The value of using full duplex on the DARN 2 meter, 440, and 1200 Mhz Repeater system

Submitted By Marty Voll, N6VI ..... with additions by Dick, K6VGP.

To all ARES and DARN members;

You may have heard references to running “duplex” as a good operating practice on DARN, and especially on ARES nets conducted over the DARN repeater system. Here’s what it means, why it’s helpful and how to do it. How often have you heard someone ask for a radio check? If you get in the habit of using duplex you will simply know how your signal sounds, as well as knowing if you are doubling over someone else .... all  by listening to yourself coming out of another nearby radio at a low level, .... incredibly valuable and very easy, even in the car.

You’re all familiar with duplex operation already; it’s how land line telephones work. You can talk and hear at the same time, allowing interactive conversations in which no one is talking over the other (okay, there are exceptions with some people ;-) ). This is in contrast to the common radio technique of saying “Over” and then letting go of the PTT in order to hear the other party.

When everyone is either talking or listening but not both, “collisions” happen, and each person misses what the other party just said. Such collisions are particularly disruptive on a net, where many participants are waiting for their turn to check in, for announcements and bulletins, or for the net to finish so they can get on with other activities. You’ve all heard examples: one operator is slow to respond and starts talking just as Net Control has given up on getting a reply and resumes transmitting, or multiple operators all try to check in at once. As a result, repeats are needed, check-ins are missed, and progress can slow to a crawl. If everyone on the net – and especially Net Control – could hear what’s coming o ut over the repeater system while he or she is talking (i.e., operating duplex), most of these disruptive collisions could be avoided.

Going duplex is not difficult, and it’s especially easy on a system such as DARN that transmits on several bands simultaneously. If you’re using a dual-band radio with simultaneous receive, leave one side of the radio on, say, the 2-meter output while transmitting on the 70 cm input. Just keep the microphone away from the radio’s speaker and keep the volume low enough to avoid feedback. Better yet, use headphones to listen on the two-meter side. If your main rig isn’t a true dual-bander, use a handheld to monitor the repeater output (preferably but not necessarily on a different band) when you’re transmitting. A “rubber duck” on your operating desk won’t be in the main pattern of your rooftop antenna, so even in-band monitoring may work. Marty listens either on a separate 2m rig (with headphones, his usual configuration) or on DARN’s 1.2 GHz port. Dick listens to 147.36 in his car while transmitting on one of the 440 (70cm) ports.

When operating duplex, you’ll quickly learn how much to allow for system delay between engaging the PTT and starting to speak. You will always know if you have a good signal. and you’ll know if someone else is talking at the same time you are and can back off quickly if you’re not being heard solidly. If you’re an ARES Net Control, you’ll be able to recognize off-timed check-ins or requests more quickly. You’ll know when you’re being overridden by a system ID. Bottom line: the net session will run more smoothly and take less time to complete.

Please give it a try .... it is invaluable and makes communications SO much easier,

Marty and Dick