<s>
Super	B-Algorithm
low	I-Algorithm
frequency	I-Algorithm
(	O
SLF	O
)	O
is	O
the	O
ITU	O
designation	O
for	O
electromagnetic	O
waves	O
(	O
radio	O
waves	O
)	O
in	O
the	O
frequency	O
range	O
between	O
30	O
hertz	O
and	O
300	O
hertz	O
.	O
</s>
<s>
Another	O
conflicting	O
designation	O
which	O
includes	O
this	O
frequency	O
range	O
is	O
Extremely	B-Language
Low	I-Language
Frequency	I-Language
(	O
ELF	O
)	O
,	O
which	O
in	O
some	O
contexts	O
refers	O
to	O
all	O
frequencies	O
up	O
to	O
300	O
hertz	O
.	O
</s>
<s>
It	O
became	O
operational	O
in	O
1989	O
but	O
was	O
discontinued	O
in	O
2004	O
due	O
to	O
advances	O
in	O
VLF	B-Architecture
communication	O
systems	O
.	O
</s>
<s>
Radio	O
amateurs	O
have	O
received	O
signals	O
in	O
this	O
range	O
using	O
simple	O
receivers	O
built	O
around	O
personal	B-Device
computers	I-Device
,	O
with	O
coil	O
or	O
loop	O
antennas	O
connected	O
to	O
the	O
PCs	B-Device
sound	B-Device
card	I-Device
.	O
</s>
