Using Cable Specifications Correctly

Find the attenuation entry for the frequency closest to your system’s frequency. A specification may list several frequencies, such as 1 GHz and 10 GHz; using a low-frequency figure for a microwave signal can substantially understate loss. When your exact frequency falls between listed points, consult the manufacturer’s guidance rather than assuming loss is constant or interpolating without checking whether that is appropriate.

  • Use the cable’s actual installed length, including routing slack and the path between equipment—not just the straight-line distance.
  • Check whether the data sheet reports attenuation per metre, per 100 ft, or another length.
  • Confirm that the figure applies to the cable type and operating conditions in question.
  • Add connector, adapter, and other component losses separately when making a fuller system estimate.

Attenuation figures commonly describe a cable under specified test conditions. Bends, installation details, temperature, aging, and termination quality can affect a real installation. The basic result is therefore an estimate, not a measurement of a particular assembled cable run.

Interpreting Decibels and Power

A loss in dB expresses a ratio, not an absolute power level. For power, the relationship is L = 10 log10(Pin/Pout), so the delivered power fraction is 10−L/10. A 0 dB loss corresponds to 100% power in this idealized calculation. A 10 dB loss corresponds to 10%, and a 20 dB loss corresponds to 1%.

Do not subtract a dB loss as if it were a percentage. For example, a 3 dB loss does not mean that 97% of power remains; it means roughly 50% remains. If you know input power in watts or milliwatts, multiply it by the calculated fraction to estimate output power. Keep the power units the same on both sides of that multiplication.

Limits and Common Mistakes

This calculator assumes that attenuation scales linearly with length at the chosen frequency. That is a useful approximation for cable loss, but it does not include reflections caused by impedance mismatch, connector losses, splitters, filters, or other equipment. Those effects need their own specifications or measurements.

  • Mixing length units: dB/100 ft must be paired with a length in feet in this calculator.
  • Using the wrong frequency: attenuation generally varies with frequency, so the relevant data-sheet row matters.
  • Confusing dB and dBm: dB is a ratio; dBm is an absolute power level referenced to 1 mW.
  • Ignoring added components: a cable-only result is not the total loss of a complete RF path.
  • Rounding too early: retain the calculator’s unrounded intermediate value when combining losses, then round the final result appropriately.

For a more accurate system assessment, use the actual cable specification and include each component in the signal path. A calibrated RF measurement can verify the assembled run; a calculated estimate cannot confirm connector condition or installation quality.

Frequently Asked Questions

How do I calculate microwave cable loss?
Multiply the cable’s attenuation per unit length by the cable length, using matching units. For example, dB/100 ft × feet ÷ 100 gives total loss in dB.

Does cable loss change with frequency?
Yes. Attenuation depends on frequency as well as cable design, so use a specification for the operating frequency rather than a generic value.

What does a 3 dB cable loss mean?
It means the output power is about half the input power, assuming the stated loss is the only relevant effect. It does not mean a 3% reduction.

Can I use metres instead of feet?
Yes, if both inputs use metric units: multiply attenuation in dB/m by length in metres. The calculator’s fields, however, expect dB/100 ft and feet.

Does the result include connectors?
No. It estimates cable attenuation only. Include connector and other component losses separately if you need a whole-path estimate.

Related

Formula used
in a | Cable attenuation | dB/100 ft | 3 | 0
in l | Cable length | ft | 50 | 0
out loss | Cable loss | dB | a * l / 100 | 2
out fraction | Power delivered | % | 100 * 10 ^ (-loss / 10) | 2

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

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