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Free-Space Path Loss

Key Takeaways

  • Free-space path loss describes the attenuation of signal strength that occurs as an electromagnetic wave travels through open air or a vacuum.

  • Free space path loss is fundamental in wireless communications and radio frequency engineering applications.

  • While free-space path loss primarily affects wireless signal transmission, it can also indirectly influence aspects of PCB design.

A 3D plot of free-space path loss (FSPL)

A 3D plot of free-space path loss (FSPL)
(Source: Wikipedia User Sss41)

When radio signals travel through air or a vacuum, they get weaker the farther they go. This phenomenon is called free-space path loss (FSPL), which is crucial for making wireless communications work effectively.  Engineers have to plan for it when they build wireless communications systems.

The free-space path loss can be calculated using the Friis transmission equation, which is given by:

FSPL (dB) = 20 log10(d) + 20 log10(f) + 20 log10(4𝜋/c)


d is the distance between the transmitter and receiver in meters

f is the frequency of the signal in hertz

c is the speed of light in a vacuum 

The equation considers how electromagnetic waves propagate across distances and how signals act differently depending on their frequencies. The longer the distance or the higher the frequency, the larger the FSPL gets, leading to a weaker signal. 

Free-Space Path Loss Applications

Free space path loss is fundamental to wireless communications and radio frequency engineering. Here's a quick overview of some key applications:

Applications of the Free-Space Path Loss Concept

Wireless Networks

Engineers leverage FSPL calculations to predict signal strength over different distances when planning network deployments and coverage in wireless networks like cell phones, WiFi, and satellites.

Sensor Networks

Engineers also depend on FSPL when designing communication links for nodes on wireless sensor networks, such as weather monitoring or industrial automation.

Radar Systems

A radar system detects objects using electromagnetic waves. FSPL calculations determine a radar's maximum range based on how the sign strength decreases with distance.

Radio Frequency Identification 

Radio frequency identification (RFID) systems identify and track objects wirelessly using electromagnetic waves. Factoring in FSPL allows for optimizing the range and link reliability of the RFID system.

Microwave Links

Point-to-point microwave links, commonly used for long-distance communication, rely on FSPL calculations to determine the RF link budget and assess the feasibility of establishing reliable connections.


Finally, FSPL even assists astronomers by estimating the signal strength of radio waves over vast distances.

Free-Space Path Loss and PCB Design

PCB design focuses on circuits' physical layout and electrical properties, while free-space path loss relates to wireless signal transmission.  However, FSPL can indirectly influence aspects of PCB design, for example:

  • Antenna Design: Impedance matching between PCB traces and an antenna is vital for efficient signal transfer.  Calculating FSPL may impact the choice of antenna type and design parameters to make up for signal loss.  
  • Wireless Communication Modules: Knowing FSPL helps determine the necessary power levels to enable communication, which can influence the power supply and distribution network design on the PCB. 
  • Ground Plane Design: The ground plane design is critical for RF and wireless products.  FSPL factors could guide the arrangement of ground planes and component placement to reduce interference and improve signal integrity. 
  • Transmission Line Effects: As signals travel through PCB traces, transmission line effects like impedance mismatches, reflections, and signal distortion become significant.  Controlling trace impedance is key to minimizing signal degradation.   
  • EMC/EMI Considerations: Managing electromagnetic compatibility (EMC) and electromagnetic interference (EMI) is essential in ensuring that different components on the board don't interfere.  Understanding how electromagnetic signals propagate and lose strength over distance due to FSPL can inform grounding, shielding, and layout choices. 
  • Trace Lengths and Delays: FSPL also relates to signal delays. Engineers must consider the time it takes for signals to travel through traces, especially in high-frequency applications.

FSPL doesn't directly impact PCB layout and design; it does influence decisions on wireless communication modules, antenna design, and the overall performance of RF circuits. Designers must consider these factors to optimize their PCB for reliable and efficient wireless communication.  

Cadence Has the Tools to Help with FSPL in PCB Design

Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools, including our industry-leading OrCAD PCB Designer,  for any application and any level of complexity. To learn more about free-space path loss and RF link budget calculations in system-level models in Cadence, read our white paper, System Simulation for RF Link Budget Analysis.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.