Link Budget Calculator

A free, open-source Windows GUI for RF and satellite link budget analysis. Calculate free-space path loss (FSPL), EIRP, C/N0, Eb/N0, and link margin with ITU-R atmospheric attenuation models — no MATLAB or spreadsheet required. Built as a lightweight native Win32 application in C.

Screenshot

Use Cases

• Satellite link budget — compute end-to-end C/N0 and Eb/N0 for LEO, MEO, or GEO satellite links with rain and gaseous attenuation
• RF link design — size antennas, select transmit power, and verify link margin for point-to-point microwave or telemetry links
• Classroom / self-study — learn how FSPL, antenna gain, noise temperature, and modulation interact in a real link budget
• Quick trade studies — sweep any parameter (frequency, distance, antenna diameter, rain rate) and find the zero-margin crossing instantly

Features

• 8-section input panel — frequency, modulation/coding, geometry, transmitter, TX antenna, path loss, RX antenna, RX system noise
• Antenna modes — direct gain input or parabolic dish (auto-computes gain and beamwidth)
• Geometry modes — slant range or horizontal distance with altitude
• Path loss models — simple FSPL (Friis transmission equation) or ITU-R (P.676-12 gaseous + P.838-3 rain attenuation)
• Clickable unit cycling — frequency (GHz/MHz/kHz), altitude (m/ft/km/NM), power (W/dBm/dBW)
• Sweep plot — margin vs. any parameter with zero-crossing detection and interactive crosshair
• Save / Load — .lbf scenario files for repeatable analysis
• CSV export — all inputs and computed results
• PDF report — direct PDF 1.4 output, no printer driver required
• Portable — single .exe, no installation needed

Installation

Download the latest release from the Releases page:

• LinkBudgetCalculator_vX.X.X_Setup.exe — Windows installer (no admin rights required)
• Link Budget Calculator.exe — portable, runs without installation

How It Works

The calculator implements the standard RF link budget equation:

Link Margin = EIRP - Path Loss + G/T - Required Eb/N0 - Implementation Loss

All intermediate values (EIRP, free-space path loss, atmospheric attenuation, antenna gain, system noise temperature, C/N0, Eb/N0) are computed and displayed. When ITU-R mode is selected, oxygen and water vapour attenuation follow ITU-R P.676-12 Annex 2, and rain attenuation follows ITU-R P.838-3. The sweep plot varies one parameter while holding everything else constant, plotting margin and marking the exact zero-crossing point.

FAQ

What is a link budget?

A link budget is an accounting of all gains and losses in a radio communication system — from transmitter power through antenna gains, path loss, and receiver sensitivity — to determine whether a signal arrives with enough strength to be decoded. Link Budget Calculator automates this calculation with a graphical interface.

Does it support satellite link budgets?

Yes. You can enter slant range or horizontal distance with altitude, choose between FSPL and ITU-R atmospheric models (oxygen, water vapour, rain), and compute C/N0 and Eb/N0 for any orbit — LEO, MEO, or GEO.

What path loss models are included?

Two models: simple free-space path loss (Friis equation) and ITU-R extended mode with gaseous attenuation (P.676-12 Annex 2) plus rain attenuation (P.838-3).

Do I need MATLAB or Excel?

No. Link Budget Calculator is a standalone Windows application. There is no dependency on MATLAB, Python, or spreadsheet software.

Can I generate a report?

Yes. The calculator exports PDF reports (direct PDF 1.4 generation, no print driver needed) and CSV files with all inputs and computed results.

Is it free?

Yes. MIT licensed — free for personal, educational, and commercial use.

Alternatives

Tool	GUI	Free	Offline	Sweep Plot	Standards
Link Budget Calculator	Native Win32	Yes (MIT)	Yes	Yes	ITU-R P.676, P.838
MATLAB link budget scripts	No (scripts)	No	Yes	Manual	Varies
Excel spreadsheets	Spreadsheet	Varies	Yes	Manual	Varies
SatNOGS	Web	Yes	No	No	Basic FSPL
Pathloss 5	Yes	No (commercial)	Yes	Yes	ITU-R full suite

Link Budget Calculator is the only free, offline, native GUI tool that includes ITU-R atmospheric and rain attenuation models with a built-in parameter sweep plot.

Keyboard Shortcuts

Key	Action
F5	Recalculate
Ctrl+S	Save scenario
Ctrl+O	Load scenario
Ctrl+E	Export CSV
Ctrl+P	PDF report
Ctrl+W	Sweep plot

Build

Requires MSYS2 with the MinGW-w64 toolchain.

# Install dependencies (once)
pacman -S mingw-w64-x86_64-gcc

# Build
PATH="/c/msys64/mingw64/bin:$PATH" windres app.rc -O coff -o app.res
PATH="/c/msys64/mingw64/bin:$PATH" gcc -Wall -Wextra -std=c11 -O2 \
    linkbudget_gui.c linkbudget_ui_controls.c linkbudget_ui_logic.c \
    linkbudget_ui_io.c linkbudget_pdf.c linkbudget_core.c app.res \
    -o "Link Budget Calculator" -lcomctl32 -lcomdlg32 -mwindows -lm

# Run tests
PATH="/c/msys64/mingw64/bin:$PATH" gcc -Wall -Wextra -std=c11 -O2 \
    linkbudget_core.c test_linkbudget.c -o test_linkbudget -lm
./test_linkbudget

Project Structure

File	Description
linkbudget_core.c/h	Physics engine — FSPL, EIRP, C/N0, Eb/N0, ITU-R models
linkbudget_gui.c	WinMain, WndProc, sweep window, tooltips, owner-draw buttons
linkbudget_gui_config.h	Layout constants, colours, control IDs
linkbudget_ui_controls.c/h	Section builders, control creators, panel WndProcs
linkbudget_ui_logic.c/h	Visibility, unit cycling, parameter extraction, calculation
linkbudget_ui_io.c/h	Save/load scenario, CSV export
linkbudget_pdf.c/h	PDF report generation
test_linkbudget.c	141 unit tests
version.h	Single-source version definition
installer.iss	Inno Setup installer script

Versioning

Version is defined in version.h. To release a new version, update the three defines and tag:

#define VERSION_MAJOR 1
#define VERSION_MINOR 1
#define VERSION_PATCH 1

git tag v1.1.1 && git push origin v1.1.1

CI will automatically build the installer and portable .exe and attach both to the GitHub Release.

Physics Models

Model	Standard
Free-Space Path Loss	Friis transmission equation
Oxygen attenuation	ITU-R P.676-12 Annex 2
Water vapour attenuation	ITU-R P.676-12 Annex 2
Rain attenuation	ITU-R P.838-3
Parabolic antenna gain	G = η(πD/λ)²
Antenna beamwidth	θ = 67λ/D
System noise temperature	Cascade noise model (Friis)