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The how big, how long and how many of ground radials

Medium wave broadcasters can gain significant range by adding ground radials the the base of a vertical antenna.  AM transmitters use return ground currents and the ground (and radials) are part of the antenna system.  An array of wires at or below the earth at the base of the antenna will improve the return path and increase range.

The signal strength scale is arbitrary and relative, based
on the maximum benefit derived from the highest possible
number of radials used during testing, either 120 or 60,
depending on the tester and the tests being performed.

     Controlled experiments have indicated that ground radials do not need to be much longer than the antenna is tall. A shortened antenna with loading coils will have a more compact "near field" where the majority of the antenna field is concentrated.
     The ground radials need only reach out as far as the near field extends. Field intensity decreases at the square of the distance from the base of the antenna.
     When working with the upper end of the AM band, radials around 10 feet long should be adequate.
     For lower frequencies, consider extending the radials to 20 feet.
     Use heavy wire, around 12 ga., stranded or solid core, insulated or not, whatever is available inexpensively.
     The optimum number of radials used appears to be subject to the principle of diminishing returns. Some recommend 60 or 120 radials but there is a high cost to using that many. Several hams have conducted controlled tests to determine the impact of additional radial pairs. I've plotted that data on the graph above.
     The benefit in additional signal strength appears to drop off quickly after 12 and 24 seem to yield 80% of the maximum possible benefit.
     Click here to read more about increasing range with better RF grounding for low power AM transmitters.

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