6
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
L A B E L I T A L Y S R L
- V i a S . A l l e n d e , 5 9 – 4 1 1 2 2 M O D E N A
T E L : 0 5 9 / 3 6 2 9 9 3 F A X : 0 5 9 / 3 7 6 0 5 6
P . I V A 0 2 5 7 8 7 5 0 3 6 2 C a p i t a l e S o c i a l e 3 2 . 5 0 0 E u r o
w e b - w w w . l a b e l i t a l y . c o m e - m a i l - i n f o @ l a b e l i t a l y . c o m
Polar diagram for a yagi antenna
RF ANTENNA BEAM WIDTH
There are a number of key features that can be seen from this polar diagram. The first is that there
is a main beam or lobe and a number of minor lobes. It is often useful to define the beam-width of
an RF antenna. This is taken to be angle between the two points where the power falls to half its
maximum level, and as a result it is sometimes called the half power beam-width.
ANTENNA GAIN
An RF antenna radiates a given amount of power. This is the power dissipated in the radiation
resistance of the RF antenna. An isotropic radiator will distribute this power equally in all directions.
For an antenna with a directional pattern, less power will be radiated in some directions and more
in others. The fact that more power is radiated in given directions implies that it can be considered
to have a gain. The gain can be defined as a ratio of the signal transmitted in the "maximum"
direction to that of a standard or reference antenna. This may sometimes be called the "forward
gain". The figure that is obtained is then normally expressed in decibels (dB). In theory the
standard antenna could be almost anything but two types are generally used. The most common
type is a simple dipole as it is easily available and it is the basis of many other types of antenna. In
this case the gain is often expressed as dBd i.e. gain expressed in decibels over a dipole. However
a dipole does not radiated equally in all directions in all planes and so an isotropic source is
sometimes used. In this case the gain may be specified in dBi i.e. gain in decibels over an isotropic
source. The main drawback with using an isotropic source (antenna dBi) as a reference is that it is
not possible to realize them in practice and so that figures using it can only be theoretical. However
it is possible to relate the two gains as a dipole has a gain of 2.1 dB over an isotropic source i.e.
2.1 dBi. In other words, figures expressed as gain over an isotropic source will be 2.1 dB higher
than those relative to a dipole. When choosing an antenna and looking at the gain specifications,
be sure to check whether the gain is relative to a dipole or an isotropic source, i.e. the antenna dBi
figure of the antenna dBd figure. Apart from the forward gain of an antenna another parameter
which is important is the front to back ratio. This is expressed in decibels and as the name implies
it is the ratio of the maximum signal in the forward direction to the signal in the opposite direction.
This figure is normally expressed in decibels. It is found that the design of an antenna can be
adjusted to give either maximum forward gain of the optimum front to back ratio as the two do not
normally coincide exactly. For most VHF and UHF operation the design is normally optimized for
the optimum forward gain as this gives the maximum radiated signal in the required direction.
All our antennas have gain in “dBd”
RF ANTENNA GAIN / BEAM WIDTH BALANCE
It may appear that maximizing the gain of an antenna will optimize its performance in a system.
This may not always be the case. By the very nature of gain and beam width, increasing the gain
will result in a reduction in the beam width. This will make setting the direction of the antenna more
critical. This may be quite acceptable in many applications but not in others. This balance should
be considered when designing and setting up a radio link.
