Horn
Antenna are made directional in several ways one is to use a horn shape.
This reflects the radio waves entering it to a point where a receiver is
located. However for transmitting the waves will always spread no
mater how long the horn is.
Horn
Cut view of Horn
The above is a cut out slice of a horn receiver. The red lines represent radio waves entering the horn from ahead note how they reflect off the sides and reach the receiver. The blue lines represent waves entering from a large angle note how they reflect and are sent back out of the horn. This in creases the direction of the antenna but still leave a range of angles that will be received.
To farther narrow the direction the waves take lens's may be used.
It must be remembered that electromagnetic waves are the same type of wave
as light. Therefore like light they may be focused using lenses.
The source or receiver below is at the focal point of the lens note how
the waves are leaving the lens parallel to each other and therefore will
go great distances with out scattering. While this works it has problems
mostly that is transmission the waves moving to the left are unaffected.
And will hit things behind the antenna. This could be harmful given
the possible power of the antenna.
Lens bending
radio waves.
The preferred way of making directional Antennas is to make a dish to
reflect the waves rather then to refract them with a lens.
These dishes are easy to make and unlike a lens which needs to be solid
and they need only a very thin reflective surface. When the reflector
is given a specific shape called a parabola and the receiver is placed
at the focus the waves received are parallel. This makes this antenna
the preferred type for radio telescopes and other extreme range applications.
Parabolic reflector