Friis Free-Space Path Loss
Lesson notes
Overview
Signal strength falls with the square of distance and frequency. This topic is rated intermediate and is number 69 of 93 in Engineering. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
Radio signals weaken as they spread out. The free-space path loss is FSPL = 20 log₁₀(d) + 20 log₁₀(f) + 32.4 dB, with d in kilometres and f in megahertz: doubling the distance or the frequency adds 6 dB of loss. The Friis equation uses it to find the received power from the transmitted power and antenna gains, and is the starting point for designing radio links, with extra margins for rain, buildings and fading.
Reading the relationship
The expression "FSPL = 20 log(d) + 20 log(f) + 32.4" is a compact summary of the idea. It uses the symbols F, S, P, L, d, f; name what each one stands for before you start. Notice that it contains a logarithm, which turns multiplication into addition and compresses large values. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
Always ask what the safety margin is and which assumption would break the design first. Idealised models hide losses, noise and limits.
How it connects
It builds on “Antenna Gain and Beamwidth”, the topic before it in Engineering. Once you are comfortable, continue with “Signal-to-Noise Ratio in Decibels”.
Why it matters
Engineers use this idea to design structures, machines and systems that are safe, efficient and reliable.
Where you might use it
- Revising “Friis Free-Space Path Loss” before a class, exam or certification
- Designing safe structures, machines and control systems that must work reliably
- Diagnosing faults in electrical, mechanical or signal-processing equipment
- Checking that a design meets safety margins before it is built or deployed
Frequently asked questions
What is Friis Free-Space Path Loss?
Signal strength falls with the square of distance and frequency. It is an intermediate topic in Engineering.
How do I use the Friis Free-Space Path Loss simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression FSPL = 20 log(d) + 20 log(f) + 32.4 packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Antenna Gain and Beamwidth”, the previous topic in Engineering. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.