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How MMRA Linking Works


Above is a closeup of the Billerica repeater. At the top is the 7330 repeater controller. Below that is a Motorola M120 transceiver. At the bottom is the Kenwood TK-750 repeater. The '750 consists of a receiver on 147.72 MHz and a transmitter on 147.12 MHz. The receiver sends three signals to the 7330 at the top: receiver audio, signal detect, and PL tone detect. When signal detect and PL tone detect are both active, the 7330 will key the 147.12 tranmitter and pass along the received audio to the transmitter. The 7330 also inserts its own audio to make voice announcements and identification.

So far, we have a repeater.

Now let's add in the linking. This is where the M120 kicks in. The M120 is a basic 2-way transceiver, much like the one that you may have at your home staion or in your car. The main difference is probably the fact that the M120 only has two channels. Channel 1 is configured to operation 449.925 - it receives on 449.925 and transmits on 444.925 - just like any radio that is set up to talk on HUB1.

The linking is enabled by sending a command to the 7330. When it is on, if the 7330 receives valid signal and tone from the '750 receiver, in addition to keying the '750s tranmitter it keys the M120 transmitter. The 7330 also sends the received audio to the M120 transmitter. The M120 is transmitting the 147.72 received audio into the 449.925 repeater. Anyone listening there will hear the signal from Billerica.

The M120 is half-duplex, so it will not hear itself coming back on 449.925 when the M120 is transmitting. However, when the M120 is not transmitting, it will hear anything else on 449.925. When this happens, it sends valid signals to the 7330 and passes along its own receive audio. When the 7330 hears this, it keys the 147.12 transmitter and forwards the audio from the M120. Now, anyone listening on 147.12 will hear the activity from HUB1. Full two-way linking is established!

All the other repeaters, for example Quincy, North Reading, etc. work the same way. Thus if someone is going in on the Quincy input. The Quincy's 7330 will retransmit them on 444.925. HUB1 will hear this and repeat it on 449.925. Billerica will hear 449.925 and retransmit on 147.12. So now anyone coming in on any repeater in the network will go out over all linked repeaters in the network.

In the diagram below, the path from the Quincy user who is talking to the Billerica user who is listening is highlighted in red.