CHA P-LOOP 2.0
Page 5
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.
A magnetic loop antenna radiates at all angles from horizon to zenith, making it an equally effective antenna for
both local and long-distance (DX) communication. While not specifically designed for NVIS, during field testing of
the CHA P-LOOP 2.0, several NVIS contacts were made on the 40 meter ham band during the day.
Parts of the Antenna
The CHA P-LOOP 2.0 is comprised of the following assemblies and components, see plates (2) through (4):
a.
Tuning Unit
The Tuning Unit adjusts the resonant frequency of the CHA P-LOOP 2.0 antenna.
b.
Flexible Radiator Loop
The Flexible Radiator Loop consists of a 33¾ inch diameter insulated flexible metal loop with UHF Plugs (PL-
259) at both ends.
c.
Coupling Loop
The Coupling Loop is a small aluminum loop attached to the end of the Telescoping Mast (d).
d.
Telescoping Mast
The Telescoping Mast connects the Tuning Unit (a) to the Coupling Loop (c).
