Ground system in vertical monopole

A vertical monopole (Marconi antenna) is fed between the base of a vertical wire and the earth. The earth is a working part of the antenna, so its quality and the ground system built under the antenna decide the efficiency.

Fundamentals

• Image action: a conducting ground acts as a mirror and produces an image of the monopole. The λ/4 monopole plus its image behaves like a λ/2 dipole: Rᵣ ≈ 36.5 Ω, directivity ≈ 3.28 (5.16 dBi), maximum radiation along the ground, vertically polarised.
• Return current: the antenna current returns to the base through the ground. Displacement current from the wire enters the ground around the antenna (most within λ/2 of the base) and flows radially back to the base.
• Ground loss: the soil has resistance, so this return current produces loss R_g. Efficiency:

η = Rᵣ / (Rᵣ + R_g + R_other)

With Rᵣ only ≈ 36 Ω (much less for short LF/MF masts, sometimes < 5 Ω), even a few ohms of ground loss can waste most of the power.

• Effect of ground constants: good conductivity σ (sea water, wet soil) gives a near-perfect image and low loss; dry, sandy or rocky soil gives large loss and lifts the radiation away from the ground, reducing the ground wave.

Importance of the ground

1. Completes the circuit of the antenna (acts as the second half of the dipole).
2. Decides radiation efficiency and radiation resistance.
3. Shapes the vertical pattern; good ground keeps the beam low along the earth, needed for MF ground-wave service.
4. Gives lightning protection and a stable reference.

Ground system construction

   Top view of radial ground system
          \   |   /
        \  \  |  /  /
     ------  (mast) ------   60-120 radials
        /  /  |  \  \        each ≈ λ/4 to λ/2
          /   |   \          buried 15-30 cm

1. Buried radial wires: 60–120 copper wires (or strips) run out radially from the base, each λ/4 to λ/2 long, buried 15–30 cm deep and joined at the base to a copper ring or plate. 120 radials of λ/4 is the usual broadcast standard; it brings the ground loss down to about 1–2 Ω.
2. Ground screen / mat: a copper mesh close around the base where current density is highest.
3. Counterpoise: where the soil is rocky, sandy or dry, a network of wires is stretched 2–4 m above the ground, insulated from earth, under the antenna. It acts as a capacitor plate and provides a low-loss return path.
4. Earth electrodes: copper rods or plates driven into moist soil, with salt/charcoal treatment, to keep earth resistance low.
5. Over sea water or very wet ground, a simple earth is enough because the soil itself is a good conductor.

A good ground system raises efficiency from 20–30% (poor ground) to over 80–90%, and strengthens the ground-wave field.