Antenna Cables; Considerations When Mounting Antennas Adjacent - ASCOM T942C System Installation

System Installation
Ascom Paging System
When planning the location of an indoor antenna, consideration must be given to the
possible effect of antenna radiation on other electronic equipment.
If several antennas are to be mounted on the same mast the vertical spacing between
antennas should be at least 0.5 m.
If a guying kit is used the attachment point must be at 60% of mast height. When
attaching to a roof frame or wall, at least 25% of the mast height must be included
between the two attachment points.
5.5

Antenna Cables

Keep coax cables between transmitter/receiver and antenna as short as possible. Use only
cable type RG-8 or RG-213. The attenuation in these types is about 0.8 dB/10m at VHF,
and 1.5 dB/10m at UHF (6 dB attenuation reduces coverage by 50%).
5.6

Considerations when Mounting Antennas Adjacent

When the antennas are mounted adjacent in a two-way paging system, the fixed
transmitters could disturb or block the fixed receivers. The reason is that power from the
fixed transmitters are coupled to the fixed receiver antennas.
If the antennas are mounted closer than 85 meters from each other, measures must be
taken to avoid blocking. An example is that when the antennas are mounted vertically on
the same mast, an attenuation of about 20 dB is provided. The requirement for
attenuation is 57-77 dB, therefore the unwanted signal must be attenuated an additional
37 dB. Measurements at the receiver antenna cable connector reveal the actual receiver
interference power level.
Note: When antennas are mounted vertically on the same mast, the clearance between
them must be at least 0.5 meter. Antennas can never be mounted on the same height on
the same mast.
5.6.1 Example of Solutions
• The frequency difference is greater than 10 MHz
A single cavity bandpass filter for the alarm frequency or a customer specified duplexer
is recommended. The bandpass filter is placed between the alarm antenna and the
alarm revceiver. When a duplexer is used it is possible to use the same antenna for both
alarm and paging. Note that a duplexer does not work as intended if ground loops are
present. The result is that the antenna could not always be grounded.
• The frequency difference is between 2 MHz and 10 MHz
A double cavity bandpass filter on the alarm frequency or a notch filter on the paging
frequency is recommended. Both filter types are placed between the antenna and the
fixed transmitter/fixed receiver.
• The frequency difference is between 1 MHz and 2 MHz
A notch filter is recommended as an bandpass filter is not suitable. The filter is placed
between the antenna and the fixed transmitter/fixed receiver.
• The frequency difference is less than 1 MHz
When frequencies are this close, decrease the output power on the fixed transmitters
to 100 mW and use the optimum vertical antenna placement (on top of each other).
10 October 2013 / Ver. D
TD 90227GB
10