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Radials and ground planes
Why a vertical needs a counterpoise and how many radials.
Why a vertical needs a counterpoise and how many radials.
Radials are wires laid out from the base of a vertical. They act as the antenna's other half, carrying the return current back to the feed point so it does not have to travel through the soil. Without them a quarter-wave vertical cannot work properly.
Current that flows up the element has to return to the transmitter. Ordinary soil is a poor conductor, so on its own it behaves like a resistor in series with the antenna. Power that goes into that resistance is turned into heat instead of radio waves.
The antenna's radiation resistance is only about 36 Ω, so even 10 Ω of ground loss wastes more than a fifth of your power (36 ÷ 46 = 78% efficient). The goal of a ground system is to make the loss resistance a small fraction of 36 Ω.
| Ground-mounted | Elevated | |
|---|---|---|
| Layout | Many radials on or just under the soil | A few radials, a quarter-wave long, some height above ground |
| How many | Dozens are common; each extra one helps less | Typically 2 to 4 per band |
| Why | Spreads the return current over a large area to cut soil loss | The radials are the entire counterpoise, so soil matters much less |
| Trade-off | Lots of wire; tuned length matters little | One set of radials per band; supports needed |
Gains from extra ground-mounted radials flatten out: going from a handful to a few dozen helps a lot, going from dozens to hundreds helps little. Soil quality matters too. Seawater is an excellent ground and poor dry soil is a bad one.
Elevated radials also set the feed impedance. Flat they leave it near 36 Ω, and sloping them downward raises it toward 50 Ω, a better match for coax. Neither the exact number of radials nor the exact angle is critical; plan on adding wire until the ground loss stops mattering.