FSPL Calculator - Free Space Path Loss | RFCalcLab
Calculate electromagnetic wave attenuation over distance using the FSPL formula. Essential for link budget analysis, signal strength prediction, and wireless network planning across MHz and GHz frequencies.
How to Use the FSPL Calculator
Enter the distance between the transmitter and receiver and the operating frequency. Free Space Path Loss (FSPL) assumes a clear line-of-sight without any obstacles or reflections. This tool evaluates the practical engineering form FSPL(dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.44, where distance d is in kilometres and frequency f is in megahertz; the 32.44 constant folds in the speed of light and the 4π term so you can read loss directly from your link's range and channel.
The result is shown in decibels (dB). FSPL increases by 6 dB every time you double the distance or frequency, following the inverse square law of electromagnetic propagation. For a worked example, a 1 km hop at 100 MHz gives 20·log₁₀(1) + 20·log₁₀(100) + 32.44 = 0 + 40 + 32.44 = 72.44 dB; pushing that same link to 2400 MHz over 10 km yields roughly 120 dB, which is why higher bands and longer ranges demand more transmit power or higher-gain antennas.
Note that real-world environments involve ground reflections, foliage, rain attenuation, and atmospheric absorption, so actual path loss will almost always exceed the theoretical FSPL value. Use FSPL as the baseline term in a link budget: add transmitter power and both antenna gains, then subtract this loss plus cable and miscellaneous losses to estimate received power. Because FSPL is defined between two isotropic radiators, it never includes antenna gain itself — that is accounted for separately.
Related Topics
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Frequently Asked Questions
- What is Free Space Path Loss (FSPL)?
- FSPL is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles to cause reflections or diffraction.
- How does frequency affect path loss?
- Path loss increases with frequency. Because the effective aperture of a receiving antenna decreases as frequency goes up, higher frequency signals experience more attenuation over the same distance.
- Does FSPL include antenna gain?
- No, FSPL strictly calculates the loss between two isotropic antennas. To find the received signal level, you must add the transmitter power and both antenna gains to the negative FSPL value.