Winter 2007 Issue

MICROWAVE

Tricks of the Trade Construction Methods

By Chuck Houghton,* WB6IGP

The bottom of my 1296-MHz rig showing components attached by the RTV sealant method. The wiring of devices is done by tying one end only, making the other end ready to run to the other leads. When the back lead is soldered, the wiring routing will have a neat appearance on the bottom of the chassis. The case is from a Qualcomm computer assembly. I took out the computer control devices and installed the 1296-MHz amps, mixers, and power-supply components. I was able to obtain five cases and use them for 1296, 2304, 3456, and 5760 MHz, and 10 GHz.
 

There are many tricks that can be used in the construction of microwave equipment, and quite a few of them are related to support equipment. We all would like to have milled out blocks of aluminum to use for our devices. However, the reality is that it is better to use simple, inexpensive, readily available containers in which to construct our power supplies and then assemble related switching equipment on a simple substrate. For heavy component parts, mounting the parts and equipment on an aluminum plate is most desirable. The aluminum might not be available at a reasonable price, though.

A suitable alternative that is reasonable in price is a new, blank, copper printed circuit board, which can serve almost as well as an aluminum plate. The copper board can be used for mounting small circircuitry assembled in the old, traditional “dead bug style.” With this method the component parts are attached by their ground leads for connection to the copper PC board top section ground, usually the top foil. This ground foil is used to connect the devices to common ground, and it works well. It is also easy to attach additional shields of copper material for high-gain circuits to prevent feedback paths. It’s easy to experiment to see just where feedback/oscillations on the PC board are taking place, and then add bits of copper foil pieces in the right spots to stop the feedback paths. Additional cuts in the ground foil can be made with a section of metal or a scrap section of PC board material used as a straight edge. Making these cuts in the ground foil material produces a variety of isolated islands to suit circuit connections. These isolated islands now add signal paths so other portions of components can use connectivity similar to the grounded foil by soldering these other portions of components to the now isolated copper foil.

When assembling small power supplies and types of circuits with few parts, this method works well. The same “dead bug” method of construction can be used when mounting larger components, but with a little twist.

When mounting somewhat larger components, such as DC-to-DC switching power supply devices, stack two or three of the small DC switching power supplies to the copper plate. Here is where the mounting of these power supplies and other component devices that have a flat surface can be easy. Most of these devices have connection leads on one side and a flat surface on the other side, making attachment with nuts and bolts one solution for things such as microwave mixers, amplifiers, and coax relays of the SMA type.

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