Result for
Signal strength vs distance
Below the dashed line a listener no longer gets reliable reception. Hover over the plot to read values.
Result for
Below the dashed line a listener no longer gets reliable reception. Hover over the plot to read values.
Scroll or pinch to zoom, drag to pan. Heights are exaggerated and wave tilt is drawn ×3 so you can see them.
Choose + Transmitter or + Receiver and click the map to add a site. To remove a pin, choose ✕ Remove and click it, or right-click or double-click it. Use ↺ to reset. Drag to pan, scroll to zoom.
Paths are from the selected transmitter. Choose "+ Receiver" or "+ Transmitter" on the map and click anywhere in the world to add sites. Select a row to see its path below.
See the mountains, buildings and signal level along the selected path in the Terrain profile tab.
Short explanations of each effect you can test. Open a card, then try the matching experiment in the Lab.
A ground (surface) wave is a vertically polarized wave that is guided along the boundary between air and earth. Currents induced in the ground drag the lower part of the wavefront, so it follows the earth's curvature well beyond the optical horizon. That is why MW (medium-wave) AM broadcasting gives stable daytime coverage over hundreds of kilometres.
Horizontal polarization is short-circuited by the conducting ground, so groundwave systems use vertical masts.
Losses depend on ground conductivity σ and relative permittivity εr. The lab uses the Sommerfeld–Norton flat-earth approximation (Terman's form):
Small p means little loss (sea water, low frequency). Large p means rapid attenuation (dry ground, high frequency).
p grows with frequency through both λ and x. At 200 kHz over sea the attenuation factor stays near 1 for hundreds of kilometres. At 10 MHz over dry land, groundwave is gone within a few tens of kilometres. This is why LF and MF carry broadcasting and navigation, and HF relies on the sky wave.
Ground losses tilt the wavefront forward. The power flow then has a downward component that feeds the losses in the earth. The tilt angle is roughly tan ψ ≈ 1/√|εr − j x|. Wave tilt is larger over poor ground. The scene exaggerates it three times so you can see it.
The flat-earth formula is good up to about 80/∛fMHz km. Beyond that, diffraction around the curved earth adds loss, roughly 0.04·∛f dB per km at standard refraction. Refraction in the lower atmosphere bends waves slightly. It is modelled with an effective earth radius factor k. Larger k (super-refraction in warm, humid inversions) reduces the curvature loss. k = 1 increases it.
A wave returns when f ≤ fo·sec φ (secant law). Steep rays at high frequency escape, which creates the skip zone.
By day, the D layer absorbs the MW sky wave, so only the ground wave is heard. At night the D layer vanishes and the sky wave returns strongly hundreds of kilometres away. Where ground and sky waves arrive with similar strength (within about 6 dB), their changing phase causes deep fading and distortion. The lab shades that zone on the plot.
For a land–sea path, the field is the average of forward and reverse calculations, in dB:
Over sea after a coastline, the field can actually rise for a while. This is the "recovery effect".
The lab is a teaching model, not a planning tool. Use ITU-R P.368 (groundwave curves), P.527 (ground constants), P.1147 (MF sky wave), P.533 (HF) and P.372 (radio noise) for real work. The sky wave here is a median value. Real signals fade by 5–10 dB within minutes.
| Ground (surface) wave | Vertically polarized wave guided along the earth's surface. Main MW/LF daytime service. |
| Sky wave | Wave reflected (refracted) back to earth by the ionosphere. |
| Conductivity σ | How easily the ground carries current, in S/m. Sea ≈ 5, average land ≈ 0.003, rock ≈ 0.0001. |
| Permittivity εr | How strongly the ground stores electric field energy. Matters most at higher frequencies. |
| Numerical distance p | Norton's measure of how "electrically far" a point is over lossy ground. Larger p means more loss. |
| dBµV/m | Field strength in decibels above 1 µV/m. 60 dBµV/m = 1 mV/m. Every 20 dB is ×10. |
| Critical frequency fo | Highest frequency a layer reflects at vertical incidence (foE, foF2). |
| Skip zone | Region with no ground wave and no returning sky wave. |
| Fading zone | Region where ground and sky waves are similar in strength and interfere. |
| Millington's method | Standard way to combine different ground types along one path. |
| Great-circle path | Shortest route between two points on the earth's surface. |
Several questions ask you to load an experiment in the Lab first. Your score updates as you answer.