Thank you all!

Here's what I intend to do:


- coat everything internally,
but without a space-filler


- seals

* fixed, for plugs and tieposts: a locally made silicone based strong
adhesive/sealant + external soft silicone coat

* reopenable, for the box lid: just soft silicone sealant

* wire tieposts & coax plug:
- soft silicone sealant, left to dry completely
- then wrapped in aluminum foil (no metal contact)
- then wrapped in friction tape, held in place by telephone twisted
pair;
it's another suspenders-and-belt trick I have used this to
weatherproof all sorts of antenna contacts, some lasting well over 10
years


This for keeping water out,
But, in case it does somehow get in, what about WEEP HOLES?

In article ,
says...
I have a low-power 4:1 toroid balun almost ready in a little plastic
box. In order to keep the rain out, I am sealing everything up, but I
am also considering space-filling materials.

I am leaning toward filling it with hot "wax" - actually, paraffin
from standard household candles, although a harder grade like
sometimes found in old military equipment should be better.

The pro should be that the box would not become sealed for eternity in
case I want to re-wire something in the future, while epoxy is bomb
proof, and urethane foam is also rather hard to remove.

Is there any electrical con to be considered?

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.

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.

SpamHog wrote:

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.

Agreed. However, the PU foams that we buy in DIY stores do tend to soak
up water into the outer layer, especially after the smooth surface has
been cut to trim off any excess.

Some foams are notably better than others in this respect, and it might
be better to look for the special 'closed cell' foams that are
specifically rated for use in for flotation aids.

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.

Before you do, you'd better read this:
http://www.diyfaq.org.uk/humour.html#foam

For 'canoe', read 'kayak'... and don't say that nobody warned you!


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK" wrote in message ...

Before you do, you'd better read this:
http://www.diyfaq.org.uk/humour.html#foam

For 'canoe', read 'kayak'... and don't say that nobody warned you!

Worth an epic poem!

I particularly liked the bit about the wailing and the hair removal.

Yes, I was already rather concerned about this. The so called
"mono-component" storebought PU foam is sometime better referred to as
"waterblown". It's rated 90% closed cell, which is fine giving the
tiny size of the typical cell and the all-around sealing I am going to
provide anyway.

The problem is, the expansion ratio is hugely unpredictable. It is
said to go up from 8 to over 20 "if water is present", with "present"
being left undefined, and probably undefinable as well, although
"wetting" is recommended.

Moreover, as the unlucky canoe builder found, penetration, bonding,
expansion, and setting go hand in hand, meaning the stuff performs
this remarkable routine:
- penetrate every nook and cranny
- soak up water
- stick to surfaces
- gain some texture and firmness
- arch its back and expand (for waterblown: at some ill defined rate
too!), bending out of whack anything that is not as firm as stone or
steel

I did a test in a small box (translucent, probably polyethylene,
formerly contained a bike tyre repair kit). I put in a couple of
airwound bare copper coils, half inch apart, rinsed it all in water,
and while it was wet I sprayed in some foam (_not_ to the point of
filling it), closed it, and wrapped it tight with wire to provide real
closing power. I could see that the foam had not yet filled the
cavity. The DC resistance between the two coils had dropped from
unmeasurable (2Meg) to 150k.

24h later, resistance was back up to unmeasurable, the box looked
completely filled, nice big round blobs of foam had formed all around
the lid, and all the surfaces of the box had become warped under
pressure. Chances are, the coils had been bent around as well - I
could not check if they were short of a CAT scan, for which I am not
yet equipped.

If I'll do foam my balun box - and it's still a big if - I'll first
fashion and fit a temporary, ~50% perforated lid, to give the foam an
easy way out, and to make it easier for me to remove the excess foam
later on.

I'll be very careful if I ever have to do the same to a canoe.

"Ian White, G3SEK" wrote in message ...
SpamHog wrote:

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.

Agreed. However, the PU foams that we buy in DIY stores do tend to soak
up water into the outer layer, especially after the smooth surface has
been cut to trim off any excess.

Some foams are notably better than others in this respect, and it might
be better to look for the special 'closed cell' foams that are
specifically rated for use in for flotation aids.

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.

Before you do, you'd better read this:
http://www.diyfaq.org.uk/humour.html#foam

The link brings up fond memories of a very similar debacle I saw first
hand. This one wasn't "engineered" by a home shop canoe builder, this
one was put together by none other than the U.S. Navy.

My first job out of school 40+ years ago was as a civilian mechanical
engineer with the U.S. Naval Air Engineering Center. The group I
worked in was responsible for the design and ongoing development of
carrier-based steam aircraft catapults.

Catapults are individually calibrated and periodically recalibrated by
launching minimum-budget wheeled vehicles of various weights called
"deadloads" off the catapults in into the water when the carrier is in
port. The idea is/was to create weight vs. end speed vs. steam
pressure curves which are vital bits of info when twisting the
catapult steam pressure control knobs below to get the airplanes off
the end of the catapults flying nicely instead of into the drink or on
the other end of the scale not torn apart by excessive g forces.

Deadloads are, or were at least back then, simply big steel boxes of
various weights to simulate the range of aircraft weights the
catapults dealt with. Each corner of the box was equipped with a wheel
salvaged from junk aircraft. They were divided up into internal
"watertight" compartments and hopefully they floated after a test shot
and could be reused. Some deadloads floated, some leaked or came apart
and sank. An average deadload weighed 35,000 pounds and was launched
at perhaps 115 mph.

Along came this self-proclaimed engineering genius from the next group
yonder who decided that his route to military fame and glory was to
become the guy who came up with the concept of filling the deadloads
with foamed flotation compounds and save all deadloads from the briny
deep.

At that point in time DIY foamed flotation plastics were experimental
and were not yet on the general market. But he managed to acquire some
samples of a two-part foaming compound for test purposes. He also
managed to get hold of an old deadload and parked it on the tarmac off
to the side of the main hanger and went to work drilling holes in all
the compartments. Then he went back and poured carefully measured
amounts of part 1 and 2 into each compartment.

I had no idea what was going on out back at the hanger, I wasn't
involved in it. Until a lieutenant commander who worked in our area
came down the main aisle with tears of laughter rolling down his
cheeks hollering "You gotta see it to believe it!" So off we trooped
to find out what was going on. The genius had slipped a decimal point
with his slide rule and the deadload had been loaded with ten times
more foaming agent than it should have had.

The deadload had turned into a steel ball 10-12 feet in diameter with
it wheels neatly arranged around it's equator, plastic shoots were
flying dozens of feet into the air like fireworks and all over the
tarmac, beyond priceless.

I keep wondering what that guy would have done to a balun box . .

For 'canoe', read 'kayak'... and don't say that nobody warned you!

w3rv

Brian Kelly wrote:

The deadload had turned into a steel ball 10-12 feet in diameter with
it wheels neatly arranged around it's equator, plastic shoots were
flying dozens of feet into the air like fireworks and all over the
tarmac, beyond priceless.

I keep wondering what that guy would have done to a balun box . .

Nice one!


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek