Basic Antenna Definitions and Applications Beamwidth - Defined by – 3dB 3dB power points on both vertical and horizontal planes. - Usually affects the physical ph ysical size of the antenna. Gain
- Defined as the power output relative to an isotropic antenna (Gain 0dB) or Dipole antenna (Gain 2.2dB). Front-to-Back Ratio - Defined as the amount of power in Front direction relative r elative to Back direction. - Usually approximately 20-25dB. 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 signal is desired (Coverage). - Downtilt is required to prevent interference to other coverage areas (Interference).
Downtilt For Coverage
Site A
Site B
Figure 1. Zero Downtilt between two neighbouring sites.
Design Cell Boundary using Bore Sight.
Site A
Site B
Site B Main Beam
Site A Main Beam
Figure 2. Downtilt when using RFU and Cell Boundary defined by Bore Sight.
Design Cell Boundary using -3dB Point.
Site A Site B -3dB Point
Site B Site A -3dB Point
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 d be the Total Downtilt required as per the Design. Let m be the Total Mechanical downtilt required. Let e be the Electrical Downtilt of the antenna. Let -3dB be the angle to the upper -3dB point. Let h be the antenna height. Let d be the distance to the Cell Boundary.
d
= Height d = Distance h
h
d
Figure 4 Calculation of Required Mechanical Downtilt. Design Cell Boundary should be located at angle d given by: d
-1
= tan (h/d )
For an initial Design the -3dB angle should be the Cell Boundary and hence the Total Mechanical Downtilt required is given by m
= d - e + -3dB
Downtilt for Interference Antenna Vertical Pattern
First Upper Lobe First Null
Main beam
Figure 5. Zero Downtilt between Two Neighbouring sites. First Upper Lobe Antenna Vertical Pattern First Null
Main beam
Figure 6. Implementing Maximum Downtilt for Interference.
Antenna Vertical Pattern
First Upper Lobe
First Null
Main beam
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.
Revised Boundary I
Original Design Boundary Revised Boundary II
Figure 8. Downtilts determination needs to consider natural and uneven terrain formations
