antenas

Omnidirectional Antennas

The radio antenna on your automobile is a common example of an
omnidirectional vertical antenna—a simple wire or rod oriented vertically
to match the RF radiation polarity of most radio broadcast
stations.
An ideal but theoretical omnidirectional vertical antenna would
radiate 360 degrees from a point in a spherical pattern.
This theoretical antenna employs an isotropic radiator—having no
depth, width, or height—and is used only as a reference to calculate
antenna performance—gain or loss—expressed in decibels as dBi or
decibels relative to an isotropic radiator. Your neighbors and most
environmentalists would really appreciate isotropic radiators—if
they existed.
Omnidirectional antennas are typically oriented vertically, perpendicular
to the Earth, so the signal they radiate spans out and around
across the horizon. If the antenna were oriented horizontally, parallel to
the Earth, much of the available signal would be lost, radiating into the
Earth and up into the atmosphere. We want our wireless signals more
earthbound, but not wasted into the ground either.

Directional Yagi Antennas

The popular and unsightly rooftop TV antenna is a common example
of a Yagi antenna (named after Hidetsugu Yagi, a Japanese electrical
engineer who came up with this type of antenna) or beam antenna
(so nick-named because it concentrates the RF signal into a beam of
radiated energy).
A Yagi antenna enhances the normal 1/4 wavelength dipole
antenna by adding a reflecting element behind a dipole antenna and
several directing elements. This creates a concentrated beam pattern
of radiated signal in a single direction, with minimal signal
radiation to the rear and sides of the antenna’s designed directionality
The common home rooftop television antenna is intended as a
receiving antenna covering an extremely broad range of frequencies—
from 50 MHz on up to nearly 1 GHz in one physical framework—
which accounts for the various sized and positioned elements.
It is effective, but not the efficient design you would find for a specific
application, such as two-way or amateur radio or wireless networking.
As with omnidirectional antennas designed to provide signal gain
by forcing the radiation pattern into a narrower shape, Yagi or beam
antennas do the same thing plus add the advantage of concentrating
the signal radiation into a specific direction. The simple rule is more
gain—less pattern area but stronger signal in the direction of the
pattern.