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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? |
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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 |
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