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6.9
RF Channel RFC
One of the 17 RF channels can be set by typing a letter from A to Q as shown in Table
7.1.1. This frequency will be stored in the EEPROM and set every time the modem is
switched on.
This function can be disabled by typing $ instead of a letter A to Q. The modem will then
use the DIL switches to set the RF frequency. The initial response will be faster when power
is applied or when the stand-by mode (DTR) is used. In addition the RF channel can be
selected on site without the need for a terminal.
6.10
Transmitter Key TXON
Typing Y to this option will switch on the Transmitter. It will then act as a radio beacon so
that any of the following installation and test procedures can be carried out using a RF
power meter, the RX LED, the RSSI output or a scanning receiver.
6.10.1 RF Power Output
The power output of the transmitter can be adjusted using the RF power potentiometer and
a RF power meter connected to the antenna socket.
6.10.2 ERP and SWR
The Standing Wave Ratio (SWR) of an antenna, and the antenna feeder loses can be
measured using a RF Power Meter.
A maximum transmitter power (ERP) of 500mW is allowed for the UK, MPT1329 licence
except band. Some of this power is lost in the coaxial cable between the radio modem and
the antenna. It is possible to compensate for feeder losses by using a high gain, directional,
Yagi antenna. Care must be taken not to transmit more than 500mW therefore the RF
power must be reduced to compensate for the gain of the antenna minus the losses of the
antenna feeder. The RF power potentiometer can be set so that the ERP does not exceed
the legal limit.
6.10.3 Antenna Alignment
If directional yagi antenna are used it is important to aligned them correctly. This can be
done using a compass and ordinance survey maps. The alignment can be checked by
attaching a radio modem to the yagi antenna feeder, rotating the antenna and measuring
the peak signal strength using the RSSI output with a voltmeter connected to it. This will be
in the direction of the transmitter beacon.
6.10.4 Site Survey
A site can be surveyed for the best reception conditions using a scanning receiver set to the
same RF frequency or observing the RX LED on the radio modem.
A radio modem set in the TXON mode is placed at the base station site. A second radio
modem powered from a 12V battery can be moved around the site. If the RX LED remains
lit there is sufficient signal for good data reception. The RX LED will go out if the signal
becomes too weak.