Loading…
Ground effects and height
How earth and height shape the pattern.
How earth and height shape the pattern.
Earth is part of almost every antenna you will use. Radio waves bounce off the ground, and the reflection adds to or cancels the direct wave. How high the antenna is, measured in wavelengths, and what the soil is like decide the angle the signal leaves at, the feed-point impedance, and how much power is lost in the ground.
For a good conducting ground, the reflected wave behaves as if it came from an image of the antenna, an equal and opposite copy as far below the surface as the antenna is above. A horizontal antenna's image carries current the opposite way, so at the horizon the two waves cancel and add up only at some higher angle. A vertical antenna's image carries current the same way, so they add at low angles. Real earth is lossy and a poor mirror, and the idea is only an approximation, but it explains most of what you see.
Raise a horizontal dipole and the single high lobe splits into lobes that come down toward the horizon. For an ideal ground and a dipole seen broadside, the lowest lobe peaks at this angle:
| Height | Lowest lobe peaks at |
|---|---|
| 0.25 λ | straight up (90°) |
| 0.5 λ | 30° |
| 0.75 λ | about 19° |
| 1 λ | about 14° |
On 40 m, 0.25 λ is about 34 ft and 0.5 λ about 69 ft; on 20 m the same fractions are 17 ft and 35 ft. A low wire sends most of its energy upward, excellent for nearby contacts and poor for far ones. Long-distance skywave contacts favour low takeoff angles, so higher is generally better, to the limit of what you can build.
A vertical depends on the ground for its return path. Seawater and wet, rich soil are good conductors and give strong low-angle radiation; dry sand or rock are poor. Ground losses depend mainly on soil conductivity, and a system of radials reduces them by giving the current a metal path instead.
Ground changes the feed-point impedance of an antenna. A horizontal dipole's impedance rises and falls as height changes, settling toward its free-space value only when well up in the air; see Antenna impedance and radiation resistance. Trees, buildings, fences and wiring act as imperfect ground and reflectors too, which is why measurement on site beats formulas.