VK4ZXI

Problems of different metals in a cavity resonator: Galvanic series A problem building a cavity resonator is the potential of electrolytic corrosion between different metals, such as copper and aluminium. This can be overcome by keeping the cavities dry and using a sacrificial anode, such as zinc, already part of the design. The Galvanic series for metals in sea water is shown below. From the table mixing copper and aluminium, is potentially not a good idea. However...   A cavity usually resides in the same room as the repeaters that is usually dry, hence no electrolyte to enable galvanic corrosion. Putting the cavity outside is a different story.   Another sneaky trick is to use a sacrificial anode to protect the other metals. Zinc, in the form of galvanised steel can do this. An integral part of my cavity design is the use of galvanised threaded rod to hold everything in place. A secondary function is the galvanic protection of the device with the sacri...

Tuning a commercial UHF duplexer with RigExpert antenna analyser The club is assembling a 70 cm repeater with some equipment kindly loaned to us to use; a Kenwood TKR-820 repeater and a good quality commercial six cavity band-pass duplexer. The tuning was done with a RigExpert antenna analyser that gives a plot of SWR swept across a frequency range. The analyser was able to tune each cavity, but the overall response, while good for separation was compromised, possibly by incorrect inter-connecting cable lengths. Tuning the duplexer The duplexer is a band-pass type, with six cavities, three each for RX and TX with a "T" combiner. The cavities use N connectors and thick LMR400 type cable, and are about twenty years old. The probes are set for maximum coupling, which is desirable for selectivity but for some transmission loss. As a band-pass cavity filter, each cavity has a N connector in and out, making tuning each cavity quite easy; put a 50 Ohm termination on ...

A $300 2m repeater duplexer and cavity resonators-hardware shop special Introduction I have been experimenting with cavity resonators for a 2m repeater for some months now. I had been using aluminium tube and brass fittings. However, with 150mm tube, the optimal antenna should be about 50 mm diameter, with a 3:1 ratio of diameters for impedance and performance. However, the mechanical arrangements and conduction between them became problematic. If I wrapped all the joins with aluminium foil, it worked well, but is not practical. In this post I will outline a new technique that uses shim copper (0.1mm) for all the RF parts. The aluminium tube is lined with copper and soldered to copper shim top and bottom plates, in turn attached to the main aluminium plates. The antenna goes back to a conventional fixed lower section, with a movable section to adjust tuning. The basic premise of the design is still that it can be made from readily available parts at low cost, and with basic wo...

DIY aluminium washers for cavity resonator In many cavity resonators, the input and output coils are rotatable to adjust the degree of coupling and/or transmission losses. Most mount the coaxial connector for the coil on a large washer, so it can rotate, then use screws to hold it in position. Sounds so easy, but where do you get the washers?? After wasting a few hours in off-line shopping at bolt and plumbing shops, I found that I would have to make them. Working with thin metal sheet is usually not easy. However, I devised a simple technique to quickly make them. Any metal that can be cut by a hole saw or step drill could be used, aluminium, brass, copper, even thin steel, (per my post on using galvanised steel buckets) The photo shows the process. Using the hole saw, cut the first blank. Subsequent blanks are started with the sheet over the hole in the wood made by the first and drilling from underneath (hands away from hole), then cutting the blank from above. Use thr...

Homebrew cavity resonator/duplexer for 2m repeater There has been some interest in the club establishing another 2m repeater, but lacked a duplexer. A couple of old cavity resonators were discovered in the back shed. We were able to tune these very quickly and easily using the new Chinese KC901H network analyser (more about that in another post). Why and how they work: an antenna in a box! Up to that point I had heard of cavity resonators but had little idea of how they worked or how they were made. Similarly, so was my knowledge of repeater. However, as I started to learn about them, I became quite intrigued with the technical finesse of being able to transmit and receive with the same antenna simultaneously, albeit on different frequencies. Further, as desirable repeater sites are restricted in number, many repeaters share the same site, again on different frequencies and bands. The solution to the repeater problems is in using very selective band-pass and/o...