Antenna Installation and Downtilting
When we talk about antenna then we need to understand about antenna installation and specially about antenna downtilting.lets assume that antenna installed then how you can change its position.
Its two type,
- left-right = its called azimuth change.
- up-down = its called tilt change.
Lets understand from start.
- Antenna installation configurations depend on the operators preferences.
- It is important to keep sufficient decoupling distances between antennas.
- If TX and RX direction use separated antennas, it is advisable to keep a horizontal separation between the antennas in order to reduce the TX signal power at the RX input stages.
- Network planners often have the problem that the base station antenna provides an over coverage.
- If the overlapping area between two cells is too large, increased switching between the base stations (handover) occurs.
- There may even be interference of a neighboring cell with the same frequency.
- If hopping is used in the network, then limiting the overlap is required to reduce the overall hit rate.
- In general, the vertical pattern of an antenna radiates the main energy towards the horizon.
- Only that part of the energy which is radiated below the horizon can be used for the coverage of the sector.
- Downtilting the antenna limits the range by reducing the field strength in the horizon.
- Antenna downtilting is the downward tilt of the vertical pattern towards the ground by a fixed angle measured w.r.t the horizon.
- Downtilting of the antenna changes the position of the half-power beamwidth and the first null relative to the horizon.
- Normally the maximum gain is at 0• (parallel to the horizon) and never intersects the horizon.
- A small downtilt places the beams maximum at the cell edge
- With appropriate downtilt, the received signal strength within the cell improves due to the placement of the main lobe within the cell radius and falls off in regions approaching the cell boundary and towards the reuse cell.
- There are two methods of downtilting
- Mechanical downtilting
- Electrical downtilting.
- Mechanical downtilting consists of physically rotating an antenna downward about an axis from its vertical position.
- In a mechanical downtilt as the front lobe moves downward the back lobe moves upwards.
- This is one of the potential drawbacks as compared to the electrical downtilt because coverage behind the antenna can be negatively affected as the back lobe rises above the horizon.
- Additionally, mechanical downtilt does not change the gain of the antenna at +/- 90deg from antenna horizon.
- As the antenna is given downtilt, the footprint starts changing with a notch being formed in the fronts while it spreads on the sides.
- After 10 degrees downtilt the notch effect is quiet visible and the spread on the sides are high. This may lead to interference on the sides.
- Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards.
- This allows 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. That adjusted downtilt angle is constant over the whole azimuth range.
- Variable electrical downtilt antennas are very costly.