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GB3DI-GB3OX Repeater Linking On Friday 29th September 2000, GB3DI and GB3OX were finally linked. The plans for linking these two repeaters have been around for some time, but the technical details for achieving the linking are not trivial. The two repeaters, both on 70cms are linked using in-band linking. That is, each repeater receives the other by listening to its output - while it is transmitting itself only 150kHz away. This is achieved by careful positioning of linking yagi aerials and deliberately 'deaf' linking receivers. The linking is active all the time, in effect giving a large coverage area repeater with two input and two output frequencies. The whole operation is transparent to the user, with the only obvious difference being that a station going into GB3OX will get an 'X' at the end of his over from both repeaters, and one going into GB3DI will get a 'D' from both repeaters. This useful feature does mean that users can tell which repeater other users are transmitting to - at the end of their over. When not 'up' - that is when not in use for a QSO, the repeaters operate individually, giving their respective beacons on a 10-minute cycle. When one is accessed, both then start to relay the over, and the beacon times are re-synchronised. During a QSO, when the 10 minute beacon time is up, the beacon itself is delayed until the end of that over, when both repeaters give their own callsigns on their own transmitters at high CW speed, (about 30 WPM), so as not to disrupt the QSO too much. After the beacon has been sent, the link is then re-established and the QSO can continue. The control for this is implemented by using another, extra CTCSS tone, on top of the one already in use for normal repeater operation. This extra tone is transmitted at a much lower level (about -24dB on the through audio). Associated with the linking receiver is a linking control board which recovers this low-Ievel CTCSS tone by a switched-capacitor high-pass filter, amplifier and CTCSS decoder. The detect output from the CTCSS decoder is used, together with squelch and local control inputs by a PIC to inform the main logic that linking should take place. When transmitting through audio, the CTCSS decoder is switched to transmit to produce the link CTCSS tone. The PIC also monitors the remote control channel (on the main repeater control board) and adds some extra DTMF controls to remotely enable and disable the linking system. An 'on-air' reset to the main control logic is also provided. The main repeater control system is the G8CUL logic, with a firmware update to interface to the linking control system. Operating tip: Always wait for the 'D' or 'X' before starting your transmission, or you will only be going out through one of the repeaters. |