Product Manual

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20-62 VHF & UHF TRANSMITTER
Product Manual
Sea Air and Land Communications Ltd, 10 Vanadium Place, Addington, Christchurch 8024, New Zealand June 2020
Store & forward systems
Overview
In a store and forward system one transmitter is the “source” (originating transmitter) while other
transmitters are paired with a receiver at a distant location and are used to re-transmit the
received messages on the same channel to increase the coverage area.
Several issues can arise in a store and forward system:
1. The originating transmitter must wait for repeaters to retransmit the message to avoid
transmitting at the same time as the repeater. The repeater will not receive messages from
the originating transmitter when it is also transmitting on the same channel!
2. The message transmitted by a repeater may be received by the repeater’s own receiver,
then retransmitted, causing the message to be repeated in a continuous loop.
3. If there are multiple repeaters in a system, the first repeater might receive the message
that has been repeated by the second repeater, and send it again, bouncing messages
between the two repeaters in a continuous loop.
The 20-62 has several features the can assist with store and forward systems.
Adding a source delay
To solve the first problem the 20-62 has a feature to add a delay between messages if it is the
“source” transmitter. In this case the store and forward configuration should be set to “source”.
Rejecting incoming messages during transmission
To solve the second problem the 20-62 can be configured to ignore incoming messages during
a transmission. In this case the lead_out delay can be used to specify the time after the message
during which incoming messages will be ignored. The store and forward configuration should be
set to “repeater”. This method is used where an incoming message should only be rejected if it
occurs immediately after the transmission, while allowing intended duplicate transmission to be
repeated.
Duplicate reject
When there are multiple repeaters in a system a repeater must reject all identical received
messages coming from other repeaters. There may be a significant delay between transmission
and receiving a repeat message from another repeater. The solution to this (issue 3) is to reject
all duplicate message for a specified time. However this has the effect of also rejecting
intentional repeated messages. To prevent this, the delay between intentionally repeated
messages should be longer than the message reject time, while the message reject time should
still be long enough to reject messages repeated by another repeater.