<s>
Direction	B-Device
finding	I-Device
(	O
DF	O
)	O
,	O
or	O
radio	B-Device
direction	I-Device
finding	I-Device
(	O
RDF	O
)	O
,	O
isin	O
accordance	O
with	O
International	O
Telecommunication	O
Union	O
(	O
ITU	O
)	O
defined	O
as	O
radio	B-Algorithm
location	I-Algorithm
that	O
uses	O
the	O
reception	O
of	O
radio	O
waves	O
to	O
determine	O
the	O
direction	O
in	O
which	O
a	O
radio	O
station	O
or	O
an	O
object	O
is	O
located	O
.	O
</s>
<s>
This	O
can	O
refer	O
to	O
radio	O
or	O
other	O
forms	O
of	O
wireless	O
communication	O
,	O
including	O
radar	B-Application
signals	O
detection	O
and	O
monitoring	O
(	O
ELINT/ESM	O
)	O
.	O
</s>
<s>
By	O
combining	O
the	O
direction	O
information	O
from	O
two	O
or	O
more	O
suitably	O
spaced	O
receivers	O
(	O
or	O
a	O
single	O
mobile	O
receiver	O
)	O
,	O
the	O
source	O
of	O
a	O
transmission	O
may	O
be	O
located	O
via	O
triangulation	B-Algorithm
.	O
</s>
<s>
Radio	B-Device
direction	I-Device
finding	I-Device
is	O
used	O
in	O
the	O
navigation	O
of	O
ships	O
and	O
aircraft	O
,	O
to	O
locate	O
emergency	O
transmitters	O
for	O
search	O
and	O
rescue	O
,	O
for	O
tracking	O
wildlife	O
,	O
and	O
to	O
locate	O
illegal	O
or	O
interfering	O
transmitters	O
.	O
</s>
<s>
RDF	O
was	O
important	O
in	O
combating	O
German	O
threats	O
during	O
both	O
the	O
World	O
War	O
II	O
Battle	B-Application
of	I-Application
Britain	I-Application
and	O
the	O
long	O
running	O
Battle	O
of	O
the	O
Atlantic	O
.	O
</s>
<s>
An	O
automatic	O
direction	B-Device
finder	I-Device
,	O
which	O
could	O
be	O
tuned	O
to	O
radio	O
beacons	B-Protocol
called	O
non-directional	O
beacons	B-Protocol
or	O
commercial	O
AM	O
radio	O
broadcasters	O
,	O
was	O
until	O
recently	O
,	O
a	O
feature	O
of	O
most	O
aircraft	O
,	O
but	O
is	O
now	O
being	O
phased	O
out	O
.	O
</s>
<s>
It	O
is	O
estimated	O
that	O
the	O
UK	O
's	O
advanced	O
"	O
huff-duff	O
"	O
systems	O
were	O
directly	O
or	O
indirectly	O
responsible	O
for	O
24%	O
of	O
all	O
U-boats	B-Operating_System
sunk	O
during	O
the	O
war	O
.	O
</s>
<s>
Modern	O
systems	O
often	O
used	O
phased	O
array	O
antennas	O
to	O
allow	O
rapid	O
beamforming	B-Algorithm
for	O
highly	O
accurate	O
results	O
,	O
and	O
are	O
part	O
of	O
a	O
larger	O
electronic	O
warfare	O
suite	O
.	O
</s>
<s>
Early	O
radio	B-Device
direction	I-Device
finders	I-Device
used	O
mechanically	O
rotated	O
antennas	O
that	O
compared	O
signal	O
strengths	O
,	O
and	O
several	O
electronic	O
versions	O
of	O
the	O
same	O
concept	O
followed	O
.	O
</s>
<s>
Early	O
British	O
radar	B-Application
sets	O
were	O
referred	O
to	O
as	O
RDF	O
,	O
which	O
is	O
often	O
stated	O
was	O
a	O
deception	O
.	O
</s>
<s>
Later	O
radar	B-Application
systems	I-Application
generally	O
used	O
a	O
single	O
antenna	O
for	O
broadcast	O
and	O
reception	O
,	O
and	O
determined	O
direction	O
from	O
the	O
direction	O
the	O
antenna	O
was	O
facing	O
.	O
</s>
<s>
The	O
earliest	O
experiments	O
in	O
RDF	O
were	O
carried	O
out	O
in	O
1888	O
when	O
Heinrich	O
Hertz	O
discovered	O
the	O
directionality	O
of	O
an	O
open	B-Architecture
loop	I-Architecture
of	I-Architecture
wire	I-Architecture
used	O
as	O
an	O
antenna	O
.	O
</s>
<s>
Early	O
radio	O
systems	O
generally	O
used	O
medium	B-Algorithm
wave	I-Algorithm
and	O
longwave	B-Algorithm
signals	O
.	O
</s>
<s>
Longwave	B-Algorithm
in	O
particular	O
had	O
good	O
long-distance	O
transmission	O
characteristics	O
due	O
to	O
their	O
limited	O
interaction	O
with	O
the	O
ground	O
,	O
and	O
thereby	O
provided	O
excellent	O
great	O
circle	O
route	O
ground	O
wave	O
propagation	O
that	O
pointed	O
directly	O
to	O
the	O
transmitter	O
.	O
</s>
<s>
Methods	O
of	O
performing	O
RDF	O
on	O
longwave	B-Algorithm
signals	O
was	O
a	O
major	O
area	O
of	O
research	O
during	O
the	O
1900s	O
and	O
1910s	O
.	O
</s>
<s>
For	O
longwave	B-Algorithm
use	O
,	O
this	O
resulted	O
in	O
loop	B-Architecture
antennas	I-Architecture
tens	O
of	O
feet	O
on	O
a	O
side	O
,	O
often	O
with	O
more	O
than	O
one	O
loop	O
connected	O
together	O
to	O
improve	O
the	O
signal	O
.	O
</s>
<s>
Another	O
solution	O
to	O
this	O
problem	O
was	O
developed	O
by	O
the	O
Marconi	B-Device
company	O
in	O
1905	O
.	O
</s>
<s>
A	O
separate	O
loop	B-Architecture
antenna	I-Architecture
located	O
in	O
this	O
area	O
could	O
then	O
be	O
used	O
to	O
hunt	O
for	O
the	O
direction	O
,	O
without	O
moving	O
the	O
main	O
antennas	O
.	O
</s>
<s>
Bellini-Tosi	B-Device
direction	I-Device
finders	I-Device
were	O
widespread	O
from	O
the	O
1920s	O
into	O
the	O
1950s	O
.	O
</s>
<s>
Early	O
RDF	O
systems	O
were	O
useful	O
largely	O
for	O
long	B-Algorithm
wave	I-Algorithm
signals	O
.	O
</s>
<s>
The	O
US	O
Army	O
Air	O
Corps	O
in	O
1931	O
tested	O
a	O
primitive	O
radio	B-Device
compass	I-Device
that	O
used	O
commercial	O
stations	O
as	O
the	O
beacon	B-Protocol
.	O
</s>
<s>
A	O
major	O
improvement	O
in	O
the	O
RDF	O
technique	O
was	O
introduced	O
by	O
Robert	O
Watson-Watt	O
as	O
part	O
of	O
his	O
experiments	O
to	O
locate	O
lightning	B-Architecture
strikes	O
as	O
a	O
method	O
to	O
indicate	O
the	O
direction	O
of	O
thunderstorms	O
for	O
sailors	O
and	O
airmen	O
.	O
</s>
<s>
He	O
had	O
long	O
worked	O
with	O
conventional	O
RDF	O
systems	O
,	O
but	O
these	O
were	O
difficult	O
to	O
use	O
with	O
the	O
fleeting	O
signals	O
from	O
the	O
lightning	B-Architecture
.	O
</s>
<s>
Development	O
was	O
limited	O
until	O
the	O
mid-1930s	O
,	O
when	O
the	O
various	O
British	O
forces	O
began	O
widespread	O
development	O
and	O
deployment	O
of	O
these	O
"	O
high-frequency	O
direction	B-Device
finding	I-Device
"	O
,	O
or	O
"	O
huff-duff	O
"	O
systems	O
.	O
</s>
<s>
By	O
that	O
time	O
huff-duff	O
had	O
helped	O
in	O
about	O
one-quarter	O
of	O
all	O
successful	O
attacks	O
on	O
the	O
U-boat	B-Operating_System
fleet	O
.	O
</s>
<s>
Improved	O
vacuum	O
tubes	O
and	O
the	O
introduction	O
of	O
the	O
transistor	B-Application
allowed	O
much	O
higher	O
frequencies	O
to	O
be	O
used	O
economically	O
,	O
which	O
led	O
to	O
widespread	O
use	O
of	O
VHF	B-Algorithm
and	O
UHF	O
signals	O
.	O
</s>
<s>
In	O
this	O
system	O
the	O
loop	B-Architecture
antenna	I-Architecture
is	O
replaced	O
with	O
a	O
single	O
square-shaped	O
ferrite	O
core	O
,	O
with	O
loops	O
wound	O
around	O
two	O
perpendicular	O
sides	O
.	O
</s>
<s>
Operation	O
consists	O
solely	O
of	O
tuning	O
in	O
the	O
station	O
,	O
and	O
is	O
so	O
automatic	O
that	O
these	O
systems	O
are	O
normally	O
referred	O
to	O
as	O
automatic	O
direction	B-Device
finder	I-Device
.	O
</s>
<s>
Pseudo-doppler	O
radio	B-Device
direction	I-Device
finder	I-Device
systems	O
use	O
a	O
series	O
of	O
small	O
dipole	O
antennas	O
arranged	O
in	O
a	O
ring	O
and	O
use	O
electronic	O
switching	O
to	O
rapidly	O
select	O
dipoles	O
to	O
feed	O
into	O
the	O
receiver	O
.	O
</s>
<s>
A	O
radio	B-Device
direction	I-Device
finder	I-Device
(	O
RDF	O
)	O
is	O
a	O
device	O
for	O
finding	O
the	O
direction	O
,	O
or	O
bearing	O
,	O
to	O
a	O
radio	O
source	O
.	O
</s>
<s>
The	O
act	O
of	O
measuring	O
the	O
direction	O
is	O
known	O
as	O
radio	B-Device
direction	I-Device
finding	I-Device
or	O
sometimes	O
simply	O
direction	B-Device
finding	I-Device
(	O
DF	O
)	O
.	O
</s>
<s>
An	O
automatic	O
direction	B-Device
finder	I-Device
,	O
often	O
capable	O
of	O
being	O
tuned	O
to	O
commercial	O
AM	O
radio	O
transmitters	O
,	O
is	O
a	O
feature	O
of	O
almost	O
all	O
modern	O
aircraft	O
.	O
</s>
<s>
It	O
is	O
estimated	O
that	O
the	O
UK	O
's	O
advanced	O
"	O
huff-duff	O
"	O
systems	O
were	O
directly	O
or	O
indirectly	O
responsible	O
for	O
24%	O
of	O
all	O
U-boats	B-Operating_System
sunk	O
during	O
the	O
war	O
.	O
</s>
<s>
Modern	O
systems	O
often	O
use	O
phased	O
array	O
antennas	O
to	O
allow	O
rapid	O
beam	B-Algorithm
forming	I-Algorithm
for	O
highly	O
accurate	O
results	O
.	O
</s>
<s>
Modern	O
pseudo-Doppler	B-Device
direction	I-Device
finder	I-Device
systems	O
consist	O
of	O
a	O
number	O
of	O
small	O
antennas	O
fixed	O
to	O
a	O
circular	O
card	O
,	O
with	O
all	O
of	O
the	O
processing	O
performed	O
by	O
software	O
.	O
</s>
<s>
Early	O
British	O
radar	B-Application
sets	O
were	O
also	O
referred	O
to	O
as	O
RDF	O
,	O
which	O
was	O
a	O
deception	O
tactic	O
.	O
</s>
<s>
Direction	B-Device
finding	I-Device
requires	O
an	O
antenna	O
that	O
is	O
directional	O
(	O
more	O
sensitive	O
in	O
certain	O
directions	O
than	O
in	O
others	O
)	O
.	O
</s>
<s>
A	O
simple	O
form	O
of	O
directional	O
antenna	O
is	O
the	O
loop	B-Architecture
aerial	I-Architecture
.	O
</s>
<s>
This	O
consists	O
of	O
an	O
open	B-Architecture
loop	I-Architecture
of	I-Architecture
wire	I-Architecture
on	O
an	O
insulating	O
frame	O
,	O
or	O
a	O
metal	O
ring	O
that	O
forms	O
the	O
antenna	O
's	O
loop	O
element	O
itself	O
;	O
often	O
the	O
diameter	O
of	O
the	O
loop	O
is	O
a	O
tenth	O
of	O
a	O
wavelength	O
or	O
smaller	O
at	O
the	O
target	O
frequency	O
.	O
</s>
<s>
This	O
is	O
called	O
a	O
null	O
in	O
the	O
signal	O
,	O
and	O
it	O
is	O
used	O
instead	O
of	O
the	O
strongest	O
signal	O
direction	O
,	O
because	O
small	O
angular	O
deflections	O
of	O
the	O
loop	B-Architecture
aerial	I-Architecture
away	O
from	O
its	O
null	O
positions	O
produce	O
much	O
more	O
abrupt	O
changes	O
in	O
received	O
current	O
than	O
similar	O
directional	O
changes	O
around	O
the	O
loop	O
's	O
strongest	O
signal	O
orientation	O
.	O
</s>
<s>
Since	O
the	O
null	O
direction	O
gives	O
a	O
clearer	O
indication	O
of	O
the	O
signal	O
direction	O
–	O
the	O
null	O
is	O
"	O
sharper	O
"	O
than	O
the	O
max	O
–	O
with	O
loop	B-Architecture
aerial	I-Architecture
the	O
null	O
direction	O
is	O
used	O
to	O
locate	O
a	O
signal	O
source	O
.	O
</s>
<s>
A	O
"	O
sense	O
antenna	O
"	O
is	O
used	O
to	O
resolve	O
the	O
two	O
direction	O
possibilities	O
;	O
the	O
sense	O
aerial	O
is	O
a	O
non-directional	O
antenna	O
configured	O
to	O
have	O
the	O
same	O
sensitivity	O
as	O
the	O
loop	B-Architecture
aerial	I-Architecture
.	O
</s>
<s>
The	O
Yagi-Uda	O
antenna	O
is	O
familiar	O
as	O
the	O
common	O
VHF	B-Algorithm
or	O
UHF	O
television	O
aerial	O
.	O
</s>
<s>
More	O
sophisticated	O
techniques	O
such	O
as	O
phased	O
arrays	O
are	O
generally	O
used	O
for	O
highly	O
accurate	O
direction	B-Device
finding	I-Device
systems	O
.	O
</s>
<s>
A	O
modern	O
helicopter-mounted	O
direction	B-Device
finding	I-Device
system	O
was	O
designed	O
by	O
ESL	O
Incorporated	O
for	O
the	O
U.S.	O
Government	O
as	O
early	O
as	O
1972	O
.	O
</s>
<s>
Radio	B-Device
Direction	I-Device
Finding	I-Device
works	O
by	O
comparing	O
the	O
signal	O
strength	O
of	O
a	O
directional	O
antenna	O
pointing	O
in	O
different	O
directions	O
.	O
</s>
<s>
At	O
first	O
,	O
this	O
system	O
was	O
used	O
by	O
land	O
and	O
marine-based	O
radio	O
operators	O
,	O
using	O
a	O
simple	O
rotatable	O
loop	B-Architecture
antenna	I-Architecture
linked	O
to	O
a	O
degree	O
indicator	O
.	O
</s>
<s>
Later	O
loop	B-Architecture
antenna	I-Architecture
designs	O
were	O
enclosed	O
in	O
an	O
aerodynamic	O
,	O
teardrop-shaped	O
fairing	O
.	O
</s>
<s>
In	O
ships	O
and	O
small	O
boats	O
,	O
RDF	O
receivers	O
first	O
employed	O
large	O
metal	O
loop	B-Architecture
antennas	I-Architecture
,	O
similar	O
to	O
aircraft	O
,	O
but	O
usually	O
mounted	O
atop	O
a	O
portable	O
battery-powered	O
receiver	O
.	O
</s>
<s>
In	O
use	O
,	O
the	O
RDF	O
operator	O
would	O
first	O
tune	O
the	O
receiver	O
to	O
the	O
correct	O
frequency	O
,	O
then	O
manually	O
turn	O
the	O
loop	O
,	O
either	O
listening	O
or	O
watching	O
an	O
S	O
meter	O
to	O
determine	O
the	O
direction	O
of	O
the	O
null	O
(	O
the	O
direction	O
at	O
which	O
a	O
given	O
signal	O
is	O
weakest	O
)	O
of	O
a	O
long	B-Algorithm
wave	I-Algorithm
(	O
LW	O
)	O
or	O
medium	B-Algorithm
wave	I-Algorithm
(	O
AM	O
)	O
broadcast	O
beacon	B-Protocol
or	O
station	O
(	O
listening	O
for	O
the	O
null	O
is	O
easier	O
than	O
listening	O
for	O
a	O
peak	O
signal	O
,	O
and	O
normally	O
produces	O
a	O
more	O
accurate	O
result	O
)	O
.	O
</s>
<s>
This	O
null	O
was	O
symmetrical	O
,	O
and	O
thus	O
identified	O
both	O
the	O
correct	O
degree	O
heading	O
marked	O
on	O
the	O
radio	O
's	O
compass	B-Language
rose	O
as	O
well	O
as	O
its	O
180-degree	O
opposite	O
.	O
</s>
<s>
A	O
key	O
breakthrough	O
was	O
the	O
introduction	O
of	O
a	O
secondary	O
vertical	O
whip	O
or	O
'	B-Architecture
sense	I-Architecture
 '	I-Architecture
antenna	I-Architecture
that	O
substantiated	O
the	O
correct	O
bearing	O
and	O
allowed	O
the	O
navigator	O
to	O
avoid	O
plotting	O
a	O
bearing	O
180	O
degrees	O
opposite	O
the	O
actual	O
heading	O
.	O
</s>
<s>
After	O
World	O
War	O
II	O
,	O
there	O
were	O
many	O
small	O
and	O
large	O
firms	O
making	O
direction	B-Device
finding	I-Device
equipment	O
for	O
mariners	O
,	O
including	O
Apelco	O
,	O
Aqua	O
Guide	O
,	O
Bendix	O
,	O
Gladding	O
(	O
and	O
its	O
marine	O
division	O
,	O
Pearce-Simpson	O
)	O
,	O
Ray	O
Jefferson	O
,	O
Raytheon	O
,	O
and	O
Sperry	O
.	O
</s>
<s>
The	O
pseudo-doppler	B-Device
technique	I-Device
is	O
a	O
phase	O
based	O
DF	O
method	O
that	O
produces	O
a	O
bearing	O
estimate	O
on	O
the	O
received	O
signal	O
by	O
measuring	O
the	O
doppler	O
shift	O
induced	O
on	O
the	O
signal	O
by	O
sampling	O
around	O
the	O
elements	O
of	O
a	O
circular	O
array	O
.	O
</s>
<s>
If	O
the	O
direction	B-Device
finding	I-Device
antenna	O
elements	O
have	O
a	O
directional	O
antenna	O
pattern	O
,	O
then	O
the	O
amplitude	O
may	O
be	O
included	O
in	O
the	O
comparison	O
.	O
</s>
<s>
Radio	B-Device
direction	I-Device
finding	I-Device
,	O
radio	B-Device
direction	I-Device
finder	I-Device
,	O
or	O
RDF	O
,	O
was	O
once	O
the	O
primary	O
aviation	O
navigational	O
aid	O
.	O
</s>
<s>
(	O
Range	O
and	O
Direction	B-Device
Finding	I-Device
was	O
the	O
abbreviation	O
used	O
to	O
describe	O
the	O
predecessor	O
to	O
radar	B-Application
.	O
)	O
</s>
<s>
Beacons	B-Protocol
were	O
used	O
to	O
mark	O
"	O
airways	O
"	O
intersections	O
and	O
to	O
define	O
departure	O
and	O
approach	O
procedures	O
.	O
</s>
<s>
Since	O
the	O
signal	O
transmitted	O
contains	O
no	O
information	O
about	O
bearing	O
or	O
distance	O
,	O
these	O
beacons	B-Protocol
are	O
referred	O
to	O
as	O
non-directional	O
beacons	B-Protocol
,	O
or	O
NDB	O
in	O
the	O
aviation	O
world	O
.	O
</s>
<s>
Starting	O
in	O
the	O
1950s	O
,	O
these	O
beacons	B-Protocol
were	O
generally	O
replaced	O
by	O
the	O
VOR	B-Device
system	O
,	O
in	O
which	O
the	O
bearing	O
to	O
the	O
navigational	O
aid	O
is	O
measured	O
from	O
the	O
signal	O
itself	O
;	O
therefore	O
no	O
specialized	O
antenna	O
with	O
moving	O
parts	O
is	O
required	O
.	O
</s>
<s>
Due	O
to	O
relatively	O
low	O
purchase	O
,	O
maintenance	O
and	O
calibration	O
cost	O
,	O
NDB	O
's	O
are	O
still	O
used	O
to	O
mark	O
locations	O
of	O
smaller	O
aerodromes	O
and	O
important	O
helicopter	B-Application
landing	O
sites	O
.	O
</s>
<s>
Similar	O
beacons	B-Protocol
located	O
in	O
coastal	O
areas	O
are	O
also	O
used	O
for	O
maritime	O
radio	O
navigation	O
,	O
as	O
almost	O
every	O
ship	O
is	O
(	O
was	O
)	O
equipped	O
with	O
a	O
direction	B-Device
finder	I-Device
(	O
Appleyard	O
1988	O
)	O
.	O
</s>
<s>
Very	O
few	O
maritime	O
radio	O
navigation	O
beacons	B-Protocol
remain	O
active	O
today	O
(	O
2008	O
)	O
as	O
ships	O
have	O
abandoned	O
navigation	O
via	O
RDF	O
in	O
favor	O
of	O
GPS	O
navigation	O
.	O
</s>
<s>
In	O
the	O
United	O
Kingdom	O
a	O
radio	B-Device
direction	I-Device
finding	I-Device
service	O
is	O
available	O
on	O
121.5MHz	O
and	O
243.0MHz	O
to	O
aircraft	O
pilots	O
who	O
are	O
in	O
distress	O
or	O
are	O
experiencing	O
difficulties	O
.	O
</s>
<s>
Radio	O
transmitters	O
for	O
air	O
and	O
sea	O
navigation	O
are	O
known	O
as	O
beacons	B-Protocol
and	O
are	O
the	O
radio	O
equivalent	O
to	O
a	O
lighthouse	B-Application
.	O
</s>
<s>
The	O
transmitter	O
sends	O
a	O
Morse	O
Code	O
transmission	O
on	O
a	O
Long	B-Algorithm
wave	I-Algorithm
(	O
150	O
–	O
400kHz	O
)	O
or	O
Medium	B-Algorithm
wave	I-Algorithm
(	O
520	O
–	O
1720kHz	O
)	O
frequency	O
incorporating	O
the	O
station	O
's	O
identifier	O
that	O
is	O
used	O
to	O
confirm	O
the	O
station	O
and	O
its	O
operational	O
status	O
.	O
</s>
<s>
Since	O
these	O
radio	O
signals	O
are	O
broadcast	O
in	O
all	O
directions	O
(	O
omnidirectional	O
)	O
during	O
the	O
day	O
,	O
the	O
signal	O
itself	O
does	O
not	O
include	O
direction	O
information	O
,	O
and	O
these	O
beacons	B-Protocol
are	O
therefore	O
referred	O
to	O
as	O
non-directional	O
beacons	B-Protocol
,	O
or	O
NDBs	O
.	O
</s>
<s>
As	O
the	O
commercial	O
medium	B-Algorithm
wave	I-Algorithm
broadcast	O
band	O
lies	O
within	O
the	O
frequency	O
capability	O
of	O
most	O
RDF	O
units	O
,	O
these	O
stations	O
and	O
their	O
transmitters	O
can	O
also	O
be	O
used	O
for	O
navigational	O
fixes	O
.	O
</s>
<s>
Strings	O
of	O
beacons	B-Protocol
formed	O
"	O
airways	O
"	O
from	O
airport	O
to	O
airport	O
,	O
while	O
marine	O
NDBs	O
and	O
commercial	O
AM	O
broadcast	O
stations	O
provided	O
navigational	O
assistance	O
to	O
small	O
watercraft	O
approaching	O
a	O
landfall	O
.	O
</s>
<s>
In	O
the	O
1950s	O
,	O
aviation	O
NDBs	O
were	O
augmented	O
by	O
the	O
VOR	B-Device
system	O
,	O
in	O
which	O
the	O
direction	O
to	O
the	O
beacon	B-Protocol
can	O
be	O
extracted	O
from	O
the	O
signal	O
itself	O
,	O
hence	O
the	O
distinction	O
with	O
non-directional	O
beacons	B-Protocol
.	O
</s>
<s>
However	O
the	O
low	O
cost	O
of	O
ADF	O
and	O
RDF	O
systems	O
,	O
and	O
the	O
continued	O
existence	O
of	O
AM	O
broadcast	O
stations	O
(	O
as	O
well	O
as	O
navigational	O
beacons	B-Protocol
in	O
countries	O
outside	O
North	O
America	O
)	O
has	O
allowed	O
these	O
devices	O
to	O
continue	O
to	O
function	O
,	O
primarily	O
for	O
use	O
in	O
small	O
boats	O
,	O
as	O
an	O
adjunct	O
or	O
backup	O
to	O
GPS	O
.	O
</s>
<s>
In	O
World	O
War	O
II	O
considerable	O
effort	O
was	O
expended	O
on	O
identifying	O
secret	O
transmitters	O
in	O
the	O
United	O
Kingdom	O
(	O
UK	O
)	O
by	O
direction	B-Device
finding	I-Device
.	O
</s>
<s>
The	O
direction	B-Device
finding	I-Device
and	O
interception	O
operation	O
increased	O
in	O
volume	O
and	O
importance	O
until	O
1945	O
.	O
</s>
<s>
In	O
1941	O
,	O
RSS	O
began	O
experimenting	O
with	O
spaced	O
loop	O
direction	B-Device
finders	I-Device
,	O
developed	O
by	O
the	O
Marconi	B-Device
company	O
and	O
the	O
UK	O
National	O
Physical	O
Laboratories	O
.	O
</s>
<s>
The	O
angle	O
of	O
the	O
beam	O
was	O
combined	O
with	O
results	O
from	O
a	O
radiogoniometer	B-Device
to	O
provide	O
a	O
bearing	O
.	O
</s>
<s>
The	O
loops	O
were	O
placed	O
at	O
the	O
ends	O
of	O
a	O
beam	O
,	O
all	O
of	O
which	O
was	O
located	O
inside	O
a	O
wooden	O
hut	O
with	O
the	O
electronics	O
in	O
a	O
large	O
cabinet	O
with	O
cathode	B-Device
ray	I-Device
tube	I-Device
display	I-Device
at	O
the	O
centre	O
of	O
the	O
beam	O
and	O
everything	O
being	O
supported	O
on	O
a	O
central	O
axis	O
.	O
</s>
<s>
The	O
Royal	O
Navy	O
introduced	O
a	O
variation	O
on	O
the	O
shore	O
based	O
HF	O
DF	O
stations	O
in	O
1944	O
to	O
track	O
U-boats	B-Operating_System
in	O
the	O
North	O
Atlantic	O
.	O
</s>
<s>
The	O
Royal	O
Navy	O
also	O
deployed	O
direction	B-Device
finding	I-Device
equipment	O
on	O
ships	O
tasked	O
to	O
anti-submarine	O
warfare	O
in	O
order	O
to	O
try	O
to	O
locate	O
German	O
submarines	O
,	O
e.g.	O
</s>
<s>
Captain	O
class	O
frigates	O
were	O
fitted	O
with	O
a	O
medium	O
frequency	O
direction	B-Device
finding	I-Device
antenna	O
(	O
MF/DF	O
)	O
(	O
the	O
antenna	O
was	O
fitted	O
in	O
front	O
of	O
the	O
bridge	O
)	O
and	O
high	O
frequency	O
direction	B-Device
finding	I-Device
(	O
HF/DF	O
,	O
"	O
Huffduff	O
"	O
)	O
Type	O
FH	O
4	O
antenna	O
(	O
the	O
antenna	O
was	O
fitted	O
on	O
top	O
of	O
the	O
mainmast	O
)	O
.	O
</s>
<s>
A	O
comprehensive	O
reference	O
on	O
World	O
War	O
II	O
wireless	O
direction	B-Device
finding	I-Device
was	O
written	O
by	O
Roland	O
Keen	O
,	O
who	O
was	O
head	O
of	O
the	O
engineering	O
department	O
of	O
RSS	O
at	O
Hanslope	O
Park	O
.	O
</s>
<s>
The	O
DF	O
systems	O
mentioned	O
here	O
are	O
described	O
in	O
detail	O
in	O
his	O
1947	O
book	O
Wireless	O
Direction	B-Device
Finding	I-Device
.	O
</s>
<s>
Most	O
direction	B-Device
finding	I-Device
effort	O
within	O
the	O
UK	O
now	O
(	O
2009	O
)	O
is	O
directed	O
towards	O
locating	O
unauthorised	O
"	O
pirate	B-Application
"	O
FM	O
broadcast	O
radio	O
transmissions	O
.	O
</s>
<s>
A	O
network	O
of	O
remotely	O
operated	O
VHF	B-Algorithm
direction	B-Device
finders	I-Device
are	O
used	O
mainly	O
located	O
around	O
the	O
major	O
cities	O
.	O
</s>
<s>
The	O
transmissions	O
from	O
mobile	O
telephone	O
handsets	O
are	O
also	O
located	O
by	O
a	O
form	O
of	O
direction	B-Device
finding	I-Device
using	O
the	O
comparative	O
signal	O
strength	O
at	O
the	O
surrounding	O
local	O
"	O
cell	O
"	O
receivers	O
.	O
</s>
<s>
There	O
are	O
many	O
forms	O
of	O
radio	O
transmitters	O
designed	O
to	O
transmit	O
as	O
a	O
beacon	B-Protocol
in	O
the	O
event	O
of	O
an	O
emergency	O
,	O
which	O
are	O
widely	O
deployed	O
on	O
civil	O
aircraft	O
.	O
</s>
<s>
Modern	O
emergency	O
beacons	B-Protocol
transmit	O
a	O
unique	O
identification	O
signal	O
that	O
can	O
aid	O
in	O
finding	O
the	O
exact	O
location	O
of	O
the	O
transmitter	O
.	O
</s>
<s>
Since	O
the	O
power	O
of	O
the	O
beacon	B-Protocol
is	O
so	O
low	O
the	O
directionality	O
of	O
the	O
radio	O
signal	O
is	O
dominated	O
by	O
small	O
scale	O
field	O
effects	O
and	O
can	O
be	O
quite	O
complicated	O
to	O
locate	O
.	O
</s>
<s>
Location	O
of	O
radio-tagged	O
animals	O
by	O
triangulation	B-Algorithm
is	O
a	O
widely	O
applied	O
research	O
technique	O
for	O
studying	O
the	O
movement	O
of	O
animals	O
.	O
</s>
<s>
Most	O
tracking	O
of	O
wild	O
animals	O
that	O
have	O
been	O
affixed	O
with	O
radio	O
transmitter	O
equipment	O
is	O
done	O
by	O
a	O
field	O
researcher	O
using	O
a	O
handheld	O
radio	B-Device
direction	I-Device
finding	I-Device
device	O
.	O
</s>
<s>
Phased	O
arrays	O
and	O
other	O
advanced	O
antenna	O
techniques	O
are	O
utilized	O
to	O
track	O
launches	O
of	O
rocket	B-Application
systems	O
and	O
their	O
resulting	O
trajectories	O
.	O
</s>
<s>
Events	O
hosted	O
by	O
groups	O
and	O
organizations	O
that	O
involve	O
the	O
use	O
of	O
radio	B-Device
direction	I-Device
finding	I-Device
skills	O
to	O
locate	O
transmitters	O
at	O
unknown	O
locations	O
have	O
been	O
popular	O
since	O
the	O
end	O
of	O
World	O
War	O
II	O
.	O
</s>
<s>
Many	O
of	O
these	O
events	O
were	O
first	O
promoted	O
in	O
order	O
to	O
practice	O
the	O
use	O
of	O
radio	B-Device
direction	I-Device
finding	I-Device
techniques	O
for	O
disaster	O
response	O
and	O
civil	O
defense	O
purposes	O
,	O
or	O
to	O
practice	O
locating	O
the	O
source	O
of	O
radio	O
frequency	O
interference	O
.	O
</s>
<s>
The	O
most	O
popular	O
form	O
of	O
the	O
sport	O
,	O
worldwide	O
,	O
is	O
known	O
as	O
Amateur	O
Radio	B-Device
Direction	I-Device
Finding	I-Device
or	O
by	O
its	O
international	O
abbreviation	O
ARDF	O
.	O
</s>
<s>
Another	O
form	O
of	O
the	O
activity	O
,	O
known	O
as	O
"	O
transmitter	O
hunting	O
"	O
,	O
"	O
mobile	O
T-hunting	O
"	O
or	O
"	O
fox	O
hunting	O
"	O
takes	O
place	O
in	O
a	O
larger	O
geographic	O
area	O
,	O
such	O
as	O
the	O
metropolitan	O
area	O
of	O
a	O
large	O
city	O
,	O
and	O
most	O
participants	O
travel	O
in	O
motor	O
vehicles	O
while	O
attempting	O
to	O
locate	O
one	O
or	O
more	O
radio	O
transmitters	O
with	O
radio	B-Device
direction	I-Device
finding	I-Device
techniques	O
.	O
</s>
<s>
A	O
radio	B-Device
direction-finding	I-Device
station	O
is	O
–	O
according	O
to	O
article	O
1.91	O
of	O
the	O
International	O
Telecommunication	O
Union	O
's	O
(	O
ITU	O
)	O
ITU	O
Radio	O
Regulations	O
(	O
RR	O
)	O
–	O
defined	O
as	O
"	O
a	O
radiodetermination	O
station	O
using	O
radio	O
direction-finding.	O
"	O
</s>
<s>
In	O
aircraft	O
,	O
a	O
DF	O
system	O
provides	O
additional	O
information	O
for	O
the	O
Radar	B-Application
Warning	O
Receiver	O
(	O
RWR	O
)	O
.	O
</s>
<s>
Over	O
time	O
,	O
it	O
became	O
necessary	O
to	O
improve	O
the	O
performance	O
of	O
microwave	O
DF	O
systems	O
in	O
order	O
to	O
counter	O
the	O
evasive	O
tactics	O
being	O
employed	O
by	O
some	O
operators	O
,	O
such	O
as	O
low-probability-of-intercept	O
radars	B-Application
and	O
covert	O
Data	O
links	O
.	O
</s>
<s>
Intensive	O
research	O
work	O
was	O
carried	O
out	O
in	O
the	O
1930s	O
in	O
order	O
to	O
develop	O
transmitting	O
tubes	O
specifically	O
for	O
the	O
microwave	O
band	O
which	O
included	O
,	O
in	O
particular	O
,	O
the	O
klystron	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
and	O
the	O
travelling	O
wave	O
tube	O
(	O
TWT	O
.	O
</s>
<s>
Microwave	O
signals	O
have	O
short	O
wavelengths	O
,	O
which	O
results	O
in	O
greatly	O
improved	O
target	O
resolution	O
when	O
compared	O
to	O
RF	B-Protocol
systems	O
.	O
</s>
<s>
Also	O
,	O
the	O
antennas	O
are	O
small	O
so	O
they	O
can	O
be	O
assembled	O
into	O
compact	O
arrays	O
and	O
,	O
in	O
addition	O
,	O
they	O
can	O
achieve	O
well	O
defined	O
beam	O
patterns	O
which	O
can	O
provide	O
the	O
narrow	O
beams	O
with	O
high	O
gain	O
favoured	O
by	O
radars	B-Application
and	O
Data	O
links	O
.	O
</s>
<s>
Other	O
advantages	O
of	O
the	O
newly	O
available	O
microwave	O
band	O
were	O
the	O
absence	O
of	O
fading	O
(	O
often	O
a	O
problem	O
in	O
the	O
Shortwave	O
radio	O
(	O
SW	O
)	O
band	O
)	O
and	O
great	O
increase	O
in	O
signal	O
spectrum	O
,	O
compared	O
to	O
the	O
congested	O
RF	B-Protocol
bands	O
already	O
in	O
use	O
.	O
</s>
<s>
In	O
addition	O
to	O
being	O
able	O
to	O
accommodate	O
many	O
more	O
signals	O
,	O
the	O
ability	O
to	O
use	O
Spread	B-Architecture
spectrum	I-Architecture
and	O
frequency	B-Protocol
hopping	I-Protocol
techniques	O
now	O
became	O
possible	O
.	O
</s>
<s>
Antennas	O
for	O
DF	O
have	O
to	O
meet	O
different	O
requirements	O
from	O
those	O
for	O
a	O
radar	B-Application
or	O
communication	O
link	O
,	O
where	O
an	O
antenna	O
with	O
a	O
narrow	O
beam	O
and	O
high	O
gain	O
is	O
usually	O
an	O
advantage	O
.	O
</s>
<s>
However	O
,	O
when	O
carrying	O
out	O
direction	B-Device
finding	I-Device
,	O
the	O
bearing	O
of	O
the	O
source	O
may	O
be	O
unknown	O
,	O
so	O
antennas	O
with	O
wide	O
beamwidths	O
are	O
usually	O
chosen	O
,	O
even	O
though	O
they	O
have	O
lower	O
antenna	O
boresight	O
gain	O
.	O
</s>
<s>
Preferably	O
,	O
when	O
using	O
amplitude	O
comparison	O
methods	O
for	O
direction	B-Device
finding	I-Device
,	O
the	O
main	O
lobe	O
should	O
approximate	O
to	O
a	O
Gaussian	O
characteristic	O
.	O
</s>
<s>
Transistors	B-Application
suitable	O
for	O
microwave	O
frequencies	O
became	O
available	O
towards	O
the	O
end	O
of	O
the	O
1950s	O
.	O
</s>
<s>
The	O
first	O
of	O
these	O
was	O
the	O
metal	B-Architecture
oxide	I-Architecture
semiconductor	I-Architecture
field	I-Architecture
effect	I-Architecture
transistor	I-Architecture
(	O
MOSFET	B-Architecture
)	O
.	O
</s>
<s>
Others	O
followed	O
,	O
for	O
example	O
,	O
the	O
metal-semiconductor	O
field-effect	O
transistor	B-Application
and	O
the	O
high	B-Algorithm
electron	I-Algorithm
mobility	I-Algorithm
transistor	I-Algorithm
(	O
HEMT	B-Algorithm
)	O
.	O
</s>
<s>
Initially	O
,	O
discrete	O
transistors	B-Application
were	O
embedded	O
in	O
stripline	O
or	O
microstrip	O
circuits	O
,	O
but	O
microwave	B-Algorithm
integrated	I-Algorithm
circuits	I-Algorithm
followed	O
.	O
</s>
<s>
The	O
DF	O
receiver	O
enjoys	O
a	O
detection	O
range	O
advantage	O
over	O
that	O
of	O
the	O
radar	B-Application
receiver	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
signal	O
strength	O
at	O
the	O
DF	O
receiver	O
,	O
due	O
to	O
a	O
radar	B-Application
transmission	O
,	O
is	O
proportional	O
to	O
1/R2	O
whereas	O
that	O
at	O
the	O
radar	B-Application
receiver	O
from	O
the	O
reflected	O
return	O
is	O
proportional	O
to	O
σ/R4	O
,	O
where	O
R	O
is	O
the	O
range	O
and	O
σ	O
is	O
the	O
radar	B-Application
cross-section	O
of	O
the	O
DF	O
system	O
.	O
</s>
<s>
This	O
results	O
in	O
the	O
signal	O
strength	O
at	O
the	O
radar	B-Application
receiver	O
being	O
very	O
much	O
smaller	O
than	O
that	O
at	O
the	O
DF	O
receiver	O
.	O
</s>
<s>
Consequently	O
,	O
in	O
spite	O
of	O
its	O
poor	O
sensitivity	O
,	O
a	O
simple	O
crystal-video	O
DF	O
receiver	O
is	O
,	O
usually	O
,	O
able	O
to	O
detect	O
the	O
signal	O
transmission	O
from	O
a	O
radar	B-Application
at	O
a	O
greater	O
range	O
than	O
that	O
at	O
which	O
the	O
Radar	B-Application
's	O
own	O
receiver	O
is	O
able	O
to	O
detect	O
the	O
presence	O
of	O
the	O
DF	O
system	O
.	O
</s>
<s>
In	O
practice	O
,	O
the	O
advantage	O
is	O
reduced	O
by	O
the	O
ratio	O
of	O
antenna	O
gains	O
(	O
typically	O
they	O
are	O
36dB	O
and	O
10dB	O
for	O
the	O
Radar	B-Application
and	O
ESM	O
,	O
respectively	O
)	O
and	O
the	O
use	O
of	O
Spread	B-Architecture
spectrum	I-Architecture
techniques	O
,	O
such	O
as	O
Chirp	B-Algorithm
compression	I-Algorithm
,	O
by	O
the	O
Radar	B-Application
,	O
to	O
increase	O
the	O
processing	O
gain	O
of	O
its	O
receiver	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
the	O
DF	O
system	O
can	O
regain	O
some	O
advantage	O
by	O
using	O
sensitive	O
,	O
low-noise	O
,	O
receivers	O
and	O
by	O
using	O
Stealth	O
practices	O
to	O
reduce	O
its	O
radar	B-Application
cross-section	O
,	O
as	O
with	O
Stealth	O
aircraft	O
and	O
Stealth	O
ships	O
.	O
</s>
<s>
Radars	B-Application
with	O
their	O
narrow	O
beams	O
would	O
only	O
illuminate	O
the	O
antennas	O
of	O
the	O
DF	O
system	O
infrequently	O
.	O
</s>
<s>
Furthermore	O
,	O
some	O
radars	B-Application
wishing	O
to	O
avoid	O
detection	O
(	O
those	O
of	O
smugglers	O
,	O
hostile	O
ships	O
and	O
missiles	O
)	O
would	O
radiate	O
their	O
signals	O
infrequently	O
and	O
often	O
at	O
low	O
power	O
.	O
</s>
<s>
Such	O
a	O
system	O
is	O
referred	O
to	O
as	O
a	O
low-probability-of-intercept	O
radar	B-Application
.	O
</s>
<s>
In	O
general	O
,	O
in	O
order	O
to	O
cater	O
for	O
modern	O
circumstances	O
,	O
a	O
broadband	O
microwave	O
DF	O
system	O
is	O
required	O
to	O
have	O
high	O
sensitivity	O
and	O
have	O
360°	O
coverage	O
in	O
order	O
to	O
have	O
the	O
ability	O
to	O
detect	O
single	O
pulses	O
(	O
often	O
called	O
amplitude	B-Algorithm
monopulse	I-Algorithm
)	O
and	O
achieve	O
a	O
high	O
"	O
Probability	O
of	O
Intercept	O
"	O
(	O
PoI	O
)	O
.	O
</s>
<s>
By	O
considering	O
the	O
signal	O
levels	O
on	O
a	O
logarithmic	O
scale	O
,	O
as	O
provided	O
by	O
the	O
DLVAs	O
,	O
a	O
large	O
dynamic	O
range	O
is	O
achieved	O
and	O
,	O
in	O
addition	O
,	O
the	O
direction	B-Device
finding	I-Device
calculations	O
are	O
simplified	O
when	O
the	O
main	O
lobes	O
of	O
antenna	O
patterns	O
have	O
a	O
Gaussian	O
characteristic	O
,	O
as	O
shown	O
earlier	O
.	O
</s>
<s>
Some	O
of	O
these	O
issues	O
are	O
covered	O
in	O
RF	B-Protocol
chain	O
.	O
</s>
