FCC 73.184
Revised as of November 30, 2020
Goto Year:2020 | 2022
```  §  73.184   Groundwave field strength graphs.

(a) Graphs 1 to 20 show, for each of 20 frequencies, the computed
values of groundwave field strength as a function of groundwave
conductivity and distance from the source of radiation. The groundwave
field strength is considered to be that part of the vertical component
of the electric field which has not been reflected from the ionosphere
nor from the troposphere. These 20 families of curves are plotted on
log-log graph paper and each is to be used for the range of frequencies
shown thereon. Computations are based on a dielectric constant of the
ground (referred to air as unity) equal to 15 for land and 80 for sea
water and for the ground conductivities (expressed in mS/m) given on
the curves. The curves show the variation of the groundwave field
strength with distance to be expected for transmission from a vertical
antenna at the surface of a uniformly conducting spherical earth with
the groundwave constants shown on the curves. The curves are for an
antenna power of such efficiency and current distribution that the
inverse distance (unattenuated) field is 100 mV/m at 1 kilometer. The
curves are valid for distances that are large compared to the
dimensions of the antenna for other than short vertical antennas.

(b) The inverse distance field (100 mV/m divided by the distance in
kilometers) corresponds to the groundwave field intensity to be
expected from an antenna with the same radiation efficiency when it is
located over a perfectly conducting earth. To determine the value of
the groundwave field intensity corresponding to a value of inverse
distance field other than 100 mV/m at 1 kilometer, multiply the field
strength as given on these graphs by the desired value of inverse
distance field at 1 kilometer divided by 100; for example, to determine
the groundwave field strength for a station with an inverse distance
field of 2700 mV/m at 1 kilometer, simply multiply the values given on
the charts by 27. The value of the inverse distance field to be used
for a particular antenna depends upon the power input to the antenna,
the nature of the ground in the neighborhood of the antenna, and the
geometry of the antenna. For methods of calculating the interrelations
between these variables and the inverse distance field, see “The
Propagation of Radio Waves Over the Surface of the Earth and in the
Upper Atmosphere,” Part II, by Mr. K.A. Norton, Proc. I.R.E., Vol. 25,
September 1937, pp. 1203-1237.

Note: The computed values of field strength versus distance used to
plot Graphs 1 to 20 are available in tabular form. For information on
obtaining copies of these tabulations call or write the Consumer
Affairs Office, Federal Communications Commission, Washington, DC
20554, (202) 632-7000.

(c) Provided the value of the dielectric constant is near 15, the
ground conductivity curves of Graphs 1 to 20 may be compared with
actual field strength measurement data to determine the appropriate
values of the ground conductivity and the inverse distance field
strength at 1 kilometer. This is accomplished by plotting the measured
field strengths on transparent log-log graph paper similar to that used
for Graphs 1 to 20 and superimposing the plotted graph over the Graph
corresponding to the frequency of the station measured. The plotted
graph is then shifted vertically until the plotted measurement data is
best aligned with one of the conductivity curves on the Graph; the
intersection of the inverse distance line on the Graph with the 1
kilometer abscissa on the plotted graph determines the inverse distance
field strength at 1 kilometer. For other values of dielectric constant,
the following procedure may be used to determine the dielectric
constant of the ground, the ground conductivity and the inverse
distance field strength at 1 kilometer. Graph 21 gives the relative
values of groundwave field strength over a plane earth as a function of
the numerical distance p and phase angle b. On graph paper with
coordinates similar to those of Graph 21, plot the measured values of
field strength as ordinates versus the corresponding distances from the
antenna in kilometers as abscissae. The data should be plotted only for
distances greater than one wavelength (or, when this is greater, five
times the vertical height of the antenna in the case of a
nondirectional antenna or 10 times the spacing between the elements of
a directional antenna) and for distances less than 80f1/3 MHz
kilometers (i.e., 80 kilometers at 1 MHz). Then, using a light box,
place the plotted graph over Graph 21 and shift the plotted graph
vertically and horizontally (making sure that the vertical lines on
both sheets are parallel) until the best fit with the data is obtained
with one of the curves on Graph 21. When the two sheets are properly
lined up, the value of the field strength corresponding to the
intersection of the inverse distance line of Graph 21 with the 1
kilometer abscissa on the data sheet is the inverse distance field
strength at 1 kilometer, and the values of the numerical distance at 1
kilometer, p1, and of b are also determined. Knowing the values of b
and p1 (the numerical distance at one kilometer), we may substitute in
the following approximate values of the ground conductivity and
dielectric constant.
eCFR graphic ec13no91.018.gif

(R/λ)1 = Number of wavelengths in 1 kilometer,

*   *   *   *   *

fMHz = frequency expressed in megahertz,
eCFR graphic ec13no91.019.gif

ε = dielectric constant on the ground referred to air as unity.

First solve for χ by substituting the known values of p1, (R/λ)1, and
cos b in equation (1). Equation (2) may then be solved for δ and
equation (3) for ε. At distances greater than 80/f1/3 MHz kilometers
the curves of Graph 21 do not give the correct relative values of field
strength since the curvature of the earth weakens the field more
rapidly than these plane earth curves would indicate. Thus, no attempt
should be made to fit experimental data to these curves at the larger
distances.

Note: For other values of dielectric constant, use can be made of the
computer program which was employed by the FCC in generating the curves
in Graphs 1 to 20. For information on obtaining a printout of this
program, call or write the Consumer Affairs Office, Federal
Communications Commission, Washington, DC 200554, (202) 632-7000.

(d) At sufficiently short distances (less than 55 kilometers at AM
broadcast frequencies), such that the curvature of the earth does not
introduce an additional attenuation of the waves, the curves of Graph
21 may be used to determine the groundwave field strength of
transmitting and receiving antennas at the surface of the earth for any
radiated power, frequency, or set of ground constants. First, trace the
straight inverse distance line corresponding to the power radiated on
transparent log-log graph paper similar to that of Graph 21, labelling
the ordinates of the chart in terms of field strength, and the
abscissae in terms of distance. Next, using the formulas given on Graph
21, calculate the value of the numerical distance, p, at 1 kilometer,
and the value of b. Then superimpose the log-log graph paper over Graph
21, shifting it vertically until both inverse distance lines coincide
and shifting it horizontally until the numerical distance at 1
kilometer on Graph 21 coincides with 1 kilometer on the log-log graph
paper. The curve of Graph 21 corresponding to the calculated value of b
is then traced on the log-log graph paper giving the field strength
versus distance in kilometers.

(e) This paragraph consists of the following Graphs 1 to 20 and 21.

Note: The referenced graphs are not published in the CFR, nor will they
be included in the Commission's automated rules system. For information
on obtaining copies of the graphs call or write the Consumer Affairs
Office, Federal Communications Commission, Washington, DC 20554,
Telephone: (202) 632-7000.

[ 28 FR 13574 , Dec. 14, 1963, as amended at  50 FR 18823 , May 2, 1985;  51 FR 45891 , Dec. 23, 1986;  52 FR 36878 , Oct. 1, 1987;  56 FR 64866 , Dec.
12, 1991;  57 FR 43290 , Sept. 18, 1992]

```

Goto Section: 73.183 | 73.185

Goto Year: 2020 | 2022
CiteFind - See documents on FCC website that cite this rule

Want to support this service?
Thanks!