Ground wave
| Ground wave | |
|---|---|
| What it is | Radio signals that travel near the earth's surface rather than by the ionosphere |
| Parts | Surface wave, plus the direct and ground-reflected waves (the space wave) |
| Polarization | Vertical; horizontally polarized surface waves die out quickly |
| Best frequencies | Low and medium frequencies; range falls quickly as frequency rises |
| Factors | |
| Best ground | Seawater |
| Poorest ground | Dry, sandy or rocky soil; built-up areas |
| Time of day | Little effect, unlike skywave |
| Typical antenna | Vertical with a good radial system |
Ground wave is the part of a radio signal that travels along or close to the earth's surface instead of being returned from the ionosphere. It is what carries an AM broadcast station's daytime signal across a region, and on the amateur bands it provides steady, short-range coverage that does not depend on the sun. Ground wave works best at low frequencies, with vertically polarized antennas and over ground that conducts electricity well. As frequency rises its range shrinks quickly, which is why it matters most on the lowest amateur bands.
Surface wave and space wave
Engineers divide the ground wave into two parts.
The surface wave clings to the earth and follows its curve, so it can reach beyond the horizon. The ground carries electric currents as the wave passes, and the wave loses energy to it all the way along. That loss is what limits range. It is far smaller for vertically polarized signals than for horizontal ones, whose electric field runs parallel to the ground and is nearly shorted out by it. In practice, a surface wave is a vertically polarized signal, and a vertical antenna is the natural way to launch one.
The space wave is made up of the direct signal, traveling in a straight line from antenna to antenna, and the part reflected from the ground in between. It is limited to roughly the line of sight, extended a little because the atmosphere bends signals slightly. On VHF and UHF, the space wave is almost the whole story, and hams often loosely call it ground wave. Its range depends mainly on antenna height: the radio horizon in miles is roughly 1.4 times the square root of the antenna height in feet.
What affects range
- Frequency. Ground losses rise steeply with frequency. A signal on the 160-meter band can travel many times as far by ground wave as one on the 10-meter band at the same power.
- Ground conductivity. Ground that conducts well absorbs less of the wave.
- Antenna. A vertical with an efficient system of radials puts more energy into the surface wave than a short or poorly grounded one.
- Power and noise. More power and a quieter receiving site both stretch usable range, though doubling power adds less distance than might be expected.
- Terrain. Hills and buildings absorb and shadow the signal, especially at the higher frequencies.
| Ground type | Ground-wave performance |
|---|---|
| Seawater | Excellent; ranges are greatest over the sea |
| Fresh water, marshes, moist farmland | Good |
| Average soil | Fair |
| Dry, sandy or rocky ground | Poor |
| Cities and industrial areas | Poor |
On the amateur bands
Ground wave is the main short-range mode on the 2200-meter and 630-meter bands, where it gives steady regional coverage day and night, though the low radiated power permitted there keeps distances modest.
On 160 meters, a station with a good vertical over good ground can often be heard out to roughly 100 miles by day, when the absorbing D layer of the ionosphere keeps skywave signals from traveling far. On the 80-meter band the useful daytime ground wave is shorter, typically some tens of miles, and on the higher HF bands it fades out within a few miles to a few tens of miles, depending on power, antennas and ground. Past that point a station can be inaudible until its skywave returns, much farther out, leaving a gap called the skip zone.
At night, skywave joins the ground wave on the lower bands. Where the two arrive with similar strength they add and cancel as the ionosphere shifts, so a local signal on 160 or 80 meters that was steady by day may fade in and out after dark.
The same high conductivity that makes seawater ideal for ground wave also strengthens the low-angle radiation of vertical antennas near it, which is why some operators take portable stations to the shore. For dependable regional HF coverage without verticals, horizontal antennas use NVIS instead, sending the signal almost straight up to fill in the area ground wave cannot reach.