Your phone shows full bars in one spot but loses half of them just a short walk away. Why? Because free space path loss (FSPL) grows with distance squared and doubling distance doesn’t just “weaken” the signal, it slashes power by a factor of four.
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What FSPL Really Means?
FSPL is the signal weakening that happens purely because energy spreads out as it propagates. Think of dropping a pebble in water, the ripples expand and energy per unit area decreases. For radio waves, this means the received power is inversely proportional to distance squared. Each doubling of distance adds a 6 dB loss which is enough to drop signal levels noticeably on your device. -
Frequency Makes It Worse?
Higher frequencies have smaller wavelengths which means the energy spreads more thinly across space. At the same distance, a 28 GHz 5G mmWave signal suffers far more FSPL than a 700 MHz LTE signal. That’s why low-band spectrum provides wide coverage with fewer base stations while high-band 5G requires dense deployments of small cells just to maintain usable connections. -
Bars Don’t Tell the Whole Story:
The “bars” on your phone are not a direct measurement of FSPL but a simplified RSSI or SINR representation. Two devices showing the same bars may experience very different throughput if one is at a higher frequency (with higher FSPL) or facing interference and multipath effects. In real systems, factors like antenna gain, polarization mismatch and environmental reflections combine with FSPL to decide the actual link quality. -
Critical Formulas:
a). Free-space path loss (dB):
→ FSPL = 20 log₁₀(d) + 20 log₁₀(f) + 32.44 (d in km, f in MHz)
b). Received power:
→ Pr = Pt + Gt + Gr − FSPL
c). Power density:
→ S = Pt / (4πR²)
d). Link margin:
→ LM = Pr − (Receiver Sensitivity + Noise + Fade Margin) -
Real-World Examples:
- A Wi-Fi router at 2.4 GHz covers an apartment easily but the same router at 5 GHz shows dead zones in distant rooms due to higher FSPL.
- Satellite links use large dishes and high power precisely to overcome the brutal FSPL of thousands of kilometers.
- 5G mmWave drops quickly if you step behind a building, not just because of blockage but because FSPL eats away the margin fast.
- Bluetooth earbuds rely on very short ranges since FSPL at just a few meters already challenges their milliwatt-level transmitters.
This image below shows how Free Space Path Loss (FSPL) increases with distance, following the inverse-square law. The curve demonstrates that every time the distance doubles, the signal power drops by about 6 dB which quickly weakens connectivity. It also highlights that higher frequencies like 5 GHz start with more loss in the first meter compared to 2.4 GHz, explaining why higher-band signals fade faster over distance.
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