William Mutch wrote

I've used bees wax and it has worked fine, though I have no way to
measure losses, if any. You can melt it to get your toroid free if you
need to. I have done this when wanting to put the same balun on
different feedlines.

I looked up the dielectric constants of a few materials:
http://zhangzc.jahee.com/chemcai/Die...nt%20Table.htm

air ~1

or

paraffin wax 2.1-2.5
beeswax 2.7-3.0 very close to paraffin!
urethane (not foamed) 3.2 (polyurethane?)
cast epoxy 3.6
polyvinylchloride 3.4 (common wire insulating material)

Potting a balun coil in these materials does substantially increase
stray capacitance.

The mil. circuits I saw potted in wax or paraffin were audio, not RF.

I guess foamed polyurethane has a modest dielectric constant.

I read that some commercial RF transformer (e.g. RF System's so called
Magnetic Longwire Balun) are filled with polyurethane foam. I also
found a 4X ham discussing and blessing PU foam at
http://lists.contesting.com/archives.../msg00038.html

The same article also warns against potting in epoxy, as it boosts
stray capacitance in transformers designed for air insulation. Wax and
paraffin are better than epoxy, yet they may both be troublesome.

The PU foam I can get is the common consumer "monocomponent" type. It
is a misnomer, as it requires ambient humidity to react and cure. It
will not properly foam inside completely closed environments unless
water is provided.

I tested foaming in a foot-long 1/2 in. plastic pipe, not pre-wetted,
and the result was that foam properly formed only for the first inch
or so at both ends, while the material collapsed into a film in the
middle.

Instructions correctly advise to 1) wet the surfaces the foam will be
in contact with, 2) if one needs to fill a deep cavity, deposit foam
in layers and allow each layer to set and dry.

The resulting foam is stiff and does form a light bond with plastic,
but is rather easily removed if the need arises.

My balun box is small, at most 3/4" deep, all metal contacts are
already coated with thin epoxy, and the transformer wire is PVC coated
too, so a momentary contact with water spray, soon to be soaked up by
the PU, should be OK.

I'll do another test in a small plastic box containing some wiring,
all pre-wetted. As the foam keeps expanding as it sets it may be a
good idea to avoid closing and bolting the box lid before it cured
completely, or at very least to provide some escape holes. More
later.