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 This program predicts the most
likely operating frequencies and signal levels for high frequency
(shortwave) radio propagation paths on specified days of the year and
hours of the day. It is most useful for paths between 250 km and 6000
km, but can be used with reduced accuracy for paths shorter or longer
than this.
 High frequency radio propagation
between approximately 3 Mhz and 30 Mhz is dominated by two ionospheric
layers, the E layer at a height of about 110 km above the Earth, and the
F layer, which varies between 250 km and 350 km, depending on the time
of day, season of year and solar activity. For the medium range paths
for which this program is intended, the dominant mode is via the
F-layer, although the program does consider the E-layer in order to
determine path loss. The success of each path is determined by the
transmitter power, receiver sensitivity, propagation path loss and
ambient noise. Especially at the lower frequencies during the summer
months, the ambient noise is dominated by electrical discharges due to
lightning in local and distant thunderstorms. This program computes the
thermal component of the ambient noise, but does not include the
electrical component, which must be estimated from experience and
seasonal conditions.
Radio propagation predictions
require knowledge of solar activity, which is compiled from daily
observations of the Sun and usually expressed as the 10-cm solar flux,
which has largely replaced the use of sunspot counts for this purpose.
The accuracy of the program depends on the accuracy in predicting this
number, which is usually averaged over some period of days, weeks or
months. The flux for the preceding day is broadcast by WWV/WWVH each
hour and are also available by telephone [NIS90]. Predictions for one or
more months in advance are also available in periodicals such as QST.
The program uses a routine Minimuf 3.5 developed by the U.S. Navy and
used to predict the MUF given the predicted flux, day of the year, hour
of the day and geographic coordinates of the transmitter and receiver.
This routine is reasonably accurate for the purposes here, with a
claimed RMS error of 3.8 MHz, but much smaller and less complex than the
programs used by major shortwave broadcasting organizations, such as the
Voice of America.
 In order to interpret the data
produced by this program, it is useful to consider how it is produced.
Given the MUF as predicted, the program constructs candidate ray
geometries over the shorter of the two great-circle paths between the
transmitter and receiver. The program can also do this for the longer of
these paths, but the accuracy is questionable at best. The program
constructs first the minimum-hop path constrained by the minimum takeoff
angle; that is, the minimum angle between the local horizon at the
great-circle azimuth and transmitted ray, which is a property of the
antenna and local topography. The default minimum angle is 10 degrees,
but this can be changed by an option. The program then constructs paths
for the next two higher-number hops, in order to assess multipath
conditions. For all three paths the program computes the minimum F-layer
MUF, maximum E-layer MUF and ionospheric absorption factor.
Using these data and the transmitter power,
antenna gain and frequencies specified, the program determines for each
frequency whether a geometric ray path is possible and, if so, computes
the path loss and delay. A path is possible only when the frequency is
below the F-layer MUF and the signal level is greater than the thermal
noise (2500 Hz at 290 K). The program output is in the form of a table,
with one entry for each hour of the UTC day, as described elsewhere.
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prophfv17a.exe
chramade@aol.com
