CHA SKYLOOP
(updated February, 2020)
Page 4
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
. 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 SKYLOOP is comprised of the following components, see plate (1):
a.
Matching Transformer.
The Matching Transformer provides a starting mounting point and impedance
matching for the CHA SKYLOOP antenna.
b.
Antenna Wire.
The Antenna Wire is a 265-foot length of black insulated wire.
c.
Line Winder.
The Line Winder is used to store the Antenna Wire (b) when it is not in use.
d.
Carabiner.
The Carabiner is a removable pear-shaped stainless-steel hook with a spring-loaded gate used to
attach the Isolation Loops (e) to the Transformer Eyebolt (i).
