JED - August 2014 - 32

HF SIGINT Battles the Ionosphere
By Barry Manz

W

The Journal of Electronic Defense | August 2014

32

hile every segment of the
Electromagnetic Spectrum
presents different SIGINT
challenges, the spectrum
between about 1 and 30 MHz, known as
High Frequency (HF), stands alone as
having propagation characteristics that
present unique challenges to SIGINT that
are encountered at no other frequencies. To understand the immense difficulty of HF SIGINT, it's necessary to view
this slice of spectrum in the context of
its higher frequency counterparts.
Before launching into the complex
world of HF signal propagation, it's important to point out that there are two
basic factors that make HF SIGINT so
difficult: The ever-changing nature of
signal propagation at HF and the size of
the antennas (and thus the real estate)
required to perform it. As frequency decreases, wavelength increases, so a fullwave antenna designed for reception at
1.8 MHz is about 525 ft. long. At 30 MHz
this decreases to about 29 ft., which is
still a formidable length of wire. While
it is certainly possible to create very
electrically-short antennas less than
a wavelength long, their effectiveness
is dramatically reduced. The full wavelength of wire can also be made into a
loop, but this, too, is far from ideal.
In practical terms, this means that
an HF SIGINT installation can be very
large. An excellent example is the AN/
FLR-9, first deployed in 1964, which
is a circularly-disposed antenna array
(CDAA), popularly known as the Elephant Cage of the "Wullenweber" variety, with a receiving bandwidth of 1.5 to
30 MHz and claimed accurate directionfinding of 5,000 nautical miles. During
the Cold War there were 20 of these systems deployed by the West, but by the
end of last year the last remaining site
(at Misawa Air Base in Japan), had been
decommissioned and destroyed.
The AN/FLR-9 consists of three rings
of HF antennas. The inner-most ring
covers 2 to 6 MHz and is typically 754
ft. in diameter and can include up to 40
folded dipoles. The middle ring covers 6

to 18 MHz, and the outer ring covers 18
to 30 MHz and is about 853 ft. in diameter with up to 120 sleeve monopoles. A
large wire screen acting as a counterpoise and providing resistance to interference from antennas on the other side
of the array is located below each ring
and supported by 80 towers. A horizontal ground screen about 1,200 ft. in diameter surrounds the site.

THE BIZARRE WORLD OF IONIZATION
Radio waves propagate via ground
waves or sky waves. As their name implies, ground waves at HF frequencies

travel directly from one point to another on the earth without leaving the
lower atmosphere, and are useful only
for communication at distances up to 50
or 75 miles. Beyond that distance, the
curvature of the earth impedes groundwave reception. In contrast, sky waves
do not follow the Earth's surface and
travel to the ionosphere (about 25 to 250
miles above Earth), where particles are
ionized by ultraviolet radiation emanating from the sun. The amount of ionization changes constantly with the time
of day, season, solar conditions, and
other variables.

U.S. Army AN/FLR-9 at Field Station Augsburg in Germany, decommissioned in 1998. (Photo
courtesy of Chaddy)



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
Letters
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
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