Core Networking · Study deck

Free-Space Path Loss

Picture a field sensor that works across an empty yard but fails after it is mounted behind a wet wall.

Packet Pete is your guide for this deck.

propfspl

Packet Pete, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Explain: A field RSSI reading from a clear doorway might validate the arithmetic for that moment, but it does not prove the link during inventory movement, seasonal leaf growth, rain, or a changed enclosure.
Explain: A 100 m path at 915 MHz is about 71.7 dB in free space, while the same 100 m path at 2.4 GHz is about 80.0 dB.
Explain: If the assumptions are visible, the team can decide whether to raise an antenna, move a gateway, change a band, accept a lower data rate, or schedule a field measurement.

Major section

Start With Distance Taking Energy Away

The clean distance model is only the first line of the evidence.

A gateway means the boundary system that joins local devices to another network or service.
Free-space path loss is the clean baseline: what the path costs before walls, trees, antennas, fading, or interference make it harder.
Once you know the open-air loss, every real deployment adds evidence about materials, height, frequency, antennas, and margin.

Major section

Overview: FSPL Is The Open-Air Baseline

Free-space path loss estimates how much radio power is lost as a signal spreads through unobstructed space.

FSPL is not an indoor or city-range promise.
The practical value of FSPL is that it separates a radio design into two questions.
That separation keeps teams from arguing about range as a single number.

Path-loss estimates become useful only when they are connected to transmit power, antenna gains, receiver sensitivity, and required margin.

Major section

Overview: FSPL Is The Open-Air Baseline (continued)

A 100 m path at 915 MHz is about 71.7 dB in free space, while the same 100 m path at 2.4 GHz is about 80.0 dB.

With equal transmit power and antenna gains, the higher-frequency link starts with roughly 8.4 dB less headroom before walls, antenna orientation, interference, and receiver mode are considered.
For an IoT gateway plan, the baseline should state the distance, frequency, formula convention, and receiver sensitivity before it states the conclusion.
If a designer cannot point to those assumptions, the range estimate cannot be reviewed or repaired when the site changes.

Major section

Overview: FSPL Is The Open-Air Baseline (continued)

If the assumptions are visible, the team can decide whether to raise an antenna, move a gateway, change a band, accept a lower data rate, or schedule a field measurement.

The chain starts with transmit power, which is the radio output plus antenna gain (minus any cable loss).
Materials come next: walls, glass and racks can attenuate the signal.
Received power must stay above receiver sensitivity, with a justified fade margin to spare.

Major section

Overview: FSPL Is The Open-Air Baseline (continued)

Multipath fading then moves the result up or down, and Fresnel blockage can add diffraction loss when terrain or objects sit near the path.

FrequencyAt equal distance, 2.4 GHz has about 8.4 dB more FSPL than 915 MHz because the frequency term is higher.
MarginThe deployment decision uses received power minus receiver sensitivity, with reserve for fading, walls, antenna mismatch, and aging.
With distance in meters and frequency in Hz, use 20 log10(d) + 20 log10(f): 147.55.
Free-space loss takes the first and usually the largest share, because distance and frequency both reduce received power.

Major section

Practitioner: Turn FSPL Into A Link-Budget Record

A field-ready link budget starts with transmit power, antenna gains, cable losses, FSPL, extra deployment losses, receiver sensitivity, and required margin.

The decision is not whether a theoretical signal exists.
The decision is whether enough margin remains after the site-specific losses are added.
A calculated pass still needs field validation.

A link-budget record turns the FSPL baseline into a go, redesign, or field-test decision.

Major section

Practitioner: Turn FSPL Into A Link-Budget Record (continued)

A fail sends you back to antenna height, gain, permitted power, or more gateways.

The path baseline should be followed by explicit allowances for walls, shelving, foliage, vehicle blockage, antenna mismatch, Fresnel clearance, weather exposure, and the fade margin required by the service.
The steps run in one order, and the order matters.
Accepting a link that has no reserve for fading, aging, or installation variance.

Major section

Practitioner: Turn FSPL Into A Link-Budget Record (continued)

A field RSSI reading from a clear doorway might validate the arithmetic for that moment, but it does not prove the link during inventory movement, seasonal leaf growth, rain, or a changed enclosure.

A smart-building estimate that passes in free space can still fail after wall loss, metal shelving, antenna orientation, and people moving through the path.
The useful output is a margin with assumptions, not a single range number.
This record also prevents overfitting to a single optimistic test.

Raw margin alone does not approve a wireless path. The diagram subtracts reserved margin for fading, site variation and measurement uncertainty, then calls for field validation of remaining margin.

Major section

Under The Hood: Model Boundaries Matter

The logarithmic shape is the key engineering idea.

Doubling distance adds about 6 dB of free-space loss because the wavefront area grows with the square of distance.
Increasing frequency also raises the loss term in the usual link-budget form, so a radio comparison at the same range still needs the band written down.

Path-loss exponent and site-loss assumptions change the distance term once the path is no longer free space.

Major section

Under The Hood: Model Boundaries Matter (continued)

These terms are deterministic; the uncertainty enters when the path stops looking like the model.

A useful boundary note names which losses are inside the calculation and which are still external assumptions.
Free-space spreading is inside.
Free space uses about n = 2.
Free-space loss may fit a clear outdoor path.

Major section

Under The Hood: Model Boundaries Matter (continued)

Indoor and obstructed environments often need a larger exponent plus explicit wall, body, foliage, or fade-margin allowances.

Wall attenuation, enclosure detuning, polarization mismatch, partial Fresnel blockage, body shadowing, rain fade, interference, and receiver implementation margin are outside unless the record adds them explicitly.
Free space uses 2.0; the example open-outdoor, office, and warehouse values are 2.2, 3.0, and 4.0.
Unit DisciplineThe constant changes with units.

Major section

Under The Hood: Model Boundaries Matter (continued)

The warehouse example doubles the free-space exponent, so a tenfold increase in distance adds 40 dB under that model instead of 20 dB in free space.

The exponent is not a decoration on the formula; it changes how quickly distance consumes margin.
Larger values describe environments where reflections, absorption, diffraction, and clutter make each distance increase more expensive.
A room or a path with trees needs measured factors for walls, clutter, or leaves.

Major section

Under The Hood: Model Boundaries Matter (continued)

The shadowing term X_sigma is the reminder that two paths with the same distance can still behave differently.

For IoT deployments, the under-the-hood review should therefore check model choice, unit consistency, and validation evidence together.
A model that is too harsh wastes gear, while one that is too kind hides gaps.
Environment DriftWarehouses, hospitals, farms, and city streets do not share one path-loss exponent.

Deck summary

Key takeaways

The clean distance model is only the first line of the evidence.

Free-space path loss estimates how much radio power is lost as a signal spreads through unobstructed space.
A 100 m path at 915 MHz is about 71.7 dB in free space, while the same 100 m path at 2.4 GHz is about 80.0 dB.
If the assumptions are visible, the team can decide whether to raise an antenna, move a gateway, change a band, accept a lower data rate, or schedule a field measurement.
A fail sends you back to antenna height, gain, permitted power, or more gateways.

Retrieval practice

Recall check 1 of 4

Packet Pete says: answer from memory, then check your reasoning.

Q1Use FSPL(dB) = 20 log10(distance in km) + 20 log10(frequency in MHz) + 32.45. For 1 km and 100 MHz, what is the free-space loss?

A32.45 dB.

B52.45 dB.

C72.45 dB.

D92.45 dB.

Show answer

Answer: C The distance term is zero, and the frequency term is added to the unit-specific constant.

Retrieval practice

Recall check 2 of 4

Packet Pete says: answer from memory, then check your reasoning.

Q2A 915 MHz LoRa link and a 2.4 GHz Wi-Fi link are both measured at 100 meters in clear line of sight. Which statement is correct for free-space path loss?

AThe 915 MHz link has more free-space path loss because lower-frequency signals travel farther.

BBoth links have the same free-space path loss because only distance appears in the formula.

CThe Wi-Fi link has more path loss only because Wi-Fi uses more bandwidth.

DThe 2.4 GHz link has more free-space path loss because the frequency term is higher.

Show answer

Answer: D FSPL rises with both distance and frequency, so comparing radios at the same distance still requires the frequency term.

Retrieval practice

Recall check 3 of 4

Packet Pete says: answer from memory, then check your reasoning.

Q3A Wi-Fi sensor has a free-space link margin of 30 dB, but the route crosses concrete, shelving, and a crowded corridor. What should the deployment record do next?

AApprove placement from free-space margin without surveying site losses.

BAdd site losses, recompute reserve, and verify with field measurements.

CReuse a different Wi-Fi band's indoor-loss estimate as exact here.

DUse data-sheet maximum range as final gateway spacing.

Show answer

Answer: B A practical link budget starts with FSPL, then adds deployment losses and verifies that adequate margin remains.

Retrieval practice

Recall check 4 of 4

Packet Pete says: answer from memory, then check your reasoning.

Q4Which review finding should stop a path-loss estimate from being used as deployment evidence?

AThe record states distance, frequency, formula units, receiver sensitivity, and required margin.

BThe record uses an indoor path-loss exponent and adds wall losses for a building deployment.

CThe record mixes meters with the km/MHz FSPL constant and then treats the result as a final indoor range.

DThe record requires field RSSI or packet-delivery checks after installation.

Show answer

Answer: C Path-loss estimates fail when units are inconsistent or when a free-space baseline is treated as proof for obstructed deployment conditions.

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Answers

Answer key.

C · The distance term is zero, and the frequency term is added to the unit-specific constant.
D · FSPL rises with both distance and frequency, so comparing radios at the same distance still requires the frequency term.
B · A practical link budget starts with FSPL, then adds deployment losses and verifies that adequate margin remains.
C · Path-loss estimates fail when units are inconsistent or when a free-space baseline is treated as proof for obstructed deployment conditions.