<s>
Doppler	B-Device
radio	I-Device
direction	I-Device
finding	I-Device
,	O
or	O
Doppler	O
DF	O
for	O
short	O
,	O
is	O
a	O
radio	B-Device
direction	I-Device
finding	I-Device
method	O
that	O
generates	O
accurate	O
bearing	O
information	O
with	O
a	O
minimum	O
of	O
electronics	O
.	O
</s>
<s>
It	O
is	O
best	O
suited	O
to	O
VHF	B-Algorithm
and	O
UHF	O
frequencies	O
,	O
and	O
takes	O
only	O
a	O
short	O
time	O
to	O
indicate	O
a	O
direction	O
.	O
</s>
<s>
Doppler	O
DF	O
is	O
one	O
of	O
the	O
most	O
widely	O
used	O
direction	B-Device
finding	I-Device
techniques	O
.	O
</s>
<s>
Other	O
direction	B-Device
finding	I-Device
techniques	O
are	O
generally	O
used	O
only	O
for	O
fleeting	O
signals	O
,	O
or	O
longer	O
or	O
shorter	O
wavelengths	O
.	O
</s>
<s>
Early	O
radio	B-Device
direction	I-Device
finding	I-Device
(	O
RDF	O
)	O
solutions	O
used	O
highly	O
directional	O
antennas	O
with	O
sharp	O
"	O
nulls	O
"	O
in	O
the	O
reception	O
pattern	O
.	O
</s>
<s>
A	O
common	O
RDF	O
antenna	O
design	O
is	O
the	O
loop	B-Architecture
antenna	I-Architecture
,	O
which	O
is	O
simply	O
a	O
loop	O
of	O
wire	O
with	O
a	O
small	O
gap	O
in	O
the	O
circle	O
,	O
typically	O
arranged	O
to	O
rotate	O
around	O
the	O
vertical	O
axis	O
with	O
the	O
gap	O
at	O
the	O
bottom	O
.	O
</s>
<s>
Before	O
the	O
1930s	O
,	O
radio	O
signals	O
were	O
generally	O
in	O
what	O
would	O
today	O
be	O
known	O
as	O
the	O
long	B-Algorithm
wave	I-Algorithm
spectrum	O
.	O
</s>
<s>
Direction	B-Device
finding	I-Device
with	O
rotating	O
antennas	O
is	O
difficult	O
at	O
these	O
wavelengths	O
due	O
to	O
the	O
size	O
of	O
the	O
antennas	O
.	O
</s>
<s>
A	O
great	O
advance	O
in	O
RDF	O
technique	O
was	O
introduced	O
in	O
the	O
form	O
of	O
the	O
Bellini-Tosi	B-Device
direction	I-Device
finder	I-Device
system	O
,	O
which	O
replaced	O
the	O
rotation	O
of	O
the	O
antenna	O
with	O
the	O
rotation	O
of	O
a	O
small	O
coil	O
of	O
wire	O
connected	O
to	O
two	O
non-moving	O
loop	B-Architecture
antennas	I-Architecture
.	O
</s>
<s>
The	O
loop	B-Architecture
antennas	I-Architecture
were	O
similar	O
to	O
those	O
used	O
in	O
earlier	O
systems	O
but	O
fixed	O
in	O
position	O
,	O
set	O
at	O
right	O
angles	O
to	O
each	O
other	O
to	O
form	O
a	O
cross-shaped	O
arrangement	O
.	O
</s>
<s>
By	O
rotating	O
a	O
small	O
loop	B-Architecture
antenna	I-Architecture
,	O
the	O
sense	O
coil	O
,	O
in	O
the	O
space	O
between	O
the	O
two	O
crossed	O
field	O
coils	O
,	O
one	O
could	O
perform	O
DF	O
.	O
</s>
<s>
Robert	O
Watson-Watt	O
introduced	O
the	O
next	O
major	O
advance	O
in	O
direction	B-Device
finding	I-Device
as	O
the	O
"	O
huff-duff	O
"	O
system	O
,	O
a	O
nickname	O
for	O
high-frequency	O
direction	B-Device
finding	I-Device
.	O
</s>
<s>
Huff-duff	O
was	O
used	O
in	O
about	O
one-quarter	O
of	O
all	O
successful	O
U-boat	B-Operating_System
sinking	O
.	O
</s>
<s>
This	O
will	O
produce	O
a	O
rising	O
and	O
falling	O
frequency	O
shift	O
of	O
the	O
target	O
signal	O
,	O
producing	O
a	O
frequency	B-Application
modulated	I-Application
(	O
FM	B-Application
)	O
signal	O
known	O
as	O
the	O
Doppler	O
sine	O
wave	O
.	O
</s>
<s>
The	O
FM	B-Application
signal	O
has	O
the	O
same	O
frequency	O
as	O
the	O
rotational	O
speed	O
of	O
the	O
vehicle	O
.	O
</s>
<s>
Consider	O
the	O
example	O
truck	O
hunting	O
an	O
FM	B-Application
radio	O
station	O
at	O
101.5MHz	O
while	O
driving	O
around	O
a	O
wide	O
pad	O
(	O
50m	O
radius	O
)	O
at	O
.	O
</s>
<s>
Early	O
examples	O
of	O
Doppler	O
DF	O
systems	O
date	O
to	O
at	O
least	O
1941	O
,	O
and	O
they	O
were	O
used	O
in	O
the	O
United	O
Kingdom	O
for	O
hunting	O
out	O
German	O
early	B-Algorithm
warning	I-Algorithm
radars	I-Algorithm
,	O
which	O
operated	O
at	O
250MHz	O
in	O
the	O
1.25-meter	B-Algorithm
band	I-Algorithm
.	O
</s>
<s>
A	O
significant	O
advantage	O
of	O
this	O
technique	O
is	O
that	O
it	O
requires	O
only	O
a	O
single	O
receiver	O
,	O
amplifier	O
,	O
and	O
the	O
appropriate	O
FM	B-Application
demodulator	O
.	O
</s>
<s>
Widespread	O
civilian	O
use	O
of	O
the	O
technique	O
did	O
not	O
start	O
until	O
the	O
introduction	O
of	O
practical	O
circuits	O
for	O
the	O
quadrature	B-Algorithm
detector	I-Algorithm
and	O
phase-locked	O
loop	O
,	O
both	O
introduced	O
after	O
the	O
war	O
,	O
which	O
greatly	O
simplified	O
the	O
reception	O
of	O
FM	B-Application
signals	O
.	O
</s>
<s>
Its	O
use	O
roughly	O
follows	O
the	O
spread	O
of	O
FM	B-Application
radio	O
,	O
which	O
also	O
used	O
these	O
techniques	O
.	O
</s>
<s>
This	O
is	O
the	O
pseudo-Doppler	O
direction	B-Device
finding	I-Device
technique	O
.	O
</s>
<s>
At	O
long	O
distances	O
from	O
the	O
transmitter	O
,	O
well	O
into	O
the	O
"	B-Algorithm
far	I-Algorithm
field	I-Algorithm
"	I-Algorithm
,	O
the	O
wavefronts	O
can	O
be	O
considered	O
to	O
be	O
parallel	O
.	O
</s>
