The following article serves to build an understanding about the antenna parameter known as Electrical Downtilt. Tilting antennas is an important concept for applications like cellular and multipoint base stations that require targeting of their transmission to obtain greater control over each antennas coverage area.
There are two general types of tilting mechanisms, electrical, and mechanical. While at first glance the two seem similar there are significant differences. Conventional mechanical downtilting involves a rudimentary arm bracket to tilt the antenna body forward to focus the antenna boresight into the ground below. This process has the effect of localising coverage to the target area, but has the unintentional effect of uptilting rear and sidelobes. Coverage and field density may also deviate from the radio planning model.
To counter these unplanned side effects, electrical tilting is more commonly used as it provides a consistent horizontal radiation pattern with the added benefit of downtilting side and back lobes. A combination of mechanical uptilt and electrical downtilt is occasionally used in problematic areas to provide a conventional main beam but with downtilted side and back lobes.
Definition of Electrical Tilt
An antennas electrical downtilt is defined as the angular shift in elevation of the direction of maximum gain of the antenna by a specific electrical design of the antenna. Electrical downtilt can be fixed or variable.
When disclosing electrical downtilt on test reports and datasheets, R-Spectrum follow the BASTA reporting recommendations. BASTA define the reporting of Electrical Tilt per the following (adapted) specification.
- Electrical tilt type is fixed or variable.
- For a fixed electrical tilt antenna, the nominal value in degrees.
- For a variable electrical tilt antenna, the nominal range of values in degrees.
- For omnidirectional antennas, this parameter shall be referenced to the nominal angle between the horizontal cut and the beam peak axis.
- Tolerance in degrees.
Testing & Reporting
Calculation of Electrical Downtilt is through ordinary analysis of 3D radiation patterns, or through analysis of elevation polar patterns. The beam position of maximum radiation above or below 0 provides the Electrical Downtilt value.
Positive values indicate downtilt, i.e., beam position below the horizon, and negative values indicate up-tilt.
- NGMN, "Recommendations on Base Station Antenna Standards", NGMN Alliance, N-P-BASTA v11.1, Mar. 2019.