On Mechanical Design of Radio Equipments

Jouni Verronen, oh8ro 2005


In making homebrew equipments the fun for me is in experimenting without any demands as to components, specs or deadlines.
However one hopes to have something completed, not to collect unfinished projects all over the place.
Then as time passes new better components emerge and one has new ideas how this or that part should have been done.
Time is not unlimited, either. To get along I have tried to put in more flexibility initially by proper mechanical design.

What I have found useful is a kind of unit approach resembling the way large systems, say radio base stations and measurement systems mostly are made. It is, however, simplified in that no motherboards with elaborate connectors or slide rails are used. And the units are not too complex to match the idea of flexibility.

There is more metal work than is usual in amateur designs, and also shielding is done properly from the beginning. What you gain is that you can swap any unit later with a better one or even take the whole thing apart and build it again to a larger enclosure in a more elaborate configuration. Also service and small circuit level changes are faster afterwards as you can easily take out any unit, rework and test it individually. And completing a unit is much easier than the whole underestimated thing in the first place.

An example: HF Receiver

This is an SSB/CW RX of classic architecture: a preselect filter with high quality rotary switches, a high level DBM , 9 MHz IF.. Pictures below show the basic idea in mechanics. The whole RX is divided into separate units which make sense electrically/mechanically, as LO, IF, AF, detector, BFO.. Both front and rear panels of the radio are formed of the front plates of the units, each 2 mm thick, width a multiple of 30 mm, height 130 mm. Depending on requirements a unit may be a closed metall box (say BFO, LO) or not so extensively shielded (AF-PA, power reg.).

I can replace any unit in the future if wanted without the need to touch other units much. If I wanted a better LO, I could just make a new LO-unit with 90 mm wide front plate. It is also possible to swap units of different RXs, or modify obsolete units for other purposes. Also one can start with a simple system, get it working, and enhance it to more complex and advanced later, if time and interest exist. Oldest units in this RX date back to early eighties. The less shiny, anodized front plates in the picture are newer units. As shown in the rear picture, there are also some TX circuitry included.

RX front
HF RX front

RX rear
HF RX rear

RX above
HF RX from above

Another RX

FM-RX

A simple repeater/packet monitor RX for 70 cm. The basic FM RX fits well in a 60 mm broad unit.

Equipment enclosures

Over the years I have used this loosely standardized mechanical system for many homebrew equipments, both simple and complicated.
An amateur uses what he has readily available for components to get something done. I have used cabinets of old radio equipments as Drake 4-series in the HF RX above, cases of 19 inch wide measurement equipments, an old AT PC-case, or made enclosures of sheet aluminum. Horizontal 10x10 mm aluminum rods in front and rear are connected to the main structure and the units are then attached to rods using M3 screws. Before that there is the fun of making all the holes with threads, 30 mm apart, along the rods .

One can also start building and tuning a system by using some simple open rack structure in the beginning to fasten the few units to and remove the units to better mechanics later.

Units

A typical slim unit is made of 30x30x2 mm L-profile of aluminum as a key part. An aluminum plate, 1.5 or 2.0 mm thick, is fastened to it by a couple of screws. 2-sided pc-board can be easily fastened on the plate with stand-offs.
Pieces sawed from Al-profiles of rectangular shape can be used for shielding.

For the circuitry I have mostly used the well-known dead bug construction over a 0.8 mm thick FR4 copper-clad fiberglass, upper solid Cu-foil providing the ground and the board pressed between the Al-plate and the shielding profile walls.
The dimensions used allow up to four connectors or pots or switches on the outer side vertically. For interconnections I have used SMA, SMB, MCX, RCA, row-connectors or whatever depending on requirements and availability at the moment.

These are some examples:
a wide band mixer (SBL-1 + ERA5), a temporary RX front/splitter using an old piece of a circuit board and an RF input selector.

tx-mixerpcbmixerRF-selector
.

In wider units an N x 30 mm wide 2 mm thick aluminum plate is used for the front side, and construction can vary from case to case. This is an example of a control unit for updating a DC laboratory power source using a PIC 16F870.
A frequency control unit could be made in much the same way.

control unit

Later I may show some other examples. Or more details, if there is interest around.


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