CHA 40/20 FD
Page 4
HF Propagation
HF radio provides relatively inexpensive and reliable local, regional, national, and international voice and data
communication capability. It is especially suitable for undeveloped areas where normal telecommunications are not
available, too costly or scarce, or where the commercial telecommunications infrastructure has been damaged by a
natural disaster or military conflict.
Although HF radio is a reasonably reliable method of communication, HF radio waves propagate through a complex
and constantly changing environment and are affected by weather, terrain, latitude, time of day, season, and the
11-year solar cycle. A detailed explanation of the theory of HF radio wave propagation is beyond the scope of this
operator’s manual, but an understanding
of the basic principles will help the operator decide what frequency and
antenna configuration will support their communication requirements.
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).
Low power, horizontal antenna polarization, rugged or urban terrain, dense foliage, or dry soil conditions can reduce
the range very significantly. The U.S. Army found that in the dense jungles of Vietnam, the range for ground waves
was sometimes less than one mile.
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.
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.
HF radio waves can then be reflected from the Earth to the ionosphere again during multi-hop 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 at
. 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,
