11-12-2012, 01:42 PM
Basic Antenna Principles for Mobile Communications
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Introduction
In the last few years a large technological jump has taken place in the field of mobile communications
due to the introduction of new mobile communication networks (GSM/PCN). The number of subscribers
worldwide has risen to over 150 Million. Fig. 1 shows an overview of the mobile communications services
and the relevant frequency ranges within Germany alone.
The requirements on the antennas needed for the ever expanding networks are becoming continually
higher:
– strictly defined radiation patterns for a most accurate network planning.
– growing concern for the level of intermodulation due to the radiation of many HF-carriers via
one antenna.
– dual polarization
– electrical down-tilting of the vertical diagram.
– unobtrusive design.
The following essay will give an insight into antenna theory in general, as well as the most important
types of antennas and the special methods used for GSM/PCN systems.
Theory
Antennas transform wire propagated waves into space propagated waves. They receive electromagnetic
waves and pass them onto a receiver or they transmit electromagnetic waves which have been
produced by a transmitter. As a matter of principle all the features of passive antennas can be applied
for reception and transmission alike (reciprocality). From a connection point of view the antenna
appears to be a dual gate, although in reality it is a quad gate. The connection which is not made to a
RF-cable is connected to the environment, therefore one must always note, that the surroundings of the
antenna have a strong influence on the antennas electrical features (Fig. 2).
Propagation Pattern
In most cases the propagation characteristic of an antenna can be described via elevations through the
horizontal and vertical radiation diagrams. In mobile communications this is defined by the magnetic
field components (H-plane) and the electrical field components (E-plane). Very often a 3-dimensional
description is chosen to describe a complex antenna.
Half-Power-Beam-Width
This term defines the aperture of the antenna. The HPBW is defined by the points in the horizontal and
vertical diagram, which show where the radiated power has reached half the amplitude of the main
radiation direction. These points are also called 3 dB points.
Gain
In reality one does not achieve an increment in energy via antenna gain. An antenna without gain radiates
energy in every direction. An antenna with gain concentrates the energy in a defined angle segment
of 3-dimensional space. The l/2-dipole is used as a reference for defining gain. At higher frequencies
the gain is often defined with reference to the isotropic radiator. The isotropic radiator is an non-existant
ideal antenna, which has also an omnidirectional radiation characteristc in the E-plane and H-plane.
Calculation:
Gain (with reference to the isotropic radiator dBi) = Gain (with reference to l/2-Dipole dBd) + 2.15 dB
The gain of an antenna is linked to the radiation characteristic of the antenna. The gain can be roughly
calculated by checking the HPBW`s in the horizontal and vertical planes (Fig.6).
VSWR /Return Loss
An impedance of exactly 50 Ohm can only be practically achieved at one frequency. The VSWR defines
how far the impedance differs from 50 Ohm with a wide-band antenna. The power delivered from
the transmitter can no longer be radiated without loss because of this incorrect compensation. Part of
this power is reflected at the antenna and is returned to the transmitter (Fig.7). The forward and return
power forms a standing wave with corresponding voltage minima and maxima (Umin/Umax). This wave
ratio (Voltage Standing Wave Ratio) defines the level of compensation of the antenna and was previously
measured by interval sensor measurements.