ASCOM telePROTECT 900 User guide

TD 90227GB
2006-09-18/ Ver. C
System Installation
On Site Paging System

TD 90227GB
2006-09-18/ Ver. C
System Installation
On Site Paging System
Contents
1 Introduction............................................................................................................. 1
1.1 Abbreviations and Glossary ................................................................................ 1
1.2 System Bus ........................................................................................................ 1
1.3 Data Buses ......................................................................................................... 2
1.4 Speech Bus ........................................................................................................ 2
1.5 FL-bus ................................................................................................................ 2
1.6 Wire Type ........................................................................................................... 2
1.7 Wiring Lengths .................................................................................................. 2
1.8 Extended Wiring Lengths ................................................................................... 3
2 Telephone and PBX Connection ............................................................................. 3
3 Transmitters............................................................................................................. 3
3.1 Frequency Locked Transmitters ........................................................................... 5
3.2 Without Modem to Transmitters ........................................................................ 5
3.3 With Modem to Transmitters ............................................................................. 6
4 Receivers .................................................................................................................. 7
5 Antennas.................................................................................................................. 8
5.1 UHF Antennas .................................................................................................... 9
5.2 HF Antennas ...................................................................................................... 9
5.3 Tuning Circuits and Power Divider ...................................................................... 9
5.4 Antenna Location ............................................................................................ 10
5.5 Antenna Cables ............................................................................................... 11
6 Power Supply ........................................................................................................ 11
7 Current Consumption ........................................................................................... 11
8 Addressing............................................................................................................. 12
9 System Check......................................................................................................... 14
10 Combined Systems.............................................................................................. 15

TD 90227GB
2006-09-18/ Ver. C
System Installation
On Site Paging System
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1 Introduction
The units in the On Site Paging (OSP) system are designed for wall mounting, either
adjacent to each other or individually. The units are housed in two types of cases; die-cast
aluminium for fixed receivers and slave transmitters, and ABS plastic for all other units.
Both types provide protection against dust and mechanical damage but should not be
located in areas subjected to moisture or extreme humidity.
Operating temperature ranges are -25 to +55ºC for radio equipment and 0 to +40ºC for
other modules. If possible, all units should be placed in dry locations with moderate
temperatures to facilitate service. Avoid locations that are subject to strong
electromagnetic fields, vibration, or rapid temperature changes. The procedure for the
installation of each unit is described in the installation guide for the respective unit.
1.1
OSP On Site Paging
PBX Private Branch Exchange
FL Frequency Lock
SP Speech
RI Receiver Interface
Abbreviations and Glossary
1.2 System Bus
The units communicate via serial transmission on the system bus. Adjacent units are
connected with a ready-made modular system bus cable. Twisted-pair cables are used to
connect remote units, such as a Transmitter located near the antenna, a PBX interface
located at the switchboard, a number of charging interfaces, etc.
System bus cable
Twisted-pair cable
If the bus cable is used, no other system bus connections are needed. Twisted-pairs bus
connections are made at screw terminals on the PC-board. The bus network must never
form loops.
Data buses < 3 km
Speech bus < 500 m
• All data buses are polarized.
• Maximum 32 modules to be connected in a bus network.

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• The bus network must be open, never a loop.
• Use only twisted-pair wiring for extension of the system bus.
• Maximum length is 3 km for data buses and 500 metres for the speech bus. Distances
exceeding this require modems.
1.3 Data Buses
The A-bus is used for data communication between the modules and the D-bus for
communication between Central Unit and Terminal transmitters. Both the A- and D-buses
carry constant communication. This can be seen on an oscilloscope as a pulse train. The
normal level is 8-10 Vp-p measured from A1 to A2, and D1 to D2. If several units are
connected to the buses or if the lines are long, the level will be lower but must be at least
0.6 Vp-p. The data buses are polarized.
The four data buses (A, B, C, and D) use balanced lines and operate according to EIA
standard RS-485 and CCITT recommendations V.11 and X.27.
• A- and B-buses are identical and are used for connection of interface modules.
• C-bus is used for communication between different installations, i.e. between two or
more Central Units.
• D-bus transmits pagings in transmit format from the Central Unit to one or several
Terminal Transmitters.
1.4 Speech Bus
The SP-bus is a two-way line connecting all modules that handle the speech signal. The
SP-bus is transformer coupled. The signal level during speech is about 2 Vp-p measured
between SP1 and SP2.
1.5 FL-bus
The FL-bus is a one-way speech bus to terminal transmitters. The FL-bus is used in
frequency locked systems. In contrast to the speech bus, the FL-bus is driven by a low
impedance driver. The FL-bus is polarized.
1.6 Wire Type
Only twisted-pair cables are to be used. Suitable types are, for example EKKX 4 x 2 x 0.5 or
EKKX 10 x 2 x 0.5.
1.7 Wiring Lengths
Max total wiring lengths are:
A-bus 3000 m Data communication between modules
B-bus 3000 m Data communication between modules
C-bus 3000 m Data communication between systems
D-bus 3000 m Data communication from central to terminal transmitters
FL-bus 3000 m 1-way SP-bus to terminal transmitters
R-line 1000 m Speech and data between fixed receiver and receiver interface
SP-bus 500 m 2-way speech bus

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On Site Paging System
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1.8 Extended Wiring Lengths
If max wiring lengths of the data buses A, B, C and D exceed 3000 m, or if there are more
than 32 units on a bus, the data bus signal must be amplified. On private lines the Bus
Converter is used, and on leased lines (or very long private lines) the Data Modem is used.
If max wiring length of the SP-bus and R-line exceeds 500 m respectively 1000 m, the Bus
Amplifier is used for 1-way speech transmission on private lines and the Speech Modem is
used for 2-way transmission on leased lines.
If max wiring length of the FL-bus exceeds 3000 m the Bus Amplifier is used.
Line Equipment
• Bus Converter T938BC2, for A-, B-, C- and D-bus on private lines.
• Data Modem T938D, for A-, B-, C- and D-bus on leased lines and for very long private
lines.
• Bus Amplifier T938BA, for 1-way transmission on private lines.
• Speech Modem T938S, for 2-way speech transmission on leased lines.
2 Telephone and PBX Connection
A telephone set or a PBX can be used as control unit via a PBX Interface module.
The PBX Interface accepts both dialling pulses and DTMF tones and can be connected to
either trunk or extension lines. It communicates with other OSP modules via the A-bus and
the SP-bus if speech is desired.
PBX
Figure 1. Telephone set or PBX can be connected to the PBX Interface.
Note that additional PBX hardware and/or software may be required to use all functions of
the paging system.
3 Transmitters
There are two types of transmitters; Terminal and Slave transmitters. The Terminal
transmitter(s), is controlled by the Central Unit and receives pagings in transmit format on
the D-bus. Slave transmitters are used to increase coverage in installations where coaxial
cable from the driving transmitter can be run without too much difficulty.
The D-bus is electrically identical with the A-bus, i.e. maximum 32 units connected with
twisted-pair wiring with a maximum total length of 3 kilometres. If speech is used, the SP-
bus must be connected to the terminal transmitters. Note that the total length of the SP-

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bus without modem must not exceed 500 metres. If the SP-bus is 1-way, the length can be
increased to max 3 km with bus amplifier T938BA.
D-bus 3 km
Terminal
Transmitter
Slave
Driver
Slave
Transmitter
Central
Unit
SP-bus 500 m
Figure 2. Max D- and SP-bus length.
In HF installations a Power Divider is required between every transmitter for driving the
Slave Transmitters, and in UHF systems a Slave Driver is used between the Terminal
Transmitter and the first Slave Transmitter in series. The Slave transmitters have one output
and they can be connected in series or used as line amplifiers. However, in most cases it is
also advisable to connect an antenna to the line amplifier, especially in speech systems
where speech quality depends directly on coverage.
Slave transmitters are connected with coaxial cable RG-8 or RG-213 with a maximum
length of 700 metres. Distances exceeding this require line amplifiers. A separate power
supply feeds the slave, but if the slave is mounted close to other units the power can be
taken from one of them.
D-bus 3 km
Terminal
Transmitter
Slave
Driver
Slave
Transmitter
Central
Unit
SP-bus 500 m
700 m
Slave
Transmitter
700 m
Power
Divider 700 m Power
Divider 700 m
D-bus 3 km
Terminal
Transmitter Slave
Transmitter
Central
Unit
SP-bus 500 m
Slave
Transmitter
HF
UHF
Slave
Transmitter
700 m
Slave
Transmitter
700 m
Figure 3. Max coaxial cable length.
Equipment for Slave Transmitters
• Power Divider MPT-50, with one output used in HF installations.
• Slave Driver U952DR, with two outputs used in UHF installations.

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Slave Driver U952DR
The Slave Driver has one antenna output and two outputs for driving Slave Transmitters
from an UHF Terminal Transmitter, U952T. It also obtain a proper level at the Slave
Transmitter input. The Slave Driver operates in the 53 - 65 MHz band to reduce coaxial
cable losses.
3.1 Frequency Locked Transmitters
The Reference Module, that contains a highly stable reference tone generator, is placed
near the Central Unit. Each transmitter is fitted with the combined SM/FL (Speech/
Frequency Lock) T952SM/FL module. The SM/FL module uses the reference tone to correct
carrier frequency. A Slave Driver is needed if the Terminal transmitter is to drive Slave
transmitters.
Equipment for frequency locking
• Combined Speech-/ Frequency Lock Module T952SM/FL is to be plugged onto the
Terminal Transmitter’s circuit board.
• Reference Module T938RM with drive circuits for FL-bus, controlled by the Central
Unit.
• Bus Amplifier T938BA = T938RM without ref. tone generator, with drive circuits for the
FL-bus.
• Modem Compensator T938MC compensates for time delay in a T938S modem-pair.
Older transmitters that can be frequency locked are the H950T HF Terminal Transmitter,
with circuit board number 40291 or higher, and U951T UHF Terminal Transmitter. They
both use Frequency Locking Module T950FL.
Systems with frequency locked transmitters fall into two basic categories: those without
modems to the transmitters, and those that have at least one modem-pair. Three twisted-
pairs carry all communication between the central and terminal transmitter site.
3.2 Without Modem to Transmitters
The transmitters are connected to the D-bus and FL-bus. If the max length for the FL-bus is
exceeded, the transmitter’s speech bus must be connected to the FL-bus on the Reference
Module or Bus Amplifier. Several transmitters may be connected to the D-bus provided the

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3 km max total length is not exceeded. Each Reference Module and Bus Amplifier has two
FL-bus outputs. The D- and SP/FL-buses must be correctly polarized.
A-bus R-line
FL-bus
D-bus
Central
Unit
PBX
Interface
Reference
Modul
Receiver
Interface
to terminal site
R-line
FL-bus
D-bus
to central site
Fixed
Receiver
Terminal
Transmitter
Figure 4. System solution: 2-way speech, private lines without modem.
3.3 With Modem to Transmitters
Data and Speech modem introduces delay in the transmitter signal. Transmitter signals “in
the air” will differ and cause distortion in areas of overlapping coverage. A basic rule in
planning an installation is to delay the signals to transmitters equally.
The transmitter code on the D-bus is always generated by the Central Unit. Be sure that
the same number of modem-pairs are used between the Central Unit and each Terminal
transmitter. Several Terminal transmitters can be connected to one Data modem if the
location is suitable.
Synchronize the data modems for the D-bus, see the figure below:
Reference
Module
Control Synchronizing D-line
Delayed buses
direct to terminal site
A-bus
SP-bus
D-bus
System bus to
other units
SP-line
A-line
D-line
Lines via modem
at terminal site
R-line
D-bus
FL-bus
Central
Unit
Data
Modem
Data
Modem
Speech
Modem
Modem
Compensator
Data
Modem
Data
Modem
Receiver
Interface
Figure 5. Private lines with Speech Modem.
Speech signals may come from several different units, for example PBX Interface, Receiver
Interface or Control Keyboard. If speech modems are used with any transmitters, the
modem compensator must be used at those terminal sites where speech is fed into the

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system. The Modem Compensator delays speech that is fed in locally by an amount
equivalent to the delay produced in a speech modem-pair with up to 3 km cabling
between them. The output signal from the Modem Compensator is DFL, “Delayed
Frequency Lock”, and has the same electrical characteristics as the FL-bus. If necessary,
DFL can be amplified by the Bus Amplifier. DFL is connected to the speech bus in the
Terminal Transmitter. Several modem compensators can be connected in series to
compensate for several modem-pairs in series.
DFL
R-line
D-line
SP-line
A-line
A- ,B- ,SP-bus
Speech
Modem
Modem
Compensator
Fixed
Receiver
Terminal
Transmitter
Data
Modem
Receiver
Interface
Data
Modem
Figure 6. Terminal site with SP-bus.
4 Receivers
Talk-back Receivers are always connected to a RI (Receiver Interface), which in turn is
connected to the A- and SP-buses. Up to four receivers may be connected to the same RI,
each with a separate twisted-pair that must not exceed 1 km. Move the RI closer to the
receivers if the distance exceeds 1 km or use modems to extend the A-, B- and SP-buses of
the RI.
Note that the twisted-pair (R-line) between each receiver and its interface is not included
in the system modular bus and must therefore be connected separately, even when the
units are mounted together. When the Receiver is not activated, this line always carries a
quality tone of about 2.4 Vp-p measured between L1 and L2. When the receiver is

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activated, the signal on the line is a combination of speech, data, subtone, and quality
tone. The line is not polarized
D-, SP-bus
A-, SP-bus
1 km
1 km
Receiver
Interface
Fixed
Receiver
Terminal
Transmitter
Fixed
Receiver
.
Figure 7. Receivers connected to the Central Unit via the Receiver Interface.
5 Antennas
Several types of antennas are used in the OSP System, such as ground-plane, dipole and
helix antennas but the choice of type should be made to suit the actual site. In some cases
other types of antennas may be required due to the terrain or climatic conditions. Note
that dipoles must be ordered for the correct frequency.
In general, 900-system antennas can be grounded if necessary, but large potential
differences between different ground points can cause problems. Therefore, antenna
grounding should be avoided if possible.
HF
VHF
UHF
25% of
mast height
GP 4103
CXL-2
1,25 m
CXL-70
0,9 m
Wall brackets FM-103
Mast pipes Ø 50 mm x 3 m (2 ea)
0,5 m
0,5 m
2 m
Figure 8. Antenna reliability measures.

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5.1 UHF Antennas
UHF transmitters in the 900 system use a CXL-70, a broad band half wave coaxial antenna
for 406-470 MHz. The antenna is dc-short-circuited, and has an “N” type connector.
CXL-70 is mounted at the top of an antenna mast or with a supporting boom. The boom
should be at least 0.5 m.
CXL-70 is designed to be insulated from the mast but is delivered with a nut and washer
that are screwed onto the antenna connector if the antenna ground (=coax shield) is to be
dc-connected to the mast.
Note: The nut and washer are normally not used.
In systems with talk-back, UHF/VHF or UHF/UHF, a common base antenna with duplex
filters can simplify the antenna installation of receiver/transmitter pairs.
5.2 HF Antennas
HF installations can use the following types of antennas:
GP-25L is a 25-175 MHz ground-plane antenna for mounting on a mast. The antenna is
frequency adjusted by cutting the fibre-glass elements to the correct length according to
the diagram that is delivered with the antenna. The antenna has a UHF type connector.
GP 4103 is a 26-47 MHz, dc-grounded ground-plane antenna for mounting on a mast.
The antenna must be ordered for the correct frequency. The antenna has a UHF type
connector.
SAT-1 is a compact helix antenna about 1 metre long for the 27 MHz band that is often
used indoors because of its short length. The antenna has a UHF type connector. The
manufacturer recommends a few different coaxial cable lengths but the same low
standing wave ratio can be achieved by moving the antenna to different possible
mounting locations (standing-wave metre required).
Another alternative is a two-band antenna for HF/UHF plus a duplex filter.
5.3 Tuning Circuits and Power Divider
MFJ-901B
Terminal
Transmitter
Central
Unit
Figure 9. Antenna tuner in HF system.
MFJ-901B, the antenna tuning unit for HF is connected between the transmitter and
antenna.

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If an antenna cannot be installed in a normal manner or if the coaxial cable has an
excessively high standing-wave ratio, the antenna tuner can be used to match the
transmitter impedance to the antenna (for example boat installations, culverts, or cellars).
MPT-50
MAT-50MAT-50
OUT
IN
MPT-50
IN
80%
20%IN 20% IN
80%
20%IN 20%
Terminal
Transmitter
Slave
Transmitter
Central
Unit
Slave
Transmitter
Figure 10. Power divider in HF system.
Antenna transformer MAT-50 is used in HF installations at the output of the transmitters
to provide galvanic isolation from the antenna and protection from over-voltage and
transients caused by lightning.
Power divider MPT-50 is used in HF installations to divide output power from a transmitter;
20% to the next slave transmitter and 80% to the antenna, or to divide power to two
antennas. The two outputs are galvanically isolated from the input so that an MAT-50 is
not needed when MPT-50 is used.
5.4 Antenna Location
An outdoor antenna should be located as centrally and unobstructed as possible within
the designed coverage area. The antenna must not be placed near metal-clad chimneys or
similar structures.
Three alternatives are:
1 On the roof; provides good coverage around the building but usually provides
diminished coverage within the building.
2 On the ground floor; theoretically provides good coverage in the entire building but
may result in uneven coverage due to absorption by floors and other building
structures.
3 On the roof of a lower nearby building; provides good coverage of entire building if
the building is not too wide.
When planning an installation, a coverage test from the intended antenna site should
always be carried out.
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.

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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%.)
6 Power Supply
All units use 12.5 V DC and can be powered separately, but normally a common power
supply feeds all the units that are placed together in one location. The power supply is
connected to screw terminals on one of the units and then connected in series to the
other units. The wiring from the power supply must be dimensioned so that the voltage
drop to the modules does not exceed 0.5 V at max load. If the total loading exceed the
capacity of the connected power supply, an additional power supply should be used, see
below.
Power cables
Figure 11. Units powered by two power supplies.
7 Current Consumption
When calculating total current load always use the max consumption for each unit:
Control Keyboards max Load (A) Fixed Transmitters max load (A)
T900 0.7 H952T 2.0
T901 0.5 U952T 2.5
H901 1.3 Power Amplifiers
H954 15
U954 10

Line Equipment max Load (A) Talk-Back Equipment max load (A)
T938D 0.4 T981RI 0.3
T938S 0.15 U980 0.3
T938MC 0.1 V980 0.3
T938RM 0.1
T938BA 0.1
T938BC2 0.25
Central Equipment
and Interfaces
max Load (A) Charging Equipment max Load (A)
T942C 1.0 T961 (all versions) 0.15
T941AM8 0.3 T966 0.1
T941AM32 0.3 T967MC 0.75
T940PT 0.5 T967EC 0.75
T942SI 0.5 T962M 1.0
T940PX 0.5 T962E 1.0
T942PX 0.5 T963A/B 1.0
MBC-5 0.1
Storage Equipment Charging Interface
T966SR – T966CI 0.7*
T967M 0.35
T967E 0.35
T967MP –
T967EP –
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* T966 charging racks connected to charging interface T966CI are powered via the
12-conductor cable. Charging racks not connected to the interface must be powered
separately.
Linear power supplies are generally recommended, specially for the transmitters
(7410, 5A) and at locations with strong electromagnetic fields.
System interface module T840IU is powered by the T840 central unit in which it is placed.
8 Addressing
All units connected to the system bus must have an address to enable communication
with other modules. The address must be set with the address switch before power is
applied to the unit, as the address is read and stored at start-up. At start-up the function
indicator (LED) on the unit starts to blink at about 8 Hz and after max 90 seconds lights
continually. This indicates that communication with the central unit has been established.

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The address consists of two hexadecimal digits that are selected by the eight sections of
the address switch. Sections 5-8 of the switch select the first (most significant) digit and
sections 1-4 select the second digit.
1
Sections 1 - 4
second hex digit (D)
Hex address 5 D
OFF ON 1
88
4
5Sections 5 - 8
first hex digit (5)
Figure 12. Address switch.
The two hex digits provide a total of 256 possible addresses in the system. The figure
below shows how to set the hex digits 0-F.
OFF ON
Hex 0
OFF ON
Hex 8
Hex 1
Hex 9
Hex 2
Hex A
Hex 3
Hex B
Hex 4
Hex C
Hex 5
Hex D
Hex 6
Hex E
Hex 7
Hex F
1
OFF ON
Hex 0
Hex address 00
8
Hex 0
1
OFF ON
Hex 1
Hex address 01
8
Hex 0
1
OFF ON
Hex F
Hex address 1F
8
Hex 1
Examples:
Figure 13. Hexadecimal address settings.
The Central Unit always has address 00 and other modules can then be addressed in
ascending sequence. Each address must be unique within the installation regardless of
which bus it is connected to. Transmitters in non-speech systems must have hex 0 as the
first (most significant) digit unless sequential transmission is used.
Note: In speech systems the transmitters must be in the address range for speech
transmitters (hex address 80-FF).

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For several reasons an address register should be kept for each installation, with the name,
address, and location of each unit.
Address Register
0 0
0 1
0 2
0 3
0 4
T942C
T940PX
T840IU
T966CI
U952T
Attic, head office
Exchange room, head office
Exchange room, head office
Reception, head office
Beneath the roof, storeroom
Address (hex) Unit Location
Figure 14. Example of an address register.
When using sequential transmission, the sequence number is set by sections 5-8 of the
address switch. The central transmitter always transmits in sequence 0 (address 00) and
the transmitters that are meant to transmit simultaneously should also have hex 0 as the
first digit in the address. Next sequence uses hex 1 as the first address digit and so on.
Each sequence can have up to 16 transmitters. Sequential transmission is used to avoid
interference between the transmitters but cannot be used in speech systems.
Address Register
0 0
0 1
0 2
0 3
0 4
1 0
2 0
T942C
T942PX
T840IU
T966CI
U952T
U952T
U952T
Attic, head office
Exchange room, head office
Exchange room, head office
Reception, head office
Beneath the roof, storeroom
Cellar, head office
Culvert, head office
Address (hex) Unit Location
Figure 15. Example of an address register in an installation with sequential
transmission.
9 System Check
After the system installation, a functional check is made according to the installation guide
for each unit.
Initiate a paging and check that the pocket receiver responds correctly. Repeat this for
every unit that can generate pagings including pocket transceivers. In speech systems
make sure that the voice messages are clear. Check all other functions such as absent
indication, busy tone, mobile-to-mobile calls etc.
Note: The battery saving time (set in the PUP or PDM depending on handset) must be set
to a battery saving time of 0.5, 2 or 5 seconds. The mode 0 seconds is for handset test
purpose only and is not supported by the system. Keeping the mode 0 seconds during

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normal operation decreases the performance of the handset and also radically decreases
its battery operating time. Be sure that the battery saving time in the handset and the
Central Unit is identical.
Note: Messages and alarms sent from the system to the handset, must be transmitted
with at least one repetition in order to secure a reliable system.
If a malfunction occurs, check that the functional indicators (LEDs) on all units show a
steady light. If not proceed as follows:
• No light: No power, check power supply. Hardware fault, contact your dealer.
• Slow blink (0.5 Hz): Program error. Restart the unit by switching the power off and on.
• Fast blink (8 Hz): Communication error. Check bus polarity and addressing of the unit.
• One blink every fourth second: Unit in test mode. Contact your dealer.
Check the coverage area by transmitting pagings (with speech if applicable) and compare
with the expected coverage area. If coverage is not as expected check the installation and
location of the antenna. If this does not help, contact your dealer. In multi-transmitter
installations each transmitter must be checked separately. This should be done by an
authorized technician.
10 Combined Systems
In combined 800/900 systems interconnection module T840IU is installed in the T840
central unit and connected to the OSP system bus via twisted-pair wiring (A and possibly
SP-buses).
Charging racks may be used separately, or connected to a T966CI charging interface if
high current charging and/or absent indication is desired.
Charging
Interface
Charging Rack
T840IU
Terminal
Transmitter
Central
Unit
Figure 16. Combined systems.
• Rack and interface are connected via a ready-made 12-conductor cable.
• Maximum distance between rack and interface is 1.5 m.
• Each charging interface has connections for four charging racks.
• The charging interface is connected to the A-bus if absent indication is desired.

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An essential difference between the old 800 system and the OSP systems is that the 800
system must be grounded at only one point if the units are dc coupled. This must be
considered when planning a combined system installation.
T850 U952T840
Figure 17. An example of an installation that cannot function due to grounding.
Both transmitters are mounted on grounded masts. Ground currents cause interference in
the current loop between T840LM and T850.
There are two solutions for this:
• Install antenna transformer MAT-50 on transmitter T850.
• Install modem T838 between T840LM and T850.