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.
 

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