Microwave Link Planning
Free space path loss in dB across distance, frequency, and antenna gain. Solve for FSPL, distance, or frequency, with full link budget, fade margin, Fresnel zone clearance, and presets for WiFi, LTE, 5G NR, VHF, UHF, ISM, and microwave bands.
Free space path loss is the foundation propagation loss every RF link plan starts from. It quantifies the attenuation a signal experiences travelling between two isotropic antennas in vacuum, governed by the inverse square spreading of the wavefront and a frequency dependent term that captures how higher frequency wavefronts couple less effectively into a fixed receive aperture. Every additional propagation effect (rain, gas, diffraction, multipath, clutter) is layered on top of FSPL. Get the FSPL number wrong and the rest of the budget is wrong by the same amount.
The noIM₃ Free Space Path Loss Calculator computes FSPL using the standard logarithmic form. FSPL in dB equals 20 log of distance plus 20 log of frequency plus a constant. For distance in km and frequency in MHz the constant is 32.45. For distance in km and frequency in GHz it is 92.45. For distance in metres and frequency in MHz it is minus 27.55. The calculator handles all unit families and surfaces the FSPL number directly. A bidirectional solver inverts the relationship so you can solve for the maximum distance achievable at a given FSPL budget, or for the required frequency to meet a distance and FSPL constraint.
Beyond the FSPL itself, the workspace covers the full link picture. Antenna gain compensation at both ends gives EIRP and received power. System loss and fade margin reduce the link margin against a configured receiver sensitivity. First Fresnel zone radius and percentage clearance support line of sight analysis for terrestrial microwave, fixed wireless access, and point to point work where terrain and obstacles intrude into the radiation envelope. Built in presets cover WiFi 2.4 GHz and 5 GHz, LTE, 5G NR, VHF, UHF, ISM 915 MHz, and microwave backhaul bands, so common feasibility questions are one click away.
Standard logarithmic formula implemented across all unit families. FSPL in dB equals 20 log of distance plus 20 log of frequency plus a unit dependent constant (32.45 for km and MHz, 92.45 for km and GHz, minus 27.55 for metres and MHz, and so on). All unit conversions are handled automatically and the constant is selected to match the input units.
Solve for FSPL given distance and frequency, or invert the relationship to solve for maximum achievable distance at a given FSPL budget, or for the required frequency to meet a distance and FSPL constraint. Useful for the early sizing question of how far a link can reach against a target loss, or which frequency band can deliver a required reach within an allowed budget.
Transmit and receive antenna gains in dBi feed directly into the link budget. EIRP equals transmit power plus transmit antenna gain. Received power equals EIRP minus FSPL plus receive antenna gain minus system losses. All quantities surfaced together so the link budget is an obvious read down rather than a multi step calculation.
Shows transmit power, transmit antenna gain, FSPL, receive antenna gain, system losses, fade margin, received power, configured receiver sensitivity, and resulting link margin in dB. Identifies underperforming links and supports the design discussion around antenna size, transmit power, and frequency band selection.
Configurable fade margin is subtracted from the link margin so the budget reflects realistic operational availability rather than only the static free space case. Useful for terrestrial microwave links where multipath and rain require an explicit margin allocation, and for fixed wireless access where seasonal or weather driven fading needs to be planned for.
Computes first Fresnel zone radius at any point along the path from frequency and distance. Returns percentage clearance against a configured obstacle height, supporting line of sight analysis for terrestrial microwave and point to point links. Useful for confirming whether a candidate path needs antenna height adjustment, terrain clearance work, or alternative routing.
Built in presets for WiFi 2.4 GHz, WiFi 5 GHz, LTE bands, 5G NR sub 6 GHz, VHF, UHF, ISM 915 MHz, and microwave backhaul bands. Each preset populates frequency, typical transmit power, antenna gain, and receiver sensitivity for the common configuration so a usable answer is one click away. Manual entry remains available for unusual band selections, amateur radio, or millimetre wave deployments.
Charts show FSPL versus distance, FSPL versus frequency, and received power versus distance. Useful for understanding the rate at which margin is consumed by range, identifying the maximum operating distance against a receive sensitivity threshold, and producing visuals that explain link feasibility to non specialist stakeholders.
Runs entirely in your browser. No frequency, distance, antenna, or transmit power data is submitted to a server. Useful for commercially confidential infrastructure work and environments where information security policy prohibits sending engineering data to third party services.
Reach for the Free Space Path Loss Calculator in any of the following situations.
FSPL in dB equals 20 log of distance plus 20 log of frequency plus a unit dependent constant. For distance in km and frequency in MHz the constant is 32.45. For distance in km and frequency in GHz it is 92.45. For distance in metres and frequency in MHz it is minus 27.55. For distance in metres and frequency in GHz it is 32.45 (with appropriate unit handling). The calculator selects the constant automatically based on the input unit families so you do not need to keep the conversion in your head.
FSPL assumes ideal line of sight conditions with no obstructions, no terrain, no atmospheric absorption, no multipath, and that both ends of the link are in the far field of each antenna. It is the right model for satellite links above the atmosphere, for short range line of sight links, and as the upper bound starting point for any link budget. For terrestrial links over significant range, you also need rain attenuation (P.530, P.838), atmospheric gas absorption (P.676), and diffraction over terrain (P.526), which the noIM₃ Link Planner handles to ITU P.530.
The FSPL Calculator focuses on the propagation loss in dB as a function of distance and frequency, plus the surrounding link budget context. The Friis Transmission Calculator works the same equation in its complete received power form (Pr equals Pt plus Gt plus Gr minus FSPL) with simpler inputs. Use FSPL when you want the loss number and full link budget breakdown including fade margin and Fresnel clearance. Use Friis when you want a quick free space received power and signal strength classification.
FSPL is the free space upper bound and is one input into a full link budget. The Link Budget Calculator extends the analysis to include feeder losses, miscellaneous losses, fade margin, atmospheric and rain attenuation, and link availability against a target. Use FSPL when you only need the propagation loss and basic link margin. Use the Link Budget Calculator for the full operational budget that goes into a design package.
First Fresnel zone radius F1 in metres equals 17.32 times square root of (d1 times d2 over (d times f in GHz)), where d1 and d2 are the distances from each end to the obstacle in km, and d is the total path length in km. For midpoint clearance, this simplifies to 8.66 times square root of (d divided by f in GHz). Output includes the radius and the percentage clearance against a configured obstacle height for line of sight assessment.
Fade margin is the headroom in the link budget reserved to absorb time varying propagation effects (multipath fading, rain, atmospheric variation) without dropping below the receiver sensitivity threshold. The calculator subtracts the configured fade margin from the link margin output so the reported number reflects realistic operational availability rather than only the static free space case. Typical microwave designs use 20 to 40 dB fade margin depending on frequency and availability target.
No. The calculator runs entirely in your browser. No frequency, distance, antenna gain, or transmit power data is submitted to a server. Useful for commercially confidential infrastructure work and environments where information security policy prohibits sending engineering data to third party services.
Yes. The bidirectional solver inverts the relationship so you can input an FSPL budget plus distance to solve for the required frequency, or input an FSPL budget plus frequency to solve for the maximum achievable distance. Useful for early sizing questions where the question is reversed (how far can I reach, or what frequency do I need) rather than what is the loss.
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