Ground Conductivity and HF Antennas: What Soil Does to Your Signal

Two radio amateurs build the same vertical antenna with the same radials. One lives on a sandy hill, the other by the sea. The one by the sea works DX with a signal that seems to be 6 decibels stronger, and neither of them did anything different. The difference is the ground. Everything that is not the antenna and the sky, and in particular the soil under and around the antenna, takes part in the radiation. Ground conductivity and HF antenna performance are linked in a way that is easy to underestimate: the earth reflects, absorbs and carries currents, and the amount of each depends on what the earth is made of and how wet it is. This article explains the electrical properties of the ground in plain terms, shows what they do to vertical and horizontal antennas, and lists the practical steps that bring the most improvement when the ground is poor.

What Ground Is, Electrically

For radio frequencies, the ground is a lossy medium. It conducts a little, it stores some electric energy like an insulator, and its properties vary from one place to the next. Two numbers describe it. The first is the conductivity, written with the Greek letter sigma and measured in siemens per metre, which tells how easily current flows. The second is the relative permittivity, also called the dielectric constant, which tells how much the material stores electric energy compared with a vacuum. The idea of conduction in a material is described in the article on electrical resistivity and conductivity.

Antenna modelling programs use typical values. The figures below are those that are commonly offered as defaults in programs such as NEC and EZNEC, and they give a good idea of the range.

The range is wide: sea water conducts a thousand times better than dry sand. Between a good soil and a poor one, the conductivity differs by a factor of thirty, and since moisture is the main variable, the same plot of land can move across several rows of the table in a year.

Two Ways the Ground Affects an Antenna

The ground influences the antenna in two separate regions, and it is helpful to keep them apart. The first is the near field, the region within a wavelength or so of the antenna. Here, the antenna currents induce currents in the soil, and those currents flow through a resistance and heat the ground. That is a loss. A vertical antenna with a few short radials sends a large part of its power into the soil near its base, and the efficiency can fall to a few tens of percent. The cure is a better ground system, which brings the return currents back in metal and not through the earth.

The second is the far field, the region from tens to hundreds of metres out, where the wave that leaves the antenna at a low angle meets the ground and is reflected. The reflection decides how much of the wave at a given takeoff angle is added to the direct wave, and it depends on the conductivity and permittivity of the soil out there, not under the antenna. A poor soil reflects badly at low angles, where the wave grazes the ground, and a good soil or sea water reflects well. This is the effect that makes the shoreline site so attractive, and it is the reason why a perfect radial system close to the antenna cannot remove the dependence on the soil. Both aspects are visible in the modelling of the antenna described in the article on EZNEC and 4nec2, where changing the ground type in the program changes the pattern.

Vertical Antennas: The Most Sensitive Case

A vertical monopole depends on the ground more than any other simple antenna. Its radiation at low angles, the angles that matter for DX, travels over the ground and is reflected from it. With average ground, a quarter-wave vertical with a good radial field has a peak gain at a takeoff angle of around 20 to 25 degrees, and the gain at 10 degrees is reduced compared with a perfect ground. With poor ground, the peak gain drops by several decibels and the maximum moves to a higher angle. Over sea water, the gain is close to that of the ideal case, with the maximum at a low angle, and the improvement relative to average ground can be more than 5 decibels at 10 degrees. This is the reason for the old observation that a vertical at the beach “works wonders”.

What can be done at an inland site? The near-field loss is under your control. A large number of radials, 16 to 60 on or just above the ground, reduces the loss in the soil close to the antenna, as the article on vertical antenna radials shows, and raised radials, a metre or two above the ground and tuned to length, reduce the dependence on the soil even more, since the ground below them is shielded by the radial plane. The far-field reflection is outside your control, and the loss at low angles in poor soil is a property of the site. That is a reason to treat the vertical as an antenna for the bands where its low-angle radiation is most useful, such as 160 and 80 metres, and to consider the horizontal alternatives for others.

Horizontal Antennas: Height Beats Soil

A horizontal dipole or a beam is less sensitive to ground conductivity, but it is not independent of it. The reflection from the ground creates an image of the antenna below the surface, and the pattern of the real antenna is the sum of its direct radiation and its reflected radiation. The height above the ground, measured in wavelengths, sets the phase of the sum, and therefore the takeoff angle of the main lobe, as explained in the article on antenna height and takeoff angle. The conductivity changes the strength of the reflection. For high horizontal antennas, a quarter wavelength or more above the ground, the effect of the type of soil on the gain is small, a fraction of a decibel up to one or two decibels. For low dipoles, a tenth of a wavelength or lower, the ground acts as part of the antenna, the efficiency falls and the pattern turns into a high-angle one. This is good for regional contacts and poor for DX.

The practical statement is that the higher the horizontal antenna, the less its performance depends on the ground, and the lower the antenna, the more the ground controls the result. A low wire over dry soil can be several decibels worse than the same wire over wet soil, and the difference appears in the SWR as well: the feed impedance of a low dipole changes with the soil moisture, which is why its resonance shifts after rain.

Where the Reflection Happens

A simple piece of geometry shows how far out the ground matters. A wave that leaves the antenna at a low takeoff angle and reflects once from the ground meets the soil at a distance equal to the height of the antenna divided by the tangent of that angle. For a dipole at 10 metres, a wave leaving at 10 degrees meets the ground about 57 metres away, and at 20 degrees about 27 metres away. For a beam at 30 metres, the 10 degree wave reflects 170 metres from the tower. These distances are far outside the radial field, and they cover a large area of whatever soil lies there, such as a lawn, a field, a road or a neighbour’s garden. This is why you cannot correct the far-field soil with a few radials, and why moving a high antenna by a few tens of metres can change the low-angle pattern.

Moisture, Frost and Seasons

The soil is not constant. Rain raises the conductivity of the upper layers, and a dry summer lowers it. Frozen ground behaves as a poor conductor, because ice does not conduct like liquid water, and a thick layer of snow adds a dielectric blanket. The result is that the performance of a vertical and of low wires changes through the year, and that your own notes of signal reports in different seasons may show the effect. Radio waves of HF frequencies penetrate the ground to a depth that depends on the frequency and the conductivity, known as the skin depth, from a few metres at low frequencies in poor soil to less than a metre at higher frequencies and with better conductivity. The effect is described in general terms in the article on the skin effect. The consequence is that the moisture at a depth of one or two metres can matter, and that a deep, damp layer under a dry surface is better than the surface alone suggests.

Finding Out What You Have

You do not need to measure the soil to make good decisions, but you can make a reasonable estimate. National services publish maps of effective ground conductivity, and the ground type of your location, farmland, forest, city or coast, gives a first answer. If you model your antenna, run it with two or three ground types, average, poor and good, and see how large the spread is. A big spread says that the site matters and that investment in radials, height or a different antenna type pays back. A small spread says that you can ignore the soil. For a practical test, compare the signal reports of the antenna in wet and dry weeks, or compare it with a second antenna at a distance of a few hundred metres, if you have such an option.

Practical Steps for Poor Ground

• Raise horizontal antennas. Every metre helps, up to a half wavelength. The gain at low angles rises and the dependence on the soil falls.
• Use many radials with verticals. Thirty short radials are better than four long ones in the near-field loss, and raised radials are a better choice than buried ones in poor soil.
• Avoid the ground-mounted vertical with a rod. A single ground rod is a poor RF ground. It is useful for lightning protection and not as a counterpoise.
• Choose the antenna for the band. A vertical for 160 and 80 metres and a high horizontal antenna for the higher bands is a common and effective combination.
• Move the antenna if you can. A few metres can put it over a damper, more conductive patch, away from a rocky outcrop or a concrete slab.
• Do not rely on a rooftop as a ground. A metal roof can behave as a ground plane, but a wooden or tiled roof is a poor one, and the building’s wiring can carry RF where you do not want it.

The Bottom Line

Ground conductivity and the HF antenna are inseparable. Typical soils range from 0.001 S/m for dry sand to 0.03 S/m for rich wet soil, and sea water reaches 5 S/m. The ground causes loss near the antenna and sets the reflection that shapes the low-angle pattern at a distance. Verticals depend on it most, losing several decibels at low angles over poor soil and gaining over sea water, while horizontal antennas depend on it less the higher they are. A large radial field, raised radials, height for horizontal wires and the right antenna for each band are the practical answers, and seasonal changes in moisture and frost explain why an antenna can behave differently in winter and in summer.

Frequently Asked Questions

What is a typical ground conductivity for ham radio modelling?

A common default for average ground is 0.005 S/m with a relative permittivity of 13. Very poor ground is around 0.001 S/m and good ground around 0.03 S/m.

Does ground quality matter for a dipole?

It matters little for a dipole a quarter wavelength high or more, and more for low dipoles, where the ground reduces efficiency and raises the takeoff angle.

Why is sea water so good for verticals?

Its conductivity is about a thousand times that of dry sand, so it reflects low-angle waves well. A shoreline vertical can show several decibels of gain at low angles.

Can radials make up for poor soil?

They reduce the loss near the antenna, which is a large part of the problem, but they cannot change the soil far away that sets the reflection of the low-angle wave.

Why does my antenna change after rain?

Moisture changes the conductivity and permittivity of the soil, which shifts the feed impedance and the resonance of verticals and low wires.

Is a ground rod a good RF ground?

Not by itself. It is useful for lightning and electrical safety, but an RF ground for a vertical needs a field of radials or a ground plane.