Highland Ham wrote:
Butch Magee wrote:
I'm looking for a supplier of project boxes (metal). What I need right
now is a 4 x 7 x 5 in. Or numbers close to this, abt. 1/2in either way
for a little ant. tuner I want to put together.
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Suggest you don't make the enclosure too small for a matching box (tuner).
Any inductor too near metal , will affect the inductor's Q
A rule of thumb is to keep the inductor away from the enclosure not less
than 1 diameter(of the inductor)
For this reason I am constructing an antenna matching unit in a plywood
enclosure or perhaps MDF ,although I understand MDF is no longer used in
the USA,because in contains phenolic resin. Perhaps someone in the know
on this NG can confirm this.

Frank GM0CSZ / KN6WH

Hi Frank,

It sounds like you are going to an extreme that will create as more
problems than it solves.

There is very little Q change in an aircore inductor of typical form
factors when it is brought near sheet metal unless that sheet metal is
in the core axis within a core diameter or less. The reason for this is
air does not saturate, and the magnetic flux simply moves to a better
spot.

There are only two exceptions to the generally very small Q reduction
by a coil being near metal. The first is when the inductor is operated
very near self-resonance, at which point any stray capacitance can
seriously affect coil Q. The second is when the metal totally encloses
the inductor and is so close it reduces inductance a significant
amount.

MFJ has that problem is some small tuners, where the end of a 2 or 3"
diameter coil is mounted right up against the sheetmetal (nearly zero
spacing) and the when the cover is installed the other end has metal a
fraction of an inch away. Unloaded inductor Q drops about 45% from 220
to 120 in that tuner whe the full coil is being used, and it is an
exceptionally bad layout. Had they stayed an inch or two away from the
ends of the coil it would not be an issue.

When you place components in a "transparent" box, you now have
significant new problems. You have no reliable groundplane. Wiring is
an issue. There also is no containment of fields, you have created a
small antenna in the room.

Most of the problems in tuners are caused by the tuner not have enough
capacitance or using an inductor with crappy Q to start with, not by
the box. You have to work at it pretty hard to make a design where the
box kills the Q, but it is easy to make a box that radiates harmful RF
into the area around the tuner.

73 Tom