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AM Ground System Correction Factors

AM antenna systems consist in part of an underground ground system in addition to the vertical antenna.  A standard ground system consists of 120 copper radials, each is 1/4 wavelength, buried in the earth surrounding the central tower at 3 degree intervals.  These radials look like the spokes of a bicycle wheel, but they are not connected at the outer ends.  Each radial is electrically connected to the other radials at the bottom of the base insulator beneath the vertical tower section.

Some nondirectional AM stations operate with fewer than 120 radials, or with radials truncated in some or all directions.  The table below provides correction factors applied by the staff to Figure 8, 47 CFR 73.190, to compute the theoretical efficiency of a simple vertical nondirectional AM antenna under these conditions.  The effective field for the nonstandard antenna, in mV/m, is REDUCED by the amounts specified below.  For example, a ground system consisting of 90 radials with an average length of 0.192 wavelengths, would result in a cumulative correction factor of

(-9.7 mV/m @ 1 km) + (-16.1 mV/m @ 1 km) = (-25.8 mV/m @ 1km).

Thus, the effective field strength value determined from Figure 8 would be reduced by 25.8 mV/m.

These correction factors may not be applied to towers in an AM directional array.

Radials Less than 1/4 Wavelength in Length

Average lengths of radials in wavelengths Correction factor
(mV/m @ 1 km)
0.2401 or greater 0
0.2301 to 0.2400 -3.2
0.2201 to 0.2300 -6.4
0.2101 to 0.2200 -9.7
0.2001 to 0.2100 -12.9
0.1901 to 0.2000 -16.1
0.1801 to 0.1900 -19.3
0.1701 to 0.1800 -22.5
0.1601 to 0.1700 -25.7
0.1501 to 0.1600 -29.0


AM Ground Systems with Less Than 120 Radials

Number of Radials Correction factor
(mV/m @ 1 km)
110-3.2
100-6.4
90 -9.7

Notes:

  1. For simple vertical antennae over salt water, use Figure 8 directly.  Do NOT make the above adjustments.
  2. Use the minimum permissible radiation figures shown below for antennas with roof top ground systems, T-shaped and L-shaped antennas, and for simple vertical systems having ground systems too short to fall within the range of corrections noted above, as follows:

    Class A (except Alaskan)362 mV/m @ 1 km
    Class B, D, and Alaskan A282 mV/m @ 1 km
    Class C241 mV/m @ 1 km
    47 CFR Section 73.182(m)

    Use these minimums if the corrected Figure 8 values result in effective fields below these permissible minimum values.  Please note that a nondirectional proof of performance may be required in cases where the antenna system's efficiency is questionable.  Top-loaded, sectionalized, or other types of antennas must be considered on a case-by-case basis.


Information about AM broadcast radio stations is available at the Audio Division on the FCC's website, and at Broadcast Radio Links

FCC > Media Bureau > Audio Division


Reviewed: November 4, 2014
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