Basic Antenna Definitions and Aplications

Beamwidth

            Defined by –3dB power points on both vertical and horizontal planes.

            Usually affects the physical size of the antenna.

Front-to-Back Ratio

             Defined as the amount of power in Front direction relative to Back direction.

 Usually approximately 20-25dB.

Gain

              Defined as the power output relative to an isotropic antenna(Gain 0dB) or Dipole     

              antenna (Gain 2.2dB)

Polarization

 Electromagnetic wave consists of both an E Field and H Field. 

 Polarisation usually refers to the direction of the Electric field relative to the intended direction of use for the antenna.

Downtilt       

            Downtilt is required to focus max.power where signal is desired (Coverage).

Downtilt is required to prevent interference to other coverage areas (Interference).

Down tilt For Coverage

           Figure 1.  Zero Downtilt between two neighbouring sites.

                        Figure 2.  Downtilt when using RFU and Cell Boundary defined by Bore Sight.

 

             

              Figure 3.  Downtilt when using SRFU and Cell Boundary defined by -3dB point.

Determination of Mechanical Downtilt

The following guideline can be used in determining the initial downtilt to be applied for a site in the Design phase.  Calculation of the angle to the Cell Boundary should be based on basic trigonometry theory.

Let q_d be the Total Downtilt required as per the Design.

Let q_m be the Total Mechanical downtilt required.

Let q_e be the Electrical Downtilt of the antenna.

Let q_-3dB be the angle to the upper -3dB point.

Let h be the antenna height.

Let d be the distance to the Cell Boundary.

              

Figure 4  Calculation of Required Mechanical Downtilt.

Design Cell Boundary should be located at angle q_d given by:

                                                q_d = tan^-1(h/d)

For an initial Design the -3dB angle should be the Cell Boundary and hence the Total Mechanical Downtilt required is given by

                                                q_m = q_d - q_e + q_-3dB

Downtilt for Interference

Figure 5.  Zero Downtilt between Two Neighbouring sites.

                               

Figure 6.  Implementing Maximum Downtilt for Interference.

 

Figure 7.  Over Implementing Downtilt.

Further Considerations

The general guidelines above consider only the ideal situation with flat terrain and no obstructions.  It is the responsibility of the RF Design Engineer to consider all other external factors prior to selecting an optimal design downtilt.

      

Figure 8. Downtilts determination needs to consider natural and uneven terrain formations