User's Manual

EMS Wireless Operator’s Manual
MirrorCell II CDM 812, CDM1912 Series
608112-1, Rev C
20
because it has adequate signal strength and enough unused capacity to accommodate
the repeater system.
In many urban environments, base stations are physically close together. The donor
antenna must have a narrow beamwidth so it receives and transmits signal to only the
desired BTS. Microwave dish antennas also provide the recommended front-to-back
ratio of more than 25 dB to help achieve the required antenna isolation.
Even better donor antenna front-to-side ratio is available with “high performance” or
“ultra high performance” reflector antennas. These models have a metal shroud
extending out from the face of the reflector. The inside of the shroud is covered with
microwave absorbing material to suppress side radiation. The best performance
models extend the shroud and use the highest quality absorbing material.
Unfortunately, this increases the wind loading of the antenna and requires a heavier
duty support structure. The trade-off is extremely high front-to-side ratio (frequently
40-55 dB), which allows close physical spacing between the donor and server antenna.
This close physical spacing (sometimes less than 40 feet) is excellent in areas with
stringent zoning requirements that limit the structure height.
The server antenna should provide the desired coverage and also provide the
recommended front-to-side ratio of 25 dB. Many different types of antennas will work
as a server, such as panel antennas, Yagi antennas, and, for indoor applications,
omni-directional antennas.
The installer should use these specific ways to achieve proper isolation:
 Use high gain, directional antennas with low sidelobes. This means the front-to-side
ratio should be better than 25dB. Also, use antennas with excellent antenna front to
back ratio.
 Point the donor and server antennas opposite each other; i.e., at 180 from each
other, and ensure that the antennas have minimal backlobe radiation.
 Physically separate the antennas, either vertically or horizontally.
 Use external shielding, such as wire mesh or a grounded metal plate placed
between the antennas. Reflector antenna manufacturers sell ice shields, normally
used to prevent ice from falling on the antennas during winter weather. They can
also be used to enhance isolation because they are metallic fence or mesh material.
The isolation can be frequently improved by about 10 to 20 dB by using an ice
shield.