How the MFH Coverage Engine Works

The link budget

For every point of the map the engine computes the received signal as: transmitter power, minus cable loss, plus the antenna gain in the right direction (and at the right take-off angle on HF), minus the path loss, plus the receiver antenna gain. It compares that with the noise in the bandwidth of the chosen mode and obtains a signal-to-noise margin over what the mode needs. A statistical spread turns the margin into a reliability: the chance that the link works.

Noise

Man-made, galactic and atmospheric noise are added in power, using smooth curves in the style of ITU-R P.372 for city, residential, rural and quiet sites, together with the noise figure of the receiver. On HF the external noise dominates; on UHF and above the receiver does.

Below 1.6 MHz: ground wave and night sky wave

The ground wave uses Norton’s numerical-distance approximation over the soil you choose (conductivity and permittivity from very poor ground to sea water). At night a sky-wave component reflected from the lower ionosphere is added, while by day it is strongly absorbed.

1.6 to 30 MHz: the ionosphere

• Critical frequency of the F2 layer. A parametric model driven by the solar flux (converted to a smoothed sunspot number), the local solar time with a lag after sunrise and a slow decay after sunset, the season, the geomagnetic latitude and the Kp index.
• MUF. From the geometry of each hop (spherical Earth, layer height, take-off angle). The path MUF is the lowest of its hops, and the day-to-day spread gives the chance that the frequency is below it.
• Absorption. D-layer absorption that grows with the Sun’s elevation and the sunspot number, falls with the square of the frequency and grows with the length of the ray inside the layer, plus auroral absorption with Kp. A calibration factor aligns it with typical amateur-radio path losses.
• Antenna over real ground. Fresnel reflection coefficients for the chosen soil give the elevation pattern of horizontal antennas at their height and of verticals over real ground; beams add an azimuth pattern.
• Hops. Up to about 3,600 km per hop; the engine tests the minimum number of hops and one more and keeps the better, adding a loss at each ground reflection.

30 to 3000 MHz: terrain, curvature and diffraction

• Elevation profile. For each direction the engine reads the heights of the ground from our own elevation tiles (SRTM data converted by Map For HAM), sampled finely enough to catch ridges. You choose the precision from about 600 m to about 75 m per point.
• Curvature of the Earth. The profile is corrected with the effective Earth radius k × 6371 km; k = 4/3 for the standard atmosphere.
• Obstacles. Multi-edge knife-edge diffraction after the Deygout method of ITU-R P.526, on the real profile and with the heights of both antennas above the local ground.
• Smooth earth and ground reflection. The engine takes the largest of the terrain diffraction, the plane-earth loss (two-ray model, softened on rough terrain) when the path is in view, and the spherical-earth diffraction beyond the radio horizon.
• Environment. A loss for buildings and trees at the receiver (open, rural, suburban, urban) that grows slowly with frequency.
• Variability. A small spread in line of sight, a large one in deep shadow.

What is not modelled

Sporadic-E and other anomalous HF modes, tropospheric ducting and scatter, rain and gas attenuation at microwave frequencies, the real height of buildings and trees, polarisation mismatch and the exact ionosphere of a given day. See about and limits.

Try it

Open the radio signal coverage map, or read the guides on the radio horizon and on how terrain affects coverage.