Antenna Installations Guideline (Version 1.0) The intent of this document is to provide some generic guidelines for antenna installations at a base station in a cellular system. In the following, we will address the isolation and the space diversity requirements for an antenna installations for AMPS-A and AMPS-B, ETACS and TACS, ETACS and GSM. Exceptions will apply when AMPS and ETACS or AMPS and GSM systems are co-locating at a same tower location. Isolation By isolation, we are talking about the signal attenuation from one transmitting antenna to the receiving end of another antenna installed at a base station. The isolation needs to be at lease 40 dB between a transmitting and a receiving antenna, and at least 20 dB between two transmit antennas. In order to achieve the desired isolation values, the antennas have to be placed at a certain minimum distance from each other. This distance is dependent upon the antenna type and the mounting configuration. In general, omni directional antennas require greater horizontal distance than the directional antennas, and the vertical separation provides much greater isolation compared to horizontal separation. Diversity Diversity that we talk about here is the space diversity at the receiving antenna. When the radio signals are received by two separate antennas, the signal which normally varies due to deep fading can be overcome by having two receive antennas with un-correlated signals. By the use of receive diversity, it assures that at least one of the two signals will be out of a deep fade. The diversity gain increases with distance, but in practice the separation distance is usually limited due to space constraints on a tower. Diversity gain is highest in the direction which is perpendicular to the line between the two receive antennas and is lowest when the two receive antennas are in-line with the radiating direction. Separation Spacing Requirement for Antennas The minimum isolation requirement for two antenna are: Tx - Tx Tx - Rx
20 dB 40 dB
λ, the wavelength to be used for the separation distance calculation is, λ (inches) = 11808 / Freq (Mhz) and λ (meters) = 300 / Freq (Mhz) 1.
Horizontal Antenna Spacing, S
Tx Antenna
Rx Antenna
s λ
Decoupling in dB ≈ 22 + 20 log − ( G T − G R )
1
s λ
Above equation is valid for > 10 and G T and G R < 10 dB 2.
Vertical or Collinear Antenna Separation,
S
s λ
Isolation in dB ≈ 28 + 40 log Typically 1λ < S and dBmax ≈ 70 dB “S” is spacing between ends of radiating portions of the antennas. 3.
Combined Horizontal and Vertical Separation SH θ SV
S (dB) = (V-H)
θ 90
+H
where, S is Slant Isolation V is Vertical Isolation H is Horizontal Isolation θ is angle between the antennas The preceding information was taken from Celwave Product Selection Guide 195. For additional information, please refer to pages 296 and 297. Diversity Receive Antenna Spacing For diversity reception, omni directional antennas must be spaced apart according to the following:
h ≤ 13 d where ‘h’ is the antenna height and ‘d’ is the separation distance. There is no need to go beyond 30 feet separation regardless of height, although greater separation is permissible. Also, the two receive omnidirectional antennas should be positioned in such a way that the main coverage area is in a direction roughly normal to the line connection the two antennas. The above information was taken from Autoplex document 401-200-300.
2
Other Separation Requirements AMPS-TACS/ETACS/GSM - Please refer to RF Engineering Guidelines for Operating an AMPS and TACS/ETACS System in the Same Service Area. Document # 54452-921222-01M ETACS-GSM - Please follow guidelines as supplied in this document.
