JED - August 2014 - 33

gion during daylight hours. When this
layer disappears at night, however, communication can be possible at these frequencies over longer distances.
The next furthest or "E" layer located 62 to 71 miles above the earth allows
communication to be possible during
the day, peaking around noon and dropping off at sundown, similar to that of
the D layer. The difference is that the
E layer absorbs much less energy than
the D layer, enhancing propagation. The
"F" layer is furthest from the earth (100
to 260 miles) and is what makes longdistance communication possible. This
layer typically remains highly ionized
throughout the night hours, dropping
off just before sunrise. The two factors
most influencing refraction and thus
propagation characteristics are ionization density and wavelength, and
together these factors (and others) determine whether and where a radio wave
will be refracted back to earth.
The maximum usable frequency
(muf) is the highest frequency that will
support communication between two
specific points under a specific set of
conditions. At night, the muf for communication between two points 700
miles apart might be 4 MHz but could
be 28 MHz between points twice that
distance. Another term, called "skip
distance," varies in length according
to another factor called wave angle. As
this angle decreases it reaches a point
called the critical angle at which refraction allows the radio waves to return to
earth, so any wave propagated at that
angle or below it will return to earth.
Sometimes waves take multiple hops to
and from the ionosphere that allow truly
long-distance communication beyond
about 2,500 miles to occur. This multihop propagation is also a complicating
factor in HF SIGINT.
Last but certainly not least is the effect of solar activity (sunspots), which
directly impacts the sun's output of ionizing radiation and thus the ability to
communicate at HF frequencies. This
level of activity changes according to

a well-documented phenomenon called
the sunspot cycle that peaks every 11
years or so. At the peak of the sunspot
cycle, the muf increases and HF propagation, in general, is dramatically improved and communication distance will
be much greater. All of these factors,
and others, contribute to the ability of
signals emitted on earth to be propagated and thus intercepted and geolocated
by HF SIGINT installations.
HF's chameleon-like nature can
render communication at a specific
frequency possible or impossible, even
if the communication attempt is made
on different days at the same time
of day using exactly the same equipment. At HF, changes in propagation
over time make "five nines" quality
of service almost impossible. In contrast, as operating frequency rises,
propagation becomes mostly line of
sight and propagation remains stable
over time. It's not by chance that the
frequencies used by commercial wireless, public safety, FM radio, air-theair television, land mobile, paging,
and air traffic control and avionics
begin just above 100 MHz and (currently) trail off beyond about 2.6 GHz.
The initial spectrum employed for "4G"
wireless services at 700 MHz in North
America was a windfall for wireless
carriers as signals propagate over longer distances and penetrate structures
more effectively than at their higher
frequency allocations.
At frequencies above 1 GHz, where
RF energy propagates in a more or less
straight line from point to point, either
more power from larger base stations or
more base stations (thus the roll-out of
gap-filling "small cells") are required to
serve the same coverage area. At millimeter-wave frequencies, propagation
over even short distances is difficult
and a high-gain antenna must be boresighted like a rifle to its target antenna and propagation is compounded by
signal attenuation caused by virtually
any type of impediment, from leaves to
heavy rain to skyscrapers.

The Journal of Electronic Defense | August 2014

When radio waves approach the ionosphere, their direction is changed by
a process called refraction (bending)
which can send them back to earth.
The ionosphere actually consists of multiple layers and each layer has different characteristics that affect signal
propagation. The lowest or "D" layer
lies between 37 and 57 miles above the
earth and exists only during daylight
hours, disappearing about a half hour
after sundown. Rather than reflecting
signals, this layer absorbs them and
makes signal propagation possible at
frequencies only up to the lower HF re-



JED - August 2014

Table of Contents for the Digital Edition of JED - August 2014

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Back to the Future With HF SIGIHT
HF SIGIHT Battles the Ionosphere
Technology Survey: Solid-State Power Amplifiers
Book Reviews
EW 101
Index of Advertisers
JED Quick Look
JED - August 2014 - cover1
JED - August 2014 - cover2
JED - August 2014 - 3
JED - August 2014 - 4
JED - August 2014 - 5
JED - August 2014 - The View From Here
JED - August 2014 - 7
JED - August 2014 - Conferences Calendar
JED - August 2014 - 9
JED - August 2014 - Courses Calendar
JED - August 2014 - 11
JED - August 2014 - From the President
JED - August 2014 - 13
JED - August 2014 - Letters
JED - August 2014 - The Monitor
JED - August 2014 - 16
JED - August 2014 - 17
JED - August 2014 - 18
JED - August 2014 - 19
JED - August 2014 - 20
JED - August 2014 - 21
JED - August 2014 - Washington Report
JED - August 2014 - 23
JED - August 2014 - World Report
JED - August 2014 - 25
JED - August 2014 - 26
JED - August 2014 - 27
JED - August 2014 - Back to the Future With HF SIGIHT
JED - August 2014 - 29
JED - August 2014 - 30
JED - August 2014 - 31
JED - August 2014 - HF SIGIHT Battles the Ionosphere
JED - August 2014 - 33
JED - August 2014 - 34
JED - August 2014 - 35
JED - August 2014 - 36
JED - August 2014 - Technology Survey: Solid-State Power Amplifiers
JED - August 2014 - 38
JED - August 2014 - 39
JED - August 2014 - 40
JED - August 2014 - 41
JED - August 2014 - 42
JED - August 2014 - 43
JED - August 2014 - 44
JED - August 2014 - Book Reviews
JED - August 2014 - EW 101
JED - August 2014 - 47
JED - August 2014 - 48
JED - August 2014 - Index of Advertisers
JED - August 2014 - JED Quick Look
JED - August 2014 - cover3
JED - August 2014 - cover4