•An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.
In mobile communications two main categories of antennas are used:
Omni directional antenna
These antennas are mostly used in rural areas.
In all horizontal direction these antennas radiate with equal power.
In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.
Directional antenna
These antennas are mostly used in mobile cellular systems to get higher gain compared to omnidirectional antenna and to minimise interference effects in the network.
In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.
In these type of antennas, the radiation is directed at a specific angle instead of uniformly across all azimuth angles in case of omni antennas.
Antenna Characteristic:
radiation pattern
-The main characteristics of antenna is the radiation pattern.
-The pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes.
-A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.
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antenna gain
-Antenna gain is a measure for antennas efficiency.
-Gain is the ratio of the maximum radiation in a given direction to that of a reference antenna for equal input power.
-Generally the reference antenna is a isotropic antenna.
-Gain is measured generally in “decibels above isotropic(dBi)” or “decibels above a dipole(dBd).
-An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. dBi = dBd + 2.15
-Antenna gain depends on the mechanical size, the effective aperature area, the frequency band and the antenna configuration.
-Antennas for GSM1800 can achieve some 5 to 6 dB more gain than antennas for GSM900 while maintaining the same mechanical size
front to back ratio
-It is the ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction.
-Generally antenna with a high front-to-back ratio should be used.
first null beam width
The first null beamwidth (FNBW) is the angular span between the first pattern nulls adjacent to the main lobe.
This term describes the angular coverage of the downtilted cells.
Antenna Lobes
Main lobe is the radiation lobe containing the direction of maximum radiation.
Side lobes
Half-power beamwidth
The half power beamwidth (HPBW) is the angle between the points on the main lobe that are 3dB lower in gain compared to the maximum.
Narrow angles mean good focusing of radiated power.
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Polarisation
Polarisation is the propagation of the electric field vector .
Antennas used in cellular communications are usually vertically polarised or cross polarised.
Frequency bandwidth
It is the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard.
VSWR of an antenna is the main bandwidth limiting factor.
Antenna impedance
Maximum power coupling into the antennas can be achieved when the antenna impedance matches the cables impedance.
Typical value is 50 ohms.
Mechanical size
Mechanical size is related to achievable antenna gain.
Large antennas provide higher gains but also need care in deployement and apply high torque to the antenna mast.
To solve over lapping in antennas coverage area we use antenna downtilt and antenna mechanical tilt:
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Mechanical Down Tilt
-With mechanical down tilting, the main beam will be down tilted but the back lobe will be up tilted that make interference. -With mechanical tilting you can’t change the tilt of one band alone.
This is one of the potential drawback as compared to the electrical downtilt because coverage behind the antenna can be negatively affected as the back lobe rises above the horizon.
Electric down tilt
-Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards.
-This allows the the antenna to be mounted vertically.
-Electrical downtilt is the only practical way to achieve pattern downtilting with omnidirectional antennas.
-Electrical downtilt affects both front and back lobes.
-If the front lobe is downtilted the back lobe is also downtilted by equal amount.
-Electrical downtilting also reduces the gain equally at all angles on the horizon. The that adjusted downtilt angle is constant over the whole azimuth range.
Smart Bias Tee SBT:
The Smart Bias Tee is used to inject DC voltage and to modulate and demodulate remote control signals, via the RF coaxial cable, to antenna line devices such as TMA and RCU. It is designed to comply with AISG 2.0 standard and can be used as a lightning protection device for the BTS/Node B.
Remote Control Unit(RCU)
RCU-003(II-2) is a compact and light weight actuator that mechanically connects to the electrical phase shifter of Comba VET antenna for remote adjustment of electrical down-tilt angle. It is designed to comply with AISG 2.0 standards and works with AISG compliant Node B and antenna line devices.
TYPES OF ANTENNA
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