Campbell Scientific TX312 High Data Rate GOES Transmitter - Manual (Page 12)

TX312 Transmitter
3.4 RF Connectors
3.4.1 RF Transmission Connector
The TX312 utilizes the type N female connector for RF power out. This
connector must have a proper antenna connection before transmission occurs.
Failure to use a properly matched antenna cable and antenna may cause
permanent damage to the RF amplifiers. The nominal impedance is 50 ohms,
the frequency range is approximately 400 to 403 MHz. At 100 and 300 BPS
transmission rates, the nominal EIRP is 48 dBm with an 11 dBic gain antenna.
At 1200 BPS, the nominal EIRP is 52 dBm.
3.4.2 GPS Connector
The GPS connector is an input to the TX312. Operation without an antenna
connected will not cause damage, but the transmitter will not transmit without
a valid GPS fix. The GPS connector is an SMA female. The GPS receiver
uses an active 3.3 V antenna.
The TX312 transmitter uses the GPS receiver for two functions. The precise
GPS time is used to ensure scheduled transmissions occur at the proper time.
The one-second GPS synchronization pulse is used to ensure a precise, drift-
free carrier frequency. See Section 6.3 for more information regarding GPS
and GPS antenna placement.
3.5 Power Connector
The TX312 power connector has two pins: ground and 12 V. The input power
requirement is 10.8 to 16 VDC at 3 amps. Because the TX312 can use up to 3
amps, the power should be connected directly to the battery. An in-line 7 amp
fast blow fuse can be used to help protect the transmitter. The TX312 is
shipped with a power cable that includes the fuse and a connector arrangement
that allows the transmitter to pull power directly from the battery when using
the CH100, PS100, PS12LA or CH12R power supply.
With the potential for a 3000 mA current drain, the voltage drop along the
battery power leads must be considered. The battery power leads are both
wires that run from the battery to the power input connectors of the TX312.
To calculate the voltage drop along the power leads, we must know the
resistance of the wire and the length of the wire. Usually the resistance of the
wire is listed as ohms per 1000 feet. As an example, a 24 AWG wire used by
CSI has a resistance of 23 ohms per 1000 feet. The length of the wire is the
distance the wire travels from the battery to the transmitter multiplied by two.
You must consider the current travels from the battery, to the transmitter and
back to the battery.
The TX312 will operate with a battery voltage range from 10.8 V to 16 V. A
fully charged lead acid battery will have a voltage of about 12.5 V. If the
battery is fully charged, a 1.7 V drop along the battery leads will stop the
transmitter from transmitting. At 3 amps, 1.7 V will be dropped with 0.566
ohms of resistance. Using the 24 AWG wire with 23 ohms resistance per 1000
ft, 24 feet of wire (battery power leads 12 ft long) will prevent transmission. A
reliable system that will transmit without a perfect battery voltage will
minimize voltage drop along the battery power leads. To minimize voltage
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