<s>
A	O
Bellini	B-Device
–	I-Device
Tosi	I-Device
direction	I-Device
finder	I-Device
(	O
B	O
–	O
T	O
or	O
BTDF	O
)	O
is	O
a	O
type	O
of	O
radio	B-Device
direction	I-Device
finder	I-Device
(	O
RDF	O
)	O
,	O
which	O
determines	O
the	O
direction	O
to	O
,	O
or	O
bearing	O
of	O
,	O
a	O
radio	O
transmitter	O
.	O
</s>
<s>
Earlier	O
RDF	O
systems	O
used	O
very	O
large	O
rotating	O
loop	B-Architecture
antennas	I-Architecture
,	O
which	O
the	O
B	O
–	O
T	O
system	O
replaced	O
with	O
two	O
fixed	O
antennae	O
and	O
a	O
small	O
rotating	O
loop	O
,	O
known	O
as	O
a	O
radiogoniometer	B-Device
.	O
</s>
<s>
BTDF	O
was	O
invented	O
by	O
a	O
pair	O
of	O
Italian	O
officers	O
in	O
the	O
early	O
1900s	O
,	O
and	O
is	O
sometimes	O
known	O
as	O
a	O
Marconi	B-Device
–	O
Bellini	O
–	O
Tosi	O
after	O
they	O
joined	O
forces	O
with	O
the	O
Marconi	B-Device
Company	O
in	O
1912	O
.	O
</s>
<s>
BTDF	O
was	O
the	O
most	O
prevalent	O
form	O
of	O
naval	O
direction	B-Device
finding	I-Device
from	O
the	O
1920s	O
to	O
well	O
into	O
the	O
1980s	O
,	O
and	O
was	O
used	O
as	O
a	O
major	O
part	O
of	O
early	O
long-distance	O
air	O
navigation	O
systems	O
from	O
the	O
1930s	O
until	O
after	O
World	O
War	O
II	O
.	O
</s>
<s>
During	O
the	O
war	O
,	O
new	O
techniques	O
like	O
huff-duff	O
began	O
to	O
replace	O
radiogoniometers	B-Device
in	O
the	O
intelligence	O
gathering	O
role	O
,	O
reducing	O
the	O
time	O
needed	O
to	O
take	O
an	O
accurate	O
fix	O
from	O
minutes	O
to	O
seconds	O
.	O
</s>
<s>
The	O
ability	O
to	O
inexpensively	O
process	O
radio	O
signals	O
using	O
microcontrollers	B-Architecture
allowed	O
pseudo-doppler	B-Device
direction	I-Device
finders	I-Device
to	O
take	O
over	O
most	O
of	O
the	O
radiogoniometer	B-Device
's	O
remaining	O
roles	O
from	O
the	O
1980s	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
longwave	B-Algorithm
or	O
medium	B-Algorithm
wave	I-Algorithm
signals	O
.	O
</s>
<s>
Longwave	B-Algorithm
,	O
in	O
particular	O
,	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	O
antennae	O
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>
One	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>
Direction	B-Device
finding	I-Device
could	O
then	O
be	O
carried	O
out	O
with	O
a	O
conventional	O
loop	B-Architecture
antenna	I-Architecture
placed	O
in	O
the	O
center	O
of	O
these	O
two	O
stators	O
(	O
or	O
field	O
coils	O
)	O
;	O
the	O
rotating	O
loop	O
was	O
known	O
as	O
the	O
rotor	O
(	O
or	O
sense	O
coil	O
)	O
.	O
</s>
<s>
For	O
longwave	B-Algorithm
use	O
,	O
the	O
two	O
crossed	O
antennae	O
could	O
be	O
easily	O
built	O
by	O
running	O
four	O
wires	O
from	O
a	O
single	O
mast	O
to	O
the	O
ground	O
to	O
form	O
triangular	O
shapes	O
.	O
</s>
<s>
The	O
pair	O
sold	O
the	O
patents	O
to	O
the	O
Marconi	B-Device
Company	O
in	O
February	O
1912	O
,	O
and	O
Bellini	O
joined	O
the	O
company	O
to	O
continue	O
development	O
.	O
</s>
<s>
The	O
B	O
–	O
T	O
system	O
was	O
introduced	O
around	O
the	O
same	O
time	O
as	O
the	O
first	O
triodes	O
,	O
and	O
the	O
Marconi	B-Device
partnership	O
took	O
place	O
in	O
the	O
same	O
year	O
that	O
the	O
triode	O
's	O
ability	O
to	O
amplify	O
signals	O
was	O
first	O
noticed	O
.	O
</s>
<s>
In	O
1917	O
Frank	O
Adcock	O
was	O
trying	O
to	O
solve	O
the	O
problem	O
of	O
making	O
large	O
antennae	O
suitable	O
for	O
use	O
with	O
the	O
radiogoniometer	B-Device
at	O
even	O
the	O
longest	O
wavelengths	O
.	O
</s>
<s>
An	O
antenna	O
that	O
broadcast	O
a	O
useful	O
signal	O
at	O
longwave	B-Algorithm
frequencies	O
would	O
be	O
larger	O
than	O
a	O
typical	O
aircraft	O
(	O
although	O
Zeppelins	B-Application
had	O
no	O
problems	O
)	O
and	O
even	O
higher	O
frequencies	O
in	O
the	O
high	B-Algorithm
frequency	I-Algorithm
(	O
HF	O
)	O
and	O
very	B-Algorithm
high	I-Algorithm
frequency	I-Algorithm
(	O
VHF	B-Algorithm
)	O
bands	O
were	O
highly	O
desirable	O
.	O
</s>
<s>
A	O
good	O
example	O
of	O
the	O
advantages	O
of	O
shorter	O
wavelengths	O
can	O
be	O
seen	O
on	O
the	O
Supermarine	O
Spitfire	O
,	O
which	O
started	O
WWII	O
with	O
an	O
HF	B-Algorithm
radio	I-Algorithm
that	O
broadcast	O
from	O
a	O
cable	O
antenna	O
stretched	O
from	O
the	O
cockpit	O
to	O
the	O
top	O
of	O
the	O
vertical	O
stabilizer	O
.	O
</s>
<s>
These	O
early	O
TR9D	O
sets	O
were	O
replaced	O
by	O
a	O
VHF	B-Algorithm
set	O
using	O
a	O
small	O
whip	O
antenna	O
offering	O
ranges	O
on	O
the	O
order	O
of	O
,	O
and	O
hundreds	O
miles	O
in	O
the	O
air-to-ground	O
mode	O
.	O
</s>
<s>
Two	O
stations	O
equipped	O
with	O
Marconi	B-Device
BTDF	O
sets	O
and	O
Adcock	O
antennae	O
were	O
set	O
up	O
at	O
Charleville	O
and	O
Melbourne	O
.	O
</s>
<s>
This	O
system	O
used	O
a	O
motor	O
to	O
quickly	O
spin	O
a	O
radiogoniometer	B-Device
,	O
as	O
well	O
as	O
providing	O
an	O
input	O
to	O
electronics	O
that	O
spun	O
the	O
X	O
and	O
Y	O
inputs	O
of	O
a	O
cathode	B-Device
ray	I-Device
tube	I-Device
(	O
CRT	O
)	O
.	O
</s>
<s>
In	O
the	O
UK	O
,	O
the	O
high-frequency	B-Algorithm
direction	B-Device
finding	I-Device
(	O
HFDF	O
or	O
"	O
huff-duff	O
"	O
)	O
system	O
largely	O
had	O
displaced	O
BTDF	O
by	O
about	O
1943	O
.	O
</s>
<s>
The	O
second	O
advance	O
was	O
the	O
introduction	O
of	O
the	O
automatic	O
direction	B-Device
finder	I-Device
(	O
ADF	O
)	O
,	O
which	O
completely	O
automated	O
the	O
RDF	O
procedure	O
.	O
</s>
<s>
Improvements	O
continued	O
to	O
be	O
made	O
to	O
both	O
systems	O
throughout	O
this	O
period	O
,	O
especially	O
the	O
introduction	O
of	O
solenoids	B-Algorithm
in	O
place	O
of	O
conventional	O
loops	O
in	O
some	O
roles	O
.	O
</s>
<s>
However	O
,	O
the	O
introduction	O
of	O
the	O
doppler	O
direction	B-Device
finder	I-Device
,	O
and	O
especially	O
the	O
low-cost	O
electronics	O
to	O
implement	O
it	O
,	O
led	O
to	O
the	O
disappearance	O
of	O
the	O
traditional	O
loop	O
systems	O
by	O
the	O
mid-1990s	O
.	O
</s>
<s>
Doppler	O
systems	O
use	O
fixed	O
antennae	O
,	O
like	O
BTDF	O
,	O
but	O
handle	O
the	O
direction	B-Device
finding	I-Device
via	O
signal	O
processing	O
alone	O
.	O
</s>
<s>
The	O
loop	B-Architecture
antenna	I-Architecture
uses	O
this	O
principle	O
in	O
a	O
convenient	O
and	O
mechanically	O
robust	O
form	O
.	O
</s>
<s>
The	O
sense	O
coil	O
,	O
another	O
loop	B-Architecture
antenna	I-Architecture
,	O
is	O
placed	O
in	O
the	O
area	O
between	O
the	O
field	O
coils	O
.	O
</s>
<s>
Rotating	O
the	O
sense	O
coil	O
in	O
the	O
field	O
coils	O
has	O
the	O
same	O
output	O
as	O
rotating	O
the	O
entire	O
loop	B-Architecture
antenna	I-Architecture
in	O
the	O
original	O
field	O
.	O
</s>
<s>
Since	O
this	O
was	O
fixed	O
as	O
part	O
of	O
the	O
construction	O
of	O
the	O
radiogoniometer	B-Device
,	O
it	O
was	O
simple	O
enough	O
to	O
correct	O
for	O
this	O
simply	O
by	O
moving	O
the	O
pointer	O
.	O
</s>
<s>
It	O
would	O
be	O
if	O
the	O
field	O
coils	O
consisted	O
of	O
a	O
single	O
loop	O
of	O
wire	O
,	O
but	O
as	O
they	O
actually	O
consist	O
of	O
multiple	O
windings	O
,	O
these	O
are	O
,	O
in	O
effect	O
,	O
small	O
solenoids	B-Algorithm
.	O
</s>
<s>
The	O
correction	O
is	O
never	O
perfect	O
,	O
the	O
precise	O
angles	O
had	O
to	O
be	O
experimented	O
with	O
on	O
every	O
radiogoniometer	B-Device
.	O
</s>
<s>
For	O
this	O
reason	O
a	O
variety	O
of	O
systems	O
were	O
used	O
to	O
decrease	O
the	O
sensitivity	O
of	O
the	O
radiogoniometer	B-Device
to	O
mis-tuning	O
.	O
</s>
<s>
Primary	O
among	O
these	O
was	O
the	O
aperiodic	O
aerial	O
concept	O
,	O
which	O
described	O
the	O
mechanical	O
layout	O
of	O
the	O
radiogoniometer	B-Device
's	O
internal	O
wiring	O
.	O
</s>
<s>
The	O
directional	O
qualities	O
of	O
the	O
radiogoniometer	B-Device
work	O
in	O
both	O
directions	O
;	O
it	O
can	O
be	O
used	O
to	O
determine	O
the	O
direction	O
of	O
an	O
incoming	O
signal	O
,	O
or	O
change	O
the	O
direction	O
of	O
a	O
transmission	O
.	O
</s>
<s>
During	O
early	O
experiments	O
,	O
this	O
capability	O
was	O
used	O
to	O
produce	O
a	O
radio	O
signal	O
that	O
swept	O
the	O
sky	O
like	O
a	O
lighthouse	B-Application
beam	O
,	O
allowing	O
conventional	O
radio	O
receivers	O
to	O
determine	O
their	O
location	O
by	O
timing	O
the	O
passage	O
of	O
the	O
signal	O
.	O
</s>
