CHA MPAS
Page 5
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.
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 at
www.voacap.com
. 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.
Parts of the Antenna
The CHA MPAS is comprised of the following components and sub-components, see plate (2):
a.
Hybrid Micro Base
The Hybrid Micro Base provides a mounting base and impedance matching for the CHA MPAS.
b.
Antenna Wire
The Antenna Wire is a 60 foot length of black insulated wire.
