Glossary term

Path Loss

Engineering definition of path loss covering free-space path loss, propagation loss, rain attenuation, obstruction loss, received power and validation evidence.

Branch: Telecommunications Engineering
Glossary type: metric
Content: Glossary term
Updated: Jun 26, 2026
Revision: v1.0.0 · reviewed

Definition

metric

Path loss is the reduction in signal power between a transmitter reference point and a receiver reference point caused by propagation and path effects.

Path loss is a positive dB quantity used in wireless, microwave, satellite, radar, telemetry and optical-link budgets. Free-space path loss comes from geometric spreading. Real paths can add atmospheric absorption, rain attenuation, diffraction, scattering, foliage, building penetration, polarization mismatch, antenna misalignment, connector loss, radome loss, terrain obstruction and multipath fading. A path-loss statement should name the reference points, frequency, distance, environment, model and validation evidence.

Path loss is the signal reduction between a transmitter-side reference point and a receiver-side reference point. It is usually written as a positive dB loss that is subtracted in a link budget. The term is most common in wireless, microwave, satellite, radar and telemetry work, but the same accounting discipline also appears in optical fiber and guided-wave systems.

Path loss is not receiver sensitivity and it is not link margin. Path loss describes how much signal the path removes. Receiver sensitivity describes how much signal the receiver needs. Link margin describes how much surplus remains after the link is compared with its requirement.

Free-Space Path Loss

For ideal free-space propagation, path loss can be written as:

\displaystyle L_{FS}=20\log_{10}\left(\frac{4\pi d}{\lambda}\right)

where d is path distance and \lambda is wavelength. A common engineering form using distance in kilometers and frequency in megahertz is:

FSPL_{dB}=32.44+20\log_{10}(d_{km})+20\log_{10}(f_{MHz})

The constant changes if the units change. This formula assumes free-space spreading, not buildings, trees, hills, rain, ducts, indoor walls or antenna misalignment.

Received Power

In a simple RF link budget:

P_{rx}=EIRP+G_{rx}-L_{path}-L_{misc}

where:

EIRP is effective isotropic radiated power;
G_{rx} is receive antenna gain;
L_{path} is propagation path loss;
L_{misc} covers feeder, connector, polarization, pointing, implementation or other losses.

All terms must use compatible dB references. Mixing dBm, dBW, dBi and dB without checking reference units is a common source of false margin.

Real Path Additions

A practical path-loss model may be:

L_{path}=L_{FS}+L_{atm}+L_{rain}+L_{obs}+L_{foliage}+L_{building}

where the added terms represent atmospheric absorption, rain attenuation, obstruction or diffraction, vegetation and building penetration. The right terms depend on frequency, path geometry, environment and service requirement. A short indoor link, rural telemetry path, microwave backhaul, satellite uplink and fiber route do not share one universal loss model.

Worked Example

A point-to-point wireless link operates at:

f=5800\ \text{MHz}

over:

d=12\ \text{km}

Free-space path loss is:

FSPL=32.44+20\log_{10}(12)+20\log_{10}(5800)=129.3\ \text{dB}

The link has EIRP=30 dBm, receive antenna gain G_{rx}=18 dBi, and miscellaneous losses L_{misc}=2 dB. The predicted received power is:

P_{rx}=30+18-129.3-2=-83.3\ \text{dBm}

If receiver sensitivity is -92 dBm, nominal margin is:

M=-83.3-(-92)=8.7\ \text{dB}

If rain and partial obstruction add 6 dB, the received power becomes:

P_{rx,rain}=-89.3\ \text{dBm}

and the remaining margin is:

M_{rain}=-89.3-(-92)=2.7\ \text{dB}

The clear-weather link closes, but the weather or obstruction case may be too thin for a required availability target.

Model Boundaries

Path-loss calculations are first-pass screens. Real links can deviate because antennas are not isotropic, polarization is wrong, Fresnel clearance is poor, terrain reflects energy, ducts form, cables absorb water, connectors corrode, radomes load with ice, trees grow into the path, vehicles block the line, or interference raises the effective threshold.

At high frequencies, rain attenuation, oxygen absorption, waveguide loss, connector quality and pointing accuracy can dominate. At lower frequencies, diffraction and building penetration may be better, but spectrum occupancy, antenna size and interference can become limiting.

Validation Evidence

A defensible path-loss statement includes frequency, distance, coordinate basis, antenna heights, antenna gains, polarization, feeder losses, Fresnel clearance, terrain or building model, atmospheric and rain assumptions, measurement reference plane, calibration state and uncertainty. Field validation should compare predicted received level with measured RSSI or power at the same reference boundary and should record weather, alignment, spectrum occupancy and hardware configuration.

Common Mistakes

Common mistakes include using free-space loss for an obstructed path, confusing path loss with total cable and connector loss, hiding rain attenuation inside an unexplained margin, using visual line-of-sight instead of Fresnel clearance, comparing measurements taken at different receiver reference planes, assuming higher frequency always means worse service without checking antenna gain and bandwidth, and treating a one-day field measurement as proof of all-season availability.

The practical rule is to keep path loss visible as its own term, then show how it affects received power, receiver sensitivity and link margin under the conditions that matter.

REF

Disciplines