Knowing the basics of duplexer theory are one thing, actually building one is quite another. There are several viable methods of construction, with varying degrees of performance and pros/cons. There is no way I can describe them all, so if you’re interested in details of duplexer theory and construction, the best place to go is the Repeater Builder site. If you spend some time reading and re-reading the great information available there, you can develop a very good understanding of how duplexers work, how to build them, and how to tune them.
Building a duplexer requires a good bit of research, because you can’t just download a “recipe” and parts list, and start building. For one thing, there are several design variables: band, frequency split, TX power, location. Perhaps the biggest variable is the availability of raw material; some items are very hard to find, and very expensive when you do. I think that’s why most homebrew duplexer projects are adapted to fit the materials a person happens to have on hand. That’s why having a good understanding of the theory is essential.
The basic building block of a duplexer is a resonant cavity, which is essentially a 1/4 wavelength section of coax (very BIG coax), shorted at one end. It typically looks like a big can. Inside is a smaller tube, attached to the lid of the can. The lid has one or two connectors for coax. Usually, either 4 or 6 of these cavities are cabled together to provide the needed isolation between transmitter and receiver. One interesting thing about the 1/4 wavelength resonant cavity is that you can use it to either absorb the resonant frequency (a notch filter), or pass the resonant frequency (a band pass filter), or you can even combine both actions and create a filter that both notches and passes (band-pass band-reject, or BpBr). A complete duplexer can be built with one, two or all three of these filter types combined together. It all depends on the design goals of your system. In our case, a basic four cavity system using band-pass, band-reject filters (BpBr) should work well.
How large does a resonant cavity filter need to be? Resonant cavities need to be 1/4 wavelength long, so for the 6 meter band that is 1.5 meters, or about 5 feet long. As far as the diameter, the bigger the better – up to a point; 6 to 12 inches is a good range for band pass and BpBr cavities, while smaller coax (1-7/8″ Heliax) can be used effectively for band reject (notch) filters. The pipe also needs to be made of (or at least coated with) very low resistance metal, such as silver, copper, gold, or aluminum. So, right away you can see the challenge: the materials are heavy, expensive and not commonly available.
Metals prices are very high these days, so even if you CAN find suitable pipe, it will likely cost you BIG money. This cost of raw materials is certainly a big part of the reason why it is so expensive to buy commercially made cavities. Therefore, most homebrew duplexers are adapted to whatever material someone has available. A good example is the irrigation pipe duplexer that NZ5V describes here. Another good example is the Heliax notch duplexer described here.
In my case, I’m very fortunate that being a broadcast engineer gives me access to raw materials that would otherwise be very hard to come by. The first avenue I explored was building a notch duplexer using scraps of 1-7/8″ rigid copper coax, pieces that I saved when our analog TV transmitter was removed. This approach appealed to me at first because I thought it would be possible to build it using only a hacksaw and drill press, and very few additional parts (read: cheap!). However, a notch-only duplexer affords no protection from other nearby transmitters, and we have plenty of them at our site. Perhaps the tight front end of the GE Mastr II radio would be able to deal with that, but I decided to keep looking for another option.
The next possibility was band pass and/or BpBr cavities built from 6″ rigid copper feedline. I had some discussions with another broadcast engineer friend of mine, Clint N2FMM, who built a four-can BpBr duplexer for the W2IVB 6m repeater in Colden, NY (near Buffalo) from 6″ transmission line, and the results were excellent! I knew my employer had some leftover 6″ line, and I figured it couldn’t hurt to ask. To my surprise, I was given permission to purchase enough to build four cans!
A big plus here is that one end of the pipe has a brass flange that would make it easy to attach an end plate. The performance would likely be very good, based on the success of the W2IVB duplexer. Construction would be a little trickier, because of the size and weight of the pipe. I would need help cutting and machining various parts. It would also be necessary to purchase some expensive metal stock for the end plates and center pipe flanges. Doable, but I was still in search of a more elegant (read: cheap!) solution.
Finally, I turned my attention to the old sideband filter from the analog transmitter. WSTM operated analog on VHF channel 3, which is 60 to 66 MHz, just a little bit above the 6 meter band of 50 to 54 MHz (which, by the way, is where the original TV channel 1 used to be!) In one of my earlier posts, I described how WSTM rebuilt the analog transmission facility in 2002 while installing digital. A new sideband filter was part of that installation. After the analog shutoff in 2009, the sideband filter was rolled off into a storage room and largely forgotten.
I don’t know much about filter design, so I had no idea just what was inside. From the outside, it appeared that there were adjustable plungers similar to what are used in duplexers. The cabinet size was about right for resonant cavities, because TV channel 3 runs from 60 to 66 MHz, and the cabinet was 1.3 meters tall. Screw patterns suggested that the cabinets were divided into chambers, although not all the way from top to bottom. Obviously, there is only ONE way to find out what’s really inside: Tear it apart!
Stay tuned – in our next duplexer installment, we’ll find out what’s inside the boxes!