7.4 Antennas
The main types of antenna used in telemetry applications are as follows:
Helical
End Fed Dipole
Yagi
7.4.1 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.
7.4.2 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
7.4.3 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.
8.0 TROUBLE
SHOOTING
8.1
No Data Transmission
Check that TD and RD are connected to the 9 way D Connector correctly. This can be
checked by using a voltmeter:
a)
Connect the Host to the Modem.
b)
With no signal present, measure the voltage between:
0V (Pin 5) and TD (Pin 3)
0V (Pin 5) and RD (Pin 2)
c)
Both should be between -5V to -15V.
If only one is at a negative voltage then the RD and TD connections are reversed.
8.2
No Data Reception
If the RX LED on the receiver is not lighting at the same time as the TX LED on the
transmitter then check the RF frequency DIL switches on both modems to see if they are set
to the same RF frequency.
