Also known as: radials, counterpoise, ground radials, radial system
Radials and a counterpoise are the artificial ground system placed under a vertical monopole antenna to give its current somewhere to return to.1 A monopole is only half an antenna — the “missing” half is supplied by its image in a conducting ground plane — so it needs a good electrical ground beneath it to work. Radials are wires laid out from the base like spokes; a counterpoise is a similar system of conductors that stands in for a real earth connection, often when a true ground is impractical. Together they set the antenna’s efficiency and stabilize its feedpoint.
How it works
Current fed into a monopole must complete a circuit. In an ideal installation over perfectly conducting earth, the ground acts as a mirror, reflecting the antenna’s image and supplying the return current with no loss. Real soil is far from a perfect conductor, so the return currents flow through lossy earth, adding resistance that dissipates power as heat — this ground loss is often the single largest inefficiency of a ground-mounted vertical. Radials intercept those return currents in low-resistance copper before they reach the lossy soil:
- Buried (or on-ground) radials are laid on or just under the surface. Because they work with the soil rather than resonating, more is better and length is not critical — 30 to 60+ quarter-wave radials approach the performance of a solid ground plane, while a handful gives poor efficiency. This is the classic broadcast-tower ground field.
- Elevated radials are raised above ground and do resonate. A surprisingly small number — as few as two to four tuned quarter-wave radials — can rival dozens of buried ones, because they are decoupled from the lossy earth and carry the return current in the air.
- A counterpoise is a conductor or set of conductors serving as the ground reference when no earth connection is used at all — for example a metal vehicle body under a mobile whip, or a wire grid under a portable vertical.
The ground system also fixes the feedpoint impedance. A good radial field brings a quarter-wave monopole to its ideal ~36 Ω; a poor one raises the apparent resistance (extra loss resistance in series) and can shift resonance, degrading the match.
In practice
The trade-offs are practical rather than exotic. Buried systems are mechanically robust and invisible but need a lot of wire; elevated systems are efficient with little wire but must be supported and kept clear of people. Radial length interacts with the design: for elevated systems, each radial is trimmed to a quarter wavelength; for buried systems, many shorter radials can outperform a few long ones for the same total copper. A ground-plane antenna is essentially a monopole whose radials have been formalized into a small set of elevated, often downward-sloped, elements at the feedpoint.
Relevance to SDR
For receiving, the demands on a ground system are gentler than for transmitting: external and atmospheric noise usually dominate the receiver’s own noise floor on the HF and low-VHF bands, so a lossy ground attenuates signal and noise together and the signal-to-noise ratio suffers less than raw efficiency figures suggest. Even so, a decent counterpoise stabilizes a vertical’s pattern and impedance and reduces common-mode pickup of local electrical noise, which can be the real limit on a scanner’s usable sensitivity. GopherTrunk never sees the ground system — it processes IQ samples from the SDR — but the radials under a vertical are part of the physical chain that determines how much clean signal reaches the front end, particularly for lower-frequency monopole installations.
Sources
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Counterpoise (ground system) — Wikipedia, for the role of radials and counterpoises as the return-current ground for monopole antennas. ↩