User's Manual

EMS Wireless Operator’s Manual

MirrorCell II CDM 812, CDM1912 Series

608112-1, Rev C

Reflector (Dish) Donor

Antenna

Sectorized Panel Server

Antenna

MirrorCell II Repeater,

Gain = 90 dB

Antenna Isolation + cable

loss = 85 dB

Oscillation occurs because

the repeater gain is more

than the antenna isolation

plus the cable attenuation.

Figure 4: Example of Repeater Oscillation

Good antenna characteristics, especially front-to-side and front-to-back ratio, help to

provide proper isolation between the server and donor antennas. Proper antenna

mounting location is also a key to achieving adequate antenna isolation.

The isolation between antennas must be at least 10 – 15 dB greater than the gain

setting of the repeater or the repeater could go into oscillation. Antenna isolation limits

the amount of repeater gain that can be used. Therefore, if the isolation is 90 dB, the

repeater gain should be set no higher than 80 dB to ensure proper operation.

Antenna isolation can be approximately estimated with a few calculations. Please refer

to the EMS Wireless application note “Antenna Isolation Calculations” for details.

Also, a small chart titled “Isolation Examples” shows four example installations using

specific antenna models, with the resulting calculated isolation for each.

The donor antenna selection is critical in the design of a properly functioning repeater

system because it provides the majority of the required front-to-side ratio. The donor

antenna is typically a high gain antenna, with excellent sidelobe and backlobe

suppression. Microwave dish (reflector) antennas with vertical and horizontal beam

widths of less than 15 are ideal candidates for the donor antenna. This is because the

repeater should only re-transmit signal from one base station, generally chosen