DIY aluminium washers for cavity resonator

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.

1. 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.
2. Use three "TEK" self-treading screws to hold the blank in place, and use a step (Christmas-tree) drill to cut a hole for the coaxial connector, taking care to get the correct diameter for the first. Subsequent ones use the hole in the wood as a guide.
3. Remove finished washer and clean off burrs. For thin sheet, hand files are sufficient and don't damage the washer, unlike angle grinders or what ever is at hand!

The finished washer and rotatable connector is shown in place on the cavity resonator, along with a fixed connector and a spare washer. The two TEK screws holding the washer are loosened a little to rotate the coil inside. The second cylinder top shows the size of hole needed for the connector to rotate.

The step drill was used to make the holes for the other cavities in the duplexer, although with 6 mm plate, the holes need to be drilled from both sides.

The general form of the duplexer with just the first cavity working. I used just one cavity for experiments; more in another post.