Low-cost duplexer for 6m, 2m or 70cm

 Low-cost duplexer for 6m, 2m or 70cm

I am proposing a conceptual design for a low-cost, home-made duplexer for 6m, 2m, or 70 cm. I have not built one to this design but it combines ideas from duplexers I have made or from commercial duplexers.

It is worth reading this post in conjunction with my other posts on cavity filters and duplexers; just search the terms. I have written a guide to how duplexers work and have a list of resources on the building and testing of them.

https://vk4zxi.blogspot.com/2018/05/the-black-art-of-duplexers-demystifying.html

General arrangement

The basic design of a single cavity filter is a large diameter aluminium pipe for the cavity and a 1/3 diameter aluminium pipe welded to a top plate. The coupling is mounted on the same plate. The other end of the large pipe is held to another plate by bolts for access. A capacitive hat on a threaded rod is mounted on the bottom plate to tune the cavity.

The main pipe is as large as possible to increase the Q of the filter, 150mm or bigger preferably for all bands. The resonator tube is 1/3 the diameter of the main pipe for 50 Ohm operation, as per coaxial cable. The main pipe is a quarter wavelength or longer for the desired band. The resonator is about 90 per cent of a quarter wavelength and closed at the end with welded aluminium sheet.

The capacitive hat is on a steel screw-threaded rod to increase the effective length of the resonator, hence why the resonator is shorter. The capacitive hat is a disk of aluminium sheet bolted to the tuning rod. The tuning rod needs a lock nut for stability.

The capacitive hat could be mounted on the side of the main pipe to allow a flat base for the filter to sit.

The general arrangement is shown in the picture.

Each side of a duplexer is made up of 2 or 3 cavities on separate or the same endplate.

I have prototyped the capacitive hat tuning of the resonator and it works fine, but not built a complete duplexer. It would be wise to build one cavity and check that the dimensions are workable before building all six. See some of my earliest attempts at duplexer construction.

 https://vk4zxi.blogspot.com/2015/02/a-300-2m-repeater-duplexer-and-cavity.html

I am not sure of the thermal stability of the cavity. I live in a sub-tropical area, south-east Queensland, Australia, where temperatures are not extreme. The tuning rod could be made of invar but it is not easy to buy. Possibly some invar rod could be salvaged from another cavity filter. The rod length for capacitive hat tuning is much less than that for variable resonator tuning. As the tuning rod is short, the main influence on temperature stability is the aluminium resonator and main tube. The linear thermal expansion of aluminium is 20 to 24, compared to 1.5 for invar, a big difference. I don't know how important this is and depends on the local climate.

Construction is fairly simple if there is access to a TIG welder for the aluminium. It may be possible to use aluminium brazing but the end result may not be as neat and a large gas torch is needed to heat the tubes. It may be worth tacking the first one and testing it so changes can be made as needed. It is probably worth welding all the way around to reduce resistance and to increase Q, but care is needed not to distort the pieces.

Couplings

The coupling can be any design, but for 6m or 2m needs to be a pass-reject type because of the close repeater separation. These are easy to make as a copy of a commercial design that I have discussed in other posts. https://vk4zxi.blogspot.com/2018/04/diy-2m-single-connector-pass-reject.html

https://vk4zxi.blogspot.com/2018/03/2m-duplexer-unusual-design-repair-tune.html

I would be very interested to hear of filters made to this design. Please leave a comment and a link thanks.