Free Space Path Loss Calculator
Calculate free space path loss (FSPL) in dB for radio signals between antennas. Enter distance, frequency, and gains to get path loss, wavelength, and charts.
About This Calculator
The Free Space Path Loss Calculator predicts the attenuation of a radio frequency signal propagating through free space (vacuum) between transmitting and receiving antennas. It uses the Friis transmission formula to compute the signal loss in decibels (dB) based on the distance between antennas, operating frequency, and antenna gains.
Free space path loss is a critical concept in wireless communication system design. It follows the inverse square law — signal intensity decreases proportionally to the square of the distance from the transmitter. The FSPL formula also accounts for frequency-dependent loss: higher frequency signals experience greater path loss over the same distance due to their shorter wavelength.
How the FSPL formula works
For isotropic antennas (0 dB gain), the free space path loss is calculated as FSPL (dB) = 20 log10(d) + 20 log10(f) + 92.44, where d is the distance in kilometers and f is the frequency in GHz. For antennas with directional gain, the loss is reduced by the sum of transmitter and receiver antenna gains. The calculator also computes the wavelength using λ = c / f, where c is the speed of light (299,792,458 m/s).
Regional Notes
India (IN): Telecom Regulatory Authority of India (TRAI) governs spectrum allocation. Common bands include 900 MHz (2G/4G), 1800 MHz (4G), 2300 MHz (4G/5G), and 3500 MHz (5G). WiFi uses 2.4 GHz and 5 GHz unlicensed bands. Satellite communication uses C-band (4-8 GHz) and Ku-band (12-18 GHz).
United States (US): FCC regulates spectrum. Key bands include 600-700 MHz (4G/5G), 2.5 GHz (5G), 3.5 GHz CBRS, 5.9 GHz (WiFi 6E), and 24-47 GHz (mmWave 5G). Satellite communications use C, Ku, and Ka bands. Antenna gain regulations follow FCC Part 15 and Part 25.
United Kingdom (UK): Ofcom manages spectrum. Key bands include 700 MHz (4G/5G), 800 MHz (4G), 1400 MHz, 2100 MHz, 2300 MHz, 3400-3800 MHz (5G), and 24.25-27.5 GHz. WiFi uses 2.4 GHz, 5 GHz, and the newly opened 6 GHz band (WiFi 6E).
Frequently Asked Questions
What is free space path loss?
Free space path loss (FSPL) is the attenuation of radio frequency signal strength as it travels through free space (vacuum) between a transmitter and receiver with a clear line of sight. It follows the inverse square law and increases with both distance and frequency.
How is FSPL calculated?
FSPL (dB) = 20×log₁₀(d) + 20×log₁₀(f) + 92.44 − Gₜ − Gᵣ, where d is distance in km, f is frequency in GHz, Gₜ is transmitter antenna gain in dB, and Gᵣ is receiver antenna gain in dB. For isotropic antennas, gains are 0 dB.
What is a typical FSPL for WiFi at 100 meters?
At 2.4 GHz over 0.1 km (100 m) with isotropic antennas, FSPL = 20×log₁₀(0.1) + 20×log₁₀(2.4) + 92.44 = -20 + 7.60 + 92.44 = 80.04 dB. At 5 GHz over the same distance, FSPL increases to 86.4 dB due to the higher frequency.
Does antenna gain reduce path loss?
Yes, higher antenna gain at both transmitter and receiver reduces the overall free space path loss. For example, a 44 dB satellite dish and 48 dB ground antenna reduce the effective FSPL by 92 dB compared to isotropic antennas. Antenna gain improves signal strength without increasing transmitter power.
Why does FSPL increase with frequency?
FSPL increases with frequency because higher frequency waves have shorter wavelengths, which interact more with obstacles and atmospheric particles. The Friis transmission equation shows that received power is proportional to (λ/4πd)², so as wavelength decreases with higher frequency, the received power drops and path loss increases.
What is the difference between FSPL and total path loss?
FSPL considers only the theoretical loss in free space with a clear line of sight. Total path loss includes additional losses from atmospheric absorption (oxygen, water vapor), rain fade, foliage, building penetration, and multipath interference. Real-world path loss is always higher than FSPL.
What is the Friis transmission equation?
The Friis transmission formula, developed by Harald T. Friis in 1946, gives the power received by one antenna from another antenna transmitting through free space. It states that Pᵣ / Pₜ = Gₜ Gᵣ (λ / 4πd)², where the term (λ / 4πd)² is the reciprocal of the free space path loss factor.
How is FSPL used in satellite communications?
In satellite communications, FSPL is a major factor in link budget calculations. For a geostationary satellite at 35,786 km, FSPL at 4 GHz is approximately 103.6 dB with typical antenna gains. Satellite engineers use FSPL to determine required transmitter power, antenna sizes, and modulation schemes.