Pencil Beam Antenna
Figure 1: A beam „thin like a pencil”
Although they don't form any antenna manner of their own actually:
Antennae which extremely strongly bundle in
are often described as a
pencil-beam antenna. For example the
phased array antenna
can send out a pencil beam,
therefore a beam „thin like a pencil”.
Pencil Beam antennae are used as a tracking radar. True 3D-radar sets also emit pencil beams, like the RRP-117.