DS8200-2 April ’07

©2007 RF Solutions Ltd, www.rfsolutions.co.uk

Page 17

Synthesised Radio Modem

X8200

X8200HP

RU67 type and kept as short as possible. As a rule of thumb the RF power is halved

every 10m of antenna feeder. It is better to keep the signal wire long and the antenna

feeder short.

Type of Antenna used.

Surrounding Topography.

Interference for other networks operating on the same frequency.

The Weather.

Antennas

The main types of antenna used in telemetry applications are as follows:

Helical

End Fed Dipole

Yagi

Helical Antenna

The helical stub antenna is robust, low cost and physically small. It has a gain less than unity.

Range of up to 2Km.

End Fed Dipole

The end fed dipole antenna has a unity gain. Its main application is to provide cost effective

omni-directional radiation.

Range 10 to 20 Km

Yagi

This antenna has a high gain typically twice (3dB) to ten times (10dB) the input power in the

direction of orientation. It is the type of aerial commonly used in domestic televisions.

The output power of a transmitter connected to a Yagi antenna has to be reduced to conform

to the DTI specification. This has the advantage of reducing the overall power consumed by

the transmitter without effecting the range in the direction of orientation. It also reduces

interference from other users.

The receiver signal is also amplified if a Yagi antenna is used thereby extended range in the

direction of orientation to around 20 Km line of sight.

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

selecting the Key Transmitter option at the transmitting station. A radio modem is then

attached to the yagi antenna feeder at the receiving station. By selecting the RSSI option in

the menu the yagi antenna can be rotated to a position of peak signal strength This will be in

the direction of the transmitter beacon.

Site Survey

First check the occupancy of all the radio channels at all the proposed sites. This can be

achieved by using an antenna, radio modem and PC. Select the RSSI option and observe

the bar graph. Note any activity. Go to the RF channel option and select the next RF