So why are we dedicating a blog to a boring topic like cabinets? Are they made of Valyrian steel?
Nope, aluminum. But we hope you’ll find the details interesting anyway.
Our repeater controller cabinets are made of 5052-H32 aluminum alloy sheet. It’s strong, light, formable, and commonly used for aircraft, marine, and commercial-grade parts.
You’ve heard that aluminum forms a thin layer of aluminum oxide when exposed to oxygen in the atmosphere, and this layer protects the aluminum from further oxidation and corrosion. But that’s only true of pure aluminum. Aluminum alloy offers less protection, so we passivate the parts. That is, we make them ‘passive’, or corrosion resistant.
One way to passivate aluminum alloy is with chromate conversion coating, sometimes referred to as chemical film or by the trade names Alodine or Iridite. This process chemically oxidizes the surface, resulting in a very thin oxide conversion layer. It is neither a plating process nor an anodizing process (anodizing is an electrolytic process that forms a thick, non-conductive oxide coating).
Here are the key reasons we specify chromate conversion:
1. It has excellent corrosion resistance.
2. It’s a good conductor (aluminum oxide is not).
3. It provides good adherence for paint.
For the chassis and cover, chromate is the final finish. But because it’s used to pretreat aluminum prior to powder coating, it’s not considered a ‘finish coat’ by the sheet metal industry. Color and darkness are not guaranteed and can range from a light yellow or gold to a light reddish brown. Variations come from the material being treated and the concentration, temperature, and duration of the bath. (If a clear finish is desired, the color can be bleached out with a rinse. We don’t specify a clear finish because a darker film has more protective value.)
Due to these process variations, light or dark streaks are common. Of course, such imperfections do not affect the cabinet’s ability to protect and shield the circuitry inside.
But we do want our cabinets to look good, so we include some extra steps. For example:
The chassis and cover are sanded to give them a grained appearance.
The front panel is powder coated in black to provide good contrast with white silkscreen.
There’s no visible hardware on the front panel; threaded inserts for attaching the chassis are pressed into counterbored holes on the back of the front panel.
Finally, we want the cabinet to be RF tight to keep fast digital signals from being heard by receivers at the site. To accomplish this, we avoid large holes that let RF escape. We ensure good electrical contact with a wide overlap between the chassis and the cover. And because paint is an insulator, we now mask the area behind the mounting ears to improve electrical contact with the rack.
These techniques have been around for many years. We use them because they’ve passed the test of time!