Path loss

Last Updated on May 12, 2026 by John Berry

The loss in signal as a wave propagates in an environment between two antennas is termed the path loss. The signal can propagate through, for example, the troposphere alone, or up into the ionosphere and back out again, or out into space to be reflected by the Moon. Understanding path loss is essential for predicting the performance of any amateur radio technology. It describes the total reduction in power density of an electromagnetic wave as it propagates. While free space models provide a baseline, real-world environments introduce complex physical interactions. In addition to free space path loss, you must account for four primary mechanisms: diffraction, reflection, scattering, and absorption.

The losses from those four mechanisms are expressed in decibels relative to the free space path loss. In any loss calculation, the antennas at either end of the link must be stated. There are two options: dipoles, or isotropic antennas and I discuss those on a separate page.

Free Space path loss

Signal power spreads as an expanding sphere. This results in an inverse-square relationship between power and distance (power proportional to 1/d²).The result is the Free Space path Loss (FSL) equation. When antennas are mounted in free space and propagation is through the normal atmosphere near to the surface of the earth, the path loss is given by a simple equation:

FSL(dBi) = 32.4 + 20logd + 20logf

This is the free space path loss accounting for normal spreading of the energy. d is the distance in kilometres, and f is the frequency in megahertz. The answer is given relative to isotropic antennas and denoted by the suffix dBi. 

If dipoles are used as reference, the equation becomes:

FSL(dBd) = 28.1 + 20logd + 20logf

Note that the gain term is applied at both ends, changing the constant to 28.1, and reducing the path loss by 4.3dB.

Diffraction loss

Diffraction enables radio signals to propagate around obstacles and beyond the visual horizon.

Loss due to diffraction varies from a few dBs for some trees in the foreground to maybe 50dB for the blocking of a path by the curvature or bulge of the Earth. It’s complicated to model diffraction,  requiring software and a digital elevation model that includes land use.

Diffraction loss is calculated considering obstruction of the first Fresnel zone. The narrower the obstruction (towards a perfect knife edge), the lower the excess loss. The broader and more rounded the obstruction (or collection of obstructions) the greater the loss.

Diffraction loss can be estimated from knowledge of the path profile and obstructions. It increases with frequency – trees in the foreground will be of little consequence at 144MHz, but at 1300MHz vegetation losses may be more destructive.

Reflection loss

Radio waves reflect from surfaces like the sea, the ground, and the Moon. Strictly reflection occurs where the reflecting surface is smooth relative to the wavelength. Snells Law would also be obeyed and the angle of reflection would equal the angle of incidence. If there are a plethora of disparate reflections, it can perhaps be more accurately described as scattering, and the loss described as scattering loss.

Reflection loss is described by the reflection coefficient, typically a fraction. It can of course be expressed as a dB loss over free space from the equation:

Reflection loss (dB) = 10logρ

Where ρ is the scalar reflection coefficient. The degree of loss depends on surface conductivity and signal polarisation. As an example, the reflection coefficient of the moon has been measured as about 6.5% or 0.065. As a reflection loss, that’s 12dB. Typically, reflection loss from the ground and sea during multi-hop HF communications, and from meteor trails is considered equal to zero.

Reflection can create a second signal path to your antenna. This can result in destructive interference, which significantly increases the effective path loss. Conversely, reflection can result in constructive interference and this is exploited by EME operators at Moon-rise and Moon-set. Multiple reflections from multiple reflectors result in a complex state.

Refraction and absorption losses

Absorption is the direct conversion of radio frequency energy into heat within a medium. This process removes energy from the wave permanently. Refraction itself causes no additional loss. The FSL equation applies to propagation in a normal atmosphere. But there are additional losses  through a medium like the ionosphere. Losses must be added for propagation within the ionosphere and for each entry and exit from the ionosphere.

Absorption occurs when a wave propagates a dense medium such as a rain cloud. Losses at frequencies below 1300MHz are low. Often the distance for which the wave is in the cloud is low compared to the path length and absorption is typically ignored.

Aggregating path loss

Typically, a path loss value can be aggregated by summing all the losses (and gains if there are any) in this sort of equation:

Total loss (dBi or dBd) = free space loss (dBi or dBd) + diffraction loss (dB) + reflection loss (dB) etc.

The total loss is expressed in dBi or dBd, set by the reference antennas used in the free space loss calculation. All the additional losses must be included and there may be many. Here’s a handy calculator. On the left (top on a mobile), calculate the maximum distance between two stations assuming a free space path between them. And on the right (bottom on a mobile), calculate the maximum range while adding a figure for practical excess path loss and another one for net antenna gains and feeder losses.

The total loss is used to determine if a path will work – for example the total path loss to the Moon and back is:

Total loss = free space loss up + reflection loss + free space loss down

As a value, that’s about 197dBi + 12dB + 197dBi, or about 406dBi at 432MHz. Other losses or gains then need to be added, and the Moon’s ‘billboard’ gain subtracted to give a final path loss value on the day.