It has already been mentioned that ideal omnidirectional Antennas cannot be produced
in reality. Nonetheless only antennas that focus their radiated power in a particular
spatial direction can properly be called directional antennas. At an equivalent transmit
power, they significantly improve the signal-to-noise ratio, but must be aligned on the
distant station so that in many cases a rotation facility has to be used. For directional
antennas the parameters gain, directivity and all values associated with the radiation
pattern, like front-to-back ratio, side-lobe suppression or half-power beamwidth as
already discussed give an overview about how much the radiated energy is
focused into a certain direction.
The simplest form of a directional antenna is a setup of two monopole antennas at a
predefined distance, which are fed with different phase
| Principle of a directional antenna |
In the example a distance of a quarter wavelength and a phase difference of 90° have
been chosen, resulting in a cardioid shaped radiation pattern when the
far field strengths generated by the two individual antennas are added.
| Cardiold shape radiation pattern |
Even though this configuration does not produce strongly focused radiation, it however
exhibits a sharply defined null towards the backside which can effectively be used to
suppress interfering signals.
By superimposing the diagrams obtained by combining two or several radiators
arranged at defined distances and with defined phase shifts, directional patterns can
be generated whose directivity is limited mainly by the available space to setup the
number of required radiators.
Instead of feeding the radiators via cables , the principle of
radiation coupling is mostly applied in practice, with only one radiator being fed from
the cable and the remaining elements activated by this radiator. Yagi-Uda antennas,
which are commonly used for the reception of TV and VHF sound broadcast signals,
have typically between 4 and 30 elements and yield gain values of 10 dB and more.
The possibility of changing the direction of the main beam of a highly directive antenna
by purely electronic means is utilized to an increasing extent also with antenna arrays
for very high frequencies (e.g. for satellite radio services). The antennas used are
referred to as planar antennas and mostly consist of a dipole curtain which, in
contrast to curtain antennas, is installed in front of a conductive plane. This array can
also be implemented by etching the radiators as tracks into a printed circuit board
(microstrip antenna). In this way, even large arrays of antennas can be implemented
for the microwave frequency range with high precision and efficiency.
=* by Rohde & Schwarz*=
