CHA TD 2.0
Page 4
HF radio waves propagate from the transmitting
antenna to the receiving antenna using two
methods: ground waves and sky waves. Ground
waves are composed of direct waves and
surface waves. Direct waves travel directly
from the transmitting antenna to the receiving
antenna when they are within the radio line-of-
sight. Typically, this distance is 8 to 14 miles for
field stations. Surface waves follow the
curvature of the Earth beyond the radio
horizon. They are usable, during the day and
under optimal conditions, up to around 90
miles, see table (1).
Frequency Distance Frequency Distance
2 MHz
88 miles
14 MHz
33 miles
4 MHz
62 miles
18MHz
29 miles
7 MHz
47 miles
24 MHz
25 miles
10 MHz
39 miles
30 MHz
23 miles
Table 1. Maximum Surface Wave Range by Frequency.
Sky waves are the primary method of HF radio wave propagation. HF radio waves on a frequency below
the critical frequency (found by an ionosonde) are reflected off one of the layers of the ionosphere and
back to Earth between 300 and 2,500 miles, depending upon the frequency and ionospheric conditions.
HF radio waves can then be reflected from the Earth to the ionosphere again during multihop
propagation for longer range communication. The most important thing for the operator to understand
about HF radio wave propagation is the concept of Maximum Usable Frequency (MUF), Lowest Usable
Frequency (LUF), and Optimal Working Frequency (OWF). The MUF is the frequency for which
successful communications between two points is predicted on 50% of the days of in a month. The LUF
is the frequency below which successful communications are lost due to ionospheric loses. The OWF,
which is somewhere between the LUF and around 80% of the MUF, is the range of frequencies which
can be used for reliable communication. If the LUF is above the MUF, HF sky wave propagation is
unlikely to occur.
The HF part of the Radio Frequency (RF) spectrum is usually filled with communications activity and an
experienced operator can often determine where the MUF is, and with less certainty, the LUF by
listening to where activity ends. The operator can then pick a frequency in the OWF and attempt to
establish contact. Another method is using HF propagation prediction software, such as the
Voice of
America Coverage Analysis Program (VOACAP)
, which is available at no cost to download or use online
. The operator enters the location of the two stations and the program show a
wheel with the predicted percentage of success based on frequency and time. ALE, which is the
standard for interoperable HF communications, is an automated method of finding a frequency in the
OWF and establishing and maintaining a communications link.
Even under optimal conditions, there is a gap between where ground waves end (around 40 to 90 miles)
and the sky wave returns to Earth on the first hop (around 300 miles). NVIS propagation can be used to
fill this gap. The frequency selected must be below the critical frequency, so NVIS is can normally only
be used on frequencies from around 2 to 10 MHz. Frequencies of 2
–
4 MHz are typical at night and 4
–
8 MHz during the day.
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