Construction Hints

During construction of our microwave projects, we are sometimes confronted with problems that need a stroke of genius to overcome. Sometimes a better way of doing something springs to the fore and makes the whole project easier to build and look better.
This page documents some of the short cuts that have been developed or adopted by the microwave operators of Australia. I have canvassed a few operators for their ideas and here are some of their suggestions.

Creating a bond between PCB layers

When constructing my 24 GHz project I had to find an easy way to bond ground connections on one side of the PCB, to the ground plane on the other side.
This was accomplished by making a punch from a slither of "Stanley blade" mounted into a brass handle.

The blade is ground to the required thickness and size to make slots in the PCB where the connection is required.
The punch is used over the end grain of a piece of softwood to neatly punch through the Teflon PCB.
A thread of copper foil is then threaded through the punched slots to connect the two copper surfaces together. After the foil is threaded, the PCB is smoothed and the foil soldered.

Multiple Dish Feed Mounts

This method of Dish feed is used by Robby VK3EK, which he saw in an ARRL publication, and is probably the simplest method of mounting a can feed to a dish.
Robby has used pieces of plastic drainpipe to hold the can feed at the focal point of the dish.
The sides of the plastic pipe can be cut away in the required plane so as not to attenuate the RF from the feed, although tests on a piece of pipe in a microwave indicated that the material is friendly to microwave energy. Robby warns that if you are contemplating testing material in this manner, do so when your wife is not watching.
Plastic screw fitting have been mounted to the dish centre so that the feed assembly can be screwed into position. This feed system allows feeds for different frequencies to be easily screwed into place on a single dish making changing the operating band of the dish quick and minimizing realignment of the dish.

Making Trimcaps for tuned lines

One of the modifications that we have made to the G4DDK oscillator was to change the last stage filter from ¼ wave lines to ½ wave lines.
Neil VK2EI implemented this modification to improve the output of the oscillator from the original design.
The line is tuned in the centre with a small barrel style trimmer capacitor.
These are easily made from a 10mm long piece of hardline with the centre conductor removed. The centre is tapped to take a 10BA screw, which acts as the adjustable capacitor element.
The hardline is soldered into the centre of the tuned line and a nut on the 10 BA screw is soldered to the PCB earth mat. Then the screw is adjusted to achieve the required capacitance.

Shown to the left is the modified G4DDK oscillator 2,6 GHz doubler with the three-½ wave lines and the hard line capacitors mounted in the middle of the lines.
To the right is the modified circuit with a hardline capacitor.

Poor Man's Silver Plating

Plating is preferred to lacquer finishes on PCB's in microwave projects because is allows for easier application of snowflakes, ("snow flaking" is the term we use for adding small pieces of brass or copper shim to the RF lines for optimisation.)
Gold or Silver are preferred for microwave work while tin/lead plating can be used for general PCB work.

The method we use to silver plate our PCB’s has been published in many technical publications over the years.
The formula that I use has been changed from the original because I did not have the main ingredient on hand, but I did have some silver nitrate.

The original formula was 1 part silver chloride, 1 part common salt and 1 part cream of tartar.
I use 1 part silver nitrate 0.07%, 1 part common salt and 1 part cream of tartar.
The mixture makes slurry that is rubbed onto the PCB surface. The mixture transfers the silver to the copper surface, which gives good protection to the copper surface as well as adding a highly conductive surface to the PCB.
Using silver nitrate instead of silver chloride does not seem to alter the result although some discolouration of the silver surface can occur.

Even if it does tarnish, silver oxide is a better conductor than copper oxide.
The photo shows part of the DB6NT 5 GHz transverter PCB that has been plated with this process.