[Code of Federal Regulations]
[Title 47, Volume 2]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 47CFR73.184]
[Page 54-56]
TITLE 47--TELECOMMUNICATION
CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION (CONTINUED)
PART 73_RADIO BROADCAST SERVICES--Table of Contents
Subpart A_AM Broadcast Stations
Sec. 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
[[Page 55]]
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 80f\1/3\ 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.
[[Page 56]]
[GRAPHIC] [TIFF OMITTED] TC13NO91.018
(R/[lambda])1= Number of wavelengths in 1 kilometer,
* * * * *
fMHz=frequency expressed in megahertz,
[GRAPHIC] [TIFF OMITTED] TC13NO91.019
[egr]=dielectric constant on the ground referred to air as unity.
First solve for [chi] by substituting the known values of
p1, (R/[lambda])1, and cos b in equation (1).
Equation (2) may then be solved for [delta] and equation (3) for [egr].
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]