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South County ARES: 6 Meter Repeater and Duplexer Project

Most members are aware that SCARES has had a 6 meter repeater in operation for the last year or so.

We’ve had excellent results communicating over 6 meters from and to a variety of spots that are almost impossible for UHF or VHF comms. There are 6 meter capable radios installed at a variety of our supported installations including Belmont EOC, RWC, and the Coroner’s Office. Many members now have 6 meter capable radios.

We want 6 meters to become a viable part of our emergency comm plan. Until recently, our 6 meter repeater operated in split site mode: the receiver in the Belmont hills and the transmitter in a flat area not far from the Bay. Signals received on 6 meters at the receiver site were transmitted via a UHF link down to the transmitter site and then retransmitted out on the 6 meters output frequency. Using a split site is not the best way to utilize the repeater, however. The best solution is to have a single elevated site (height is might in comms), which requires that we acquire a 6 meter duplexer.

6m Duplexer with Goal meter
76 inches tall
6 ft 6 in

You can help us by donating towards this duplexer


South County ARES, Inc. is a 501(c)(3) non profit corporation. Donations are tax deductible.

What does a duplexer do? And, why do we need one?

A duplexer is really a filter or rather a set of filters used for repeater operation. Here’s the problem: if a transmitter is operating next to a receiver on the same band, there will be interference. So much so that the receiver won’t be able to hear any radio signals while the transmitter is operating. And yet, a repeater needs to have the receiver and transmitter be able to operate simultaneously.

One solution is to run the repeater in split site mode: a distance of about a mile is typically required to prevent the interference between transmit and receive operations. This is the way our 6 meter repeater originally worked.

Using a duplexer is a better solution. It allows both the receiver and transmitter to be at the same location, because it protects the receiver from the transmitter and allows them both to use the same antenna. Operating at the same site using a duplexer also removes the need for additional controllers and linking radios required for the receiver and transmitter to communicate over a distance.

One of the reasons we didn't originally set up a 6 meter single site is because a 6 meter duplexer is BIG!! A duplexer is made up of multiple adjacent cavities each of which has to be about a halfwave high, because it must be tuned for the correct frequency. So let’s see: 6 meters is 19.685039 feet, which is 6.56168 yards. Hmmm, this means a half-wave would be 9.84 feet (really tall!). In actual fact, 6 meter duplexers run a little shorter than that, say about 76 inches. That’s still plenty tall! Up until now there was no place where we could put a duplexer, a controller, radio and antenna required for the 6 meter repeater. Recently however, we’ve been exploring a site that would accomodate this equipment.

The second reason is the expense of a 6m duplexer. Even with a “ham” discount, a new 6m duplexer costs roughly $4400 plus packing and shipping which can run as much as $1200. Here comes the BUT ... Robert WB6WGM located a 6m duplexer that was for sale at the amazing price of $2000. AND, the seller was willing to tune the duplexer to our repeater frequencies! The Board approved the purchase: with the cost of packing/shipping the total was $2800. Even at such a great price, it is still a lot of money for a group that does not collect any dues from its members.
 

Several members have already contributed towards the purchase. These contributions total around $2100. We are asking for additional donations: we’d very much appreciate your help. You may use the button above or you may visit our donation page to make a contribution (choose the 6m Duplexer Fund section).

Also, find out if your employer would be willing to contribute. If so, please contact Gary Aden K6GDA at k6gda-at-arrl dot net with details and the person to contact.

We believe a six meter repeater single site will be a great help during emergencies because it will reach into many areas that have been the very difficult for UHF/VHF comms. A big Thank You to all who can help with the purchase of the duplexer to bring us a big step closer to a single site home for the 6 meter repeater.


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