Why the ground system is half the antenna
A quarter-wave vertical is one half of a dipole. The missing half is supplied by the ground plane under it, and the current that would have flowed in the other leg flows through the earth instead. Earth is a mediocre conductor, so that current meets resistance, and the resistance sits in series with the radiation resistance at the feedpoint.
Series resistances split power in direct proportion, which makes the arithmetic unusually clean. With 36 ohms of radiation resistance and 12 ohms of ground loss, 36 of every 48 ohms is radiating: 75 percent of the transmitter output goes out and 25 percent warms the dirt. In decibels that is a 1.25 dB penalty. Get the loss down to 6 ohms and it becomes 85.7 percent and 0.67 dB.
Both resistances have to come from measurement or modelling. There is no table of ground loss by soil type worth putting in a calculator, because it depends on soil, moisture, the water table, the season, what else is buried and how the radials are laid. What the calculator can do is show you what a given loss resistance is worth in decibels, so you know what you are chasing before you buy the wire.
The wire, the weight and the day
Thirty-two radials of 32 feet is 1,024 feet of wire. At 0.14 dollars a foot that is 143.36 dollars, and at 20.2 pounds per thousand feet it is 20.68 pounds of copper. Pinned every three feet it takes 11 pins per radial, so 352 pins, which at 9 cents each adds 31.68 dollars and brings materials to 175.04 dollars.
The 352 pins are the part people underestimate. Each one is a separate act of bending down, and the field also has to be laid out radially without tangling, kept flat enough that a mower passes over it, and left where the grass will grow through it. Budget a day for a field this size and more if the ground is hard.
The 1,024 feet also covers a circle 64 feet across and about 3,217 square feet — 0.074 of an acre — and that entire area has to be free of anything you would rather not run wire across. In practice the available ground, not the theory, sets the radial length.
Reading the tip gap
Radials fan out from a point, so they are close together near the base and furthest apart at the tips. The gap at the tips is the circumference of the field divided by the number of radials. With 32 radials 32 feet long the circumference is 201.06 feet and the tips sit 6.28 feet apart, which at 7.15 MHz is 0.0457 of a wavelength.
That number is the useful guide to where the next hundred feet of wire should go. When the tips are already close in wavelength terms, adding more radials of the same length is putting wire into a region that is already reasonably covered, while the ground near the base — where the return current is densest — may still be sparse. A second set of shorter radials close in, or lengthening what is there, often does more than raising the count.
What this does not decide
How many radials to lay and how long. There is no defensible universal answer, and any page that gives you one is guessing about your soil. The inputs here are all yours, the arithmetic is takeoff and one division, and the output is what it costs and what a loss resistance you supply is worth in decibels.
Before wire goes anywhere in the ground, get a utility locate. Buried services are invisible and a locate is free or nearly free in most places. And be clear that a radial field is not a lightning ground: lightning protection and bonding for an antenna system is a separate subject governed by published standards and frequently by licensed work, and none of it is on this page.
Questions people ask
How many radials does a vertical antenna need?
This page will not tell you, because the answer depends on your soil, the water table, the season, the frequency and how much ground you have — and no table of soil types is trustworthy enough to put in a calculator. What it does instead is cost out a field you specify and show what a given ground loss resistance is worth in decibels, so you can decide how much improvement you are buying before you buy the wire.
How much wire is 32 radials of 32 feet?
1,024 feet, covering a circle 64 feet across and about 3,217 square feet, which is 0.074 of an acre. At 20.2 pounds per thousand feet that is 20.68 pounds of copper, and pinned every 3 feet it takes 11 pins per radial, so 352 pins in total. At 14 cents a foot for wire and 9 cents a pin, materials come to 175.04 dollars before the day it takes to lay.
How much does ground loss actually cost me?
The loss resistance and the radiation resistance sit in series at the feedpoint and split the power in direct proportion. With 36 ohms of radiation resistance and 12 ohms of ground loss, 75 percent radiates and 25 percent heats the ground, which is a 1.25 dB penalty. Halve the loss to 6 ohms and it becomes 85.7 percent, a 0.67 dB penalty. Both resistances have to come from your own measurement or model.
Are more radials better than longer radials?
It depends on where your field is thin, and the tip gap is the number that tells you. With 32 radials at 32 feet the tips are 6.28 feet apart, which at 7.15 MHz is 0.0457 of a wavelength — already fairly close. When the outer edge is dense but the ground near the base is not, a second set of shorter radials close in, where the return current is heaviest, usually does more per foot of wire than raising the count.
Is a radial field the same as a lightning ground?
No, and treating it as one is a mistake. A radial field exists to give radio-frequency return current a low-loss path. Lightning protection and bonding for an antenna system is a separate subject with published standards behind it, it is frequently licensed work, and it involves the whole structure and the building electrical system rather than the wire under one antenna. That belongs with people qualified to do it.