I'll try to simplify this extremely complex issue as much as possible. I'll use Radio Systems' I AM Radio transmitter's external antenna, ATU, as the basis for this scenario.
For siting the external ATU, try to implement the following generalities regarding configuration IF adequate unobstructed space exists, i.e. 10000 square feet (100'x100'), with single story dwellings surrounding that space:
1. If possible, build an earthen mound approximately 5' in height above the surrounding terrain and as level as possible on your property. Cover the mound with sod eventually to prevent erosion and encourage the retention of moisture within a couple inches below the sod's surface. The external ATU and its artificial RF ground plane will be mounted in the center of this mound. At a minimum, ensure NO obstructions exist within a circle with a 40' radius surrounding the external ATU.
2. From the center of this mound, EVENLY distribute 8 radial copper wires with the length (radius) of each wire being 10' down the gentle slope of the mound and bury them ~ 2". * Keep the slope less than 60 degrees from a horizontal plane. The length (diameter) of opposing wires will be 20'. Again, space the 8 wires EVENLY around a 360 degree circle in four pairs of two. Each pair will contain one radial directly opposite the other radial, e.g. one radial will lie north and the other one will lie south, etc. Each radial will be 45 degrees from its adjacent radial.
3. Once the mound and artificial RF ground plane are created, drive a 3' to 8' (8' is standard) copper grounding rod into the earth in the center of the artificial RF ground plane. Tie ALL of the grounds together (8 wires to ground rod and ground lug on ATU). Naturally, the ATU along with its 10' whip antenna will sit in the middle of the artificial RF ground plane along with the 3' to 8' grounding rod.
4. For radials with a length of 10', use MW (AM) transmitter frequencies from 1000 KHz to 1700 KHz. For radials with a length of of 20', use MW (AM) transmitter frequencies from 520 KHz to 990 KHz.
Congratulations, you've just located and built one of the most effective external antennas possible for unlicensed, low-power Part 15 broadcasts. These generalized requirements should be considered a MINIMUM specification.
* A 20' radius option can also be used in dry soil with poor conductivity for a diameter of 40'. There is NO need to increase the length of a radial beyond 20' for a 10' high monopole antenna.
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