How Antenna Downtilting selection for cdma ?

Downtilting is the method of effectively adjusting the vertical radiation pattern of the antenna to increase the amount of power radiated downwards. Downtilting can be used to increase the amount of coverage close to the site where “nulls” (holes) may exist due to the effective height of the antenna.

Downtilting can also be used to reduce “pilot pollution” caused by reflections or undesired RF propagation beyond a pre-determined footprint. There are principally two types of antenna downtilting possible, mechanical and electronic.

Mechanical downtilting can be achieved through the mechanical adjustment of an antenna’s physical position. The main advantage of the mechanical type of downtilting is the ease (dependent upon location) of mechanically adjusting the antenna’s direction following system optimization.

Note that any cellular or PCS network will require some degree of system optimization based upon site specific variables. The adjustment of antenna downtilt has historically been one of the principle methods of tuning system performance, therefore the system engineer should consider if the chosen antenna can be downtilted and if so, by how much?

The second method of downtilting that can used for Cellular/PCS applications is Electronic downtilt. This is the only way to implement downtilt for an Omni directional antenna. The level of electronic downtilt for an antenna is normally pre-set and ordered directly from the antenna manufacturer.

The system engineer should be aware that as electrical antenna downtilt is pre-set,
the field adjustment of downtilt and therefore vertical radiation can not normally be reduced.

The system engineer should also remember that the amount of gain in the antenna will also have a direct affect both on the physical size of the antenna and the vertical beamwidth. If a low gain antenna is utilized, the vertical beamwidth will be relatively broad and therefore the benefits of down tilting will be minimal.

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