Radial Field Planner for Vertical Antennas

The radial field is the half of a vertical antenna that nobody photographs and everybody skips. It is also the difference between a vertical that works and the many verticals that gave the design its reputation for mediocrity. Current leaving the radiator has to return through something, and if that something is soil, a large fraction of your transmitter output ends up warming the garden.

Two systems, two sets of rules

Almost every argument about radials on the air is two people describing different systems. Elevated and on-ground radial fields work by different mechanisms and follow opposite rules about count and length.

Elevated radials are the other half of the antenna

Raise the feedpoint at least eight feet, run four wires cut to a resonant quarter wavelength, and slope them down at around 45 degrees. Those four wires are radiating elements forming the counterpoise directly, so the soil beneath them barely participates and its quality barely matters.

Four is genuinely enough, which is the surprising part. The fifth and sixth radials in an elevated system add almost nothing measurable, because the currents in a symmetrical set of four already cancel their own radiation in the horizontal plane. Adding more is a common instinct and it is wasted wire.

Sloping them has a convenient side effect. A quarter-wave vertical over a perfect ground plane presents about 36 ohms; drooping the radials raises that towards 50, so a 45 degree droop typically lands close enough to 50 ohms that no matching network is needed at all. The cost is mechanical: four sloping wires at head height are a hazard in a garden people walk through, and they need real anchors.

On-ground radials are a shield

Lay wires on the soil and they stop behaving as resonant elements. The soil dominates, and the wires act as a screen intercepting return currents before those currents enter the ground where they would dissipate as heat. The measure that matters is therefore how much of the ground surface near the antenna is covered, not whether any wire is the right length.

That produces the rule that surprises people: more shorter radials beats fewer longer ones. Thirty-two radials at an eighth of a wavelength outperform eight at a quarter wavelength, using the same total wire. Return current density is highest close to the base and falls with distance, so the wire is worth most in the first few metres.

Read the top row carefully, because it describes the most common amateur vertical installation in existence. Four on-ground radials costs 3 to 4 decibels, which is more than half your transmitter power converted directly into warm soil. A 100 watt station with that ground field is radiating roughly 40 watts. Adding wire is the cheapest improvement available to it by a very wide margin.

The SWR trap

This is the single most misleading thing about verticals. Ground loss appears at the feedpoint as resistance, and it adds to the roughly 36 ohms of radiation resistance a quarter-wave vertical presents. Add 14 ohms of loss and the feedpoint reads 50 ohms exactly, giving a beautiful 1 to 1 match while nearly 30 percent of your power is being dissipated in the ground.

A properly built vertical with a large radial field frequently reads a worse SWR than a bad one, because it is closer to its true 36 ohms. If you add radials and your SWR gets slightly worse, you have almost certainly improved the antenna. Judge a vertical by measured signal reports over time, or by modelling, not by the meter. The relationship is worked through on the SWR calculator.

Building it, practically

1. Buy wire by the spool. A Paladin 14 AWG stranded copper wire, 500 ft$74.50 is 500 feet, which is 32 radials at an eighth wavelength on 40 metres with wire left over. Buying individual lengths costs several times as much. For lighter portable work, thinner wire is fine because radials carry current but no mechanical load.
2. Do not cut them all the same length. Run each as far as the garden allows in its direction. A lopsided field that covers the available ground beats a symmetrical set of four short ones every time.
3. Bond them all at a single point. A Copper grounding bus bar kit, 16 positions$21.50 or a simple ring of heavy wire around the base, with every radial connected to it and one connection to the coax braid. One bond point, no loops.
4. Pin them down and leave them on the surface. Landscape staples every few feet. Grass grows through within a season and the lawnmower stops finding them. Burying is optional and one or two inches is plenty.
5. Add a common-mode choke at the feedpoint. Twelve turns of coax on an FT-240-31 ferrite toroid core$11.67. Without it the coax shield becomes an unintended radial running in one direction, which skews the pattern and puts RF in the shack.
6. Add more wire next year. A radial field is the one part of an antenna that improves incrementally. Every wire you add helps a little, and you can keep adding for years.

For a portable vertical, a pre-cut set such as the Super Antenna MR4010 radial set$29.99 saves the measuring and packs into a bag. It is a compromise by design, since a portable field is elevated only slightly and consists of a handful of wires rather than dozens, but it is dramatically better than the nothing that most portable verticals are used with.

Soil quality changes the arithmetic

The counterintuitive line is the last two. Bad soil does not mean radials are pointless; it means they matter more, because there is more loss for them to shield you from. If your ground is genuinely poor and you cannot lay enough wire, elevating the feedpoint and using four resonant radials removes the soil from the equation almost entirely, and it is the better answer.

What a radial field is not

A radial field is a radio frequency counterpoise. It is not a lightning ground, not a safety ground, and not a substitute for either. The two systems serve different purposes and are built to different standards, though they must be bonded together.

A vertical antenna outdoors needs a proper ground electrode at its base, bonded back to the building service ground, with a listed arrester on the coax at the entry point. A hundred quarter-wave radials will not carry a lightning strike anywhere useful, and an isolated ground rod that is not bonded to the service ground creates a potential difference during a strike that is more dangerous than no rod at all. An 8 ft copper-bonded ground rod kit with clamp$54.80 at the antenna base and a Proxicast ProGrade coaxial lightning arrester, SO-239$19.95 at the entry panel are the minimum, and the whole system is described on station grounding and lightning protection.