4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation

4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation

ACMA Exam Focus: Syllabus item 7.2 — under free-space conditions radio waves travel in straight lines and spread out as they go. Examinable material covers inverse-square spreading, surface (ground) wave behaviour and its dependence on frequency and ground conductivity, the direct or space wave, the radio horizon and why it is farther than the optical horizon, and practical VHF/UHF path planning.

Free Space: Straight Lines and Spreading

The starting assumption of item 7.2 is a wave in free space — no ground, no obstructions, no ionosphere. Under those conditions a radio wave leaves the antenna and travels in a straight line at constant speed, and it spreads out as it goes.

Spreading is the key idea. The energy leaving the antenna passes outward through the surface of an ever-expanding sphere, and the area of that sphere grows as the square of the distance. The same fixed number of watts is therefore smeared over four times the area every time the distance doubles, so power density falls as the inverse square of distance:

This is free-space path loss. Nothing is absorbed or destroyed; the signal is simply thinned out. It is also why a 6 dB improvement at the antenna matters so much — it restores exactly what you lost by doubling the path length.

Three Ways a Signal Travels Near the Earth

Real paths are not free space. Near the ground three mechanisms compete:

1. Ground wave (surface wave) — clings to the surface and follows the Earth's curvature.
2. Direct wave (space wave) — a near-straight line from antenna to antenna, usually with a ground-reflected ray alongside it.
3. Sky wave — refracted back to Earth by the ionosphere, covered separately in the ionospheric propagation sections.

Ground Wave Propagation

The ground wave travels along the boundary between the air and the Earth. Its lower edge is slowed and absorbed by currents induced in the ground, so the wavefront tilts forward and loses energy, but that same interaction lets it diffract around the curvature of the Earth and reach receivers well beyond the visual horizon.

Three characteristics dominate the exam questions:

• It must be vertically polarised. A horizontally oriented electric field lies parallel to the conductive ground, which effectively short-circuits it. This is why medium-wave broadcast masts and marine stations use vertical radiators.
• Attenuation rises steeply with frequency. The lower the frequency, the farther the ground wave reaches. This is the single most examined property.
• Ground conductivity matters enormously. Salt water is an excellent conductor at around 5 siemens per metre and hardly attenuates the wave at all; dry sand, rock and the arid soils of inland Australia are poor conductors near 0.001 siemens per metre and absorb the wave rapidly.

Those figures are indicative only; the trend is what is examinable. Useful ground-wave coverage effectively disappears above the lower HF bands.

The Direct Wave and Ground Reflection

Above about 30 MHz the ionosphere normally lets signals pass straight through into space, apart from sporadic-E and other special modes. VHF and UHF work therefore relies on the direct wave, sometimes called the space wave: a straight-line path from transmitting antenna to receiving antenna.

In practice two rays usually arrive: the direct ray and a ground-reflected ray. The reflected ray travels slightly farther and suffers a phase reversal on reflection, so the two can reinforce or partly cancel. That is the cause of the rapid fluttering, or picket-fencing, heard when working a 2 m repeater from a moving car.

The Radio Horizon

Because the direct wave travels in an essentially straight line, range is limited by the curvature of the Earth rather than by transmitter power. The optical horizon — how far you can actually see — is given by d (km) = 3.57 x sqrt(h) with the antenna height h in metres.

The radio horizon is farther. The troposphere's refractive index falls gradually with altitude, so a radio wave is bent very slightly downward, following the curve of the Earth a little way past the visual limit. Under standard atmospheric conditions this is modelled by pretending the Earth is larger than it really is, multiplying its radius by the k-factor of 4/3 to give an effective radius near 8500 km. The constant in the horizon formula grows accordingly:

d (km) = 4.12 x sqrt(h) for one antenna, and for a path between two stations:

d (km) = 4.12 x ( sqrt(h1) + sqrt(h2) )

Comparing the two constants, 4.12 divided by 3.57 is about 1.15, so the radio horizon is roughly 15% farther than the optical horizon.

Worked example 1 — mountaintop repeater

A repeater antenna stands 625 m above the surrounding plain and a mobile whip is 4 m above the road. The square roots are 25 and 2, giving a total of 27, so the radio horizon is 4.12 x 27 = 111.2 km.

Worked example 2 — is the path viable?

Two home stations have masts 100 m and 25 m above local terrain and want to work a 70 km path. The square roots are 10 and 5, so the horizon is 4.12 x 15 = 61.8 km. The path lies about 8 km beyond line of sight, so contact depends on diffraction or a lift in conditions rather than a clean direct wave.

Practical VHF and UHF Path Considerations

• Height beats power. The horizon formula contains a square root of height and no power term at all. Raising an antenna from 9 m to 36 m doubles its horizon contribution; doubling transmitter power adds only 3 dB.
• Clearance, not just sightline. Obstacles intruding into the first Fresnel zone around the direct ray cause extra loss even when the path looks visually clear.
• Diffraction extends coverage. Signals bend over sharp ridges, so contacts are routinely made somewhat beyond the calculated horizon.
• Ducting can smash the limit. Temperature inversions over water, common along the Australian coast and across Bass Strait, form tropospheric ducts that carry VHF and UHF signals hundreds of kilometres.
• Higher frequencies suffer more locally. Foliage, rain and building penetration losses all worsen from 2 m to 70 cm and above.