What are the collectives thoughts on the type of material to use for a
homebrew ATU enclosure, should it be aluminium to reduce unwanted RF
radiation or plastic to reduce coupling effects possibly caused by an
aluminium enclosure.
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For QRP power levels it could be best fitted in a wooden box , simple and
easy .
For much higher power levels a metal enclosure will be better.
However ,in order not to influence inductances by the metal cabinet , a rule
of thumb says that (air) inductors should be free form any metal keeping a
distance of their diameter
Example : With a fixed air or roller inductor of say 100 mm (4 inches)
,you need an enclosure of at least 300 mm (12 inches) such that the inductor
is not less than 100mm from any metal surface.

For this reason many homebrew antenna matching units are fitted inside
non-metallic enclosures often made of perspex (plexy glass)

Frank GM0CSZ / KN6WH