LMR-240 Cable Loss Calculator and Formula

An LMR-240 cable loss calculator estimates how much radio-frequency signal is lost in a specified length of cable at a chosen frequency. Enter the frequency and cable length, and it returns the estimated attenuation in decibels (dB) and the remaining signal-power ratio; use the result as a planning estimate, not a substitute for a cable datasheet or measurement.

Formula

This estimate uses a common approximate model for LMR-240 attenuation: loss per 100 feet is 0.141 × the square root of frequency in megahertz. Total loss scales in proportion to cable length.

Loss (dB) = 0.141 × √f × (l ÷ 100)

• f is frequency in megahertz (MHz).
• l is cable length in feet (ft), measured along the cable run.
• Loss is estimated attenuation in decibels (dB).

The second output translates that loss into a power ratio: power remaining (%) = 100 × 10^{−(loss ÷ 10)}. This is the percentage of input power left after the modeled cable loss alone. The model is an approximation, not a manufacturer’s guaranteed specification. Actual attenuation depends on the cable construction, frequency, connectors, installation, and operating conditions.

Worked Example

Using the calculator’s defaults, frequency is 150 MHz and cable length is 10 ft.

1. Find the square root of the frequency: √150 ≈ 12.247.
2. Calculate loss: 0.141 × 12.247 × (10 ÷ 100) ≈ 0.1727 dB, which rounds to 0.17 dB.
3. Calculate the power remaining: 100 × 10^{−(0.1727 ÷ 10)} ≈ 96.10%.

So, under this estimate, a 10-foot run at 150 MHz loses about 0.17 dB and passes about 96.10% of the input power. The percentage does not mean that the signal’s voltage or received strength is reduced by exactly the same percentage; dB is a logarithmic measure of power ratio.

When to Use the Estimate

Use a cable-loss estimate when comparing run lengths, checking whether a planned feed line might materially affect a link budget, or deciding whether to look for a shorter route or a lower-loss cable. It can also help explain why a transmitter’s output power and the power available at an antenna are not identical.

For example, a longer run has proportionally more modeled loss at the same frequency. Raising the frequency also raises the loss in this simplified equation, because the square-root frequency term increases. These trends are useful for quick comparisons, but the result is not a complete radio-system calculation: it excludes antenna gain, mismatch, connector losses, splitters, adapters, and other components.

Reading Decibels and Power Remaining

A loss stated in dB describes a ratio, not a fixed amount of power. For power, the relationship is output/input = 10^−loss/10. A larger positive loss therefore means a smaller fraction of the input power reaches the far end. Decibels for voltage use a different relationship and depend on the impedances being compared, so do not apply the power-ratio formula directly to voltage readings without checking the measurement conditions.

The calculator reports loss to two decimal places. That rounding is convenient for display, but it can be significant when comparing small differences. The underlying cable loss is not necessarily known to hundredths of a decibel in a real installation; treat displayed precision as arithmetic precision, not measurement certainty.

What Can Change Real Cable Loss?

Manufacturer data for the exact cable type and frequency is preferable when accuracy matters. Consult the datasheet’s attenuation table or curve, and check that its stated length units match your run. Depending on the product and conditions, published values may be given per 100 ft, per 100 m, or another length.

• Length: measure the actual cable path, including routing slack, rather than straight-line distance.
• Frequency: use the operating frequency, not a band’s midpoint unless that is appropriate for your calculation.
• Connections: connectors, adapters, and transitions can add loss beyond the cable itself.
• Installation and condition: damaged cable, poor terminations, water ingress, tight bends, and environmental exposure can affect performance.
• Impedance and matching: mismatch can cause reflected power and is a separate issue from the cable’s specified attenuation.

For a more complete estimate, add the losses of each component in dB, provided the values apply to the relevant frequency and configuration. For a critical installation, verify the assembled feed line with appropriate test equipment or ask a qualified radio-frequency professional.

Common Mistakes and Limits

The calculator takes frequency in MHz and length in feet. Entering a frequency in GHz without converting it, or entering metres as though they were feet, produces a wrong answer. For reference, 1 GHz = 1,000 MHz and 1 m ≈ 3.28084 ft. Convert first, or use a calculation based on the datasheet’s units.

Another common misconception is that “LMR-240” alone determines an exact loss value. Cable designations identify a cable family or type, but published attenuation can vary by manufacturer and product specification. This calculator’s coefficient is a broad estimate; use the exact product’s documented figures when selecting or validating a run. It also does not determine safe transmitter power, cable power handling, or regulatory compliance.

Frequently Asked Questions

How much loss does LMR-240 have per 100 feet?
The estimate here is 0.141 × √(frequency in MHz) dB per 100 ft. At 150 MHz, that is about 1.73 dB per 100 ft. Check the exact cable datasheet for a product-specific value.

Does cable loss increase with frequency?
Generally, attenuation increases as frequency rises. This calculator models that trend with a square-root relationship, but real cable attenuation should be read from the relevant product’s frequency-specific data.

Can I enter cable length in metres?
Not directly in this calculator. Convert metres to feet by multiplying by approximately 3.28084. Alternatively, use the manufacturer’s attenuation per metre or per 100 metres and keep the units consistent.

Does the result include connectors?
No. It estimates cable attenuation only. Add suitable connector, adapter, or other component losses separately, using values that apply at your operating frequency.

Is the estimated power percentage the same as signal-strength percentage?
No. It is a power ratio derived from the dB loss. Voltage ratios and receiver signal indicators use different relationships or scales; do not interpret this figure as a direct percentage change in every signal measurement.

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in f | Frequency | MHz | 150 | 0 | 6000
in l | Cable length | ft | 10 | 0
out loss | Estimated cable loss | dB | 0.141 * sqrt(f) * l / 100 | 2
out ratio | Power remaining | % | 100 * 10 ^ (-(loss / 10)) | 2