User's Manual
HF NETWORKS
ENTRIES FOR AN HF NETWORK
134 ENVOY™ TRANSCEIVER REFERENCE MANUAL
Preamble
Preamble may be used in HF networks that use the Codan Selcall, Open Selcall, and
ALE/CALM call systems only. When you make a call to a specific station or group of
stations, the addresses of those stations are sent in a preamble signal before the call is
established. Transceivers that are scanning listen for the nominated dwell time on each
channel that is being scanned. If the transceiver detects a preamble signal, it pauses
scanning and determines if its address is in the preamble. If an address in the preamble
matches the self address of the listening station, and it has a matching call system, the
station stops the scan and opens the mute ready to establish the call with the other
station. If there is no matching address in the preamble, the listening station resumes
scanning.
The preamble that is sent must be transmitted for a period of time greater than or equal
to the channel dwell time multiplied by the number of channels being scanned in the
transceiver that is being called.
Preamble length
The length of preamble that is sent with a call is dependent on the number of channels
being scanned in the listening station, and the channel dwell time. If you select the
Auto value for the preamble length, the transceiver calculates this value for you.
Figure 42 shows how to calculate the minimum preamble to be sent so that listening
stations can identify if the call is intended for the station.
Figure 42: Preamble example
T
o
1
2
3
4
F
r
o
m
1
2
2
2
T
o
1
2
3
4
F
r
o
m
1
2
2
2
T
o
1
2
3
4
F
r
o
m
1
2
2
2
T
o
1
2
3
4
F
r
o
m
1
2
2
2
T
o
1
2
3
4
F
r
o
m
1
2
2
2
T
o
1
2
3
4
F
r
o
m
1
2
2
2
Preamble signal on a single frequency
Sending
transceiver
Address
1222
Listening
transceiver
(scanning)
● Global address 1234
● ALE HF network:
scanned
5 channels
default CDT
● Codan Selcall HF network:
scanned
3 channels
default CDT
Address
1234