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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
lokey -
My recommendation is to permanently connect the bottom winding to some mechanical strain-relief attached to the bottom of the secondary coilform. Otherwise, eventually you'll break the wire. I use a copper plate epoxied to the coilform, with a machine nut or threaded boss soldered or brazed to the plate. The bottom turn of the winding is soldered directly to the copper plate, and your RF ground wire is secured to the nut with a short pan-head machine screw.
The terminations for both top and bottom turns, and baffle installation, have been covered previously in another thread:
Registered Member #2553
Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97
OK, heres a couple closeups of my setup. so ur saying i have to grind a groove in the mounting base to fit the copper plate glued to the coil form? im just trying to do this right and asking lots of questions as i go along to avoid as many problems as i can.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I would wrap entire base of your secondary coil in copper or Al adhesive tape, and then the same for the base on inside so it forms a contact when secondary is tucked in.
But I've as well drawn the wire inside the secondary all the time and never had any problems - due to distribution of electric fields it's very unlikely that streamers will form inside the secondary, at least as far as I feel.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
lokey -
".....so ur saying i have to grind a groove in the mounting base to fit the copper plate glued to the coil form?"
Never said anything about grinding a groove (where did you seee that??); just form a piece of thin copper or brass plate to the radius of the coilform, solder on a brass machine nut, and epoxy the plate to the exposed coilform below the windings.
Your photos indicate that you intend to support your secondary in what looks like an inverted plastic toilet flange, and you don't have enough exposed coilform left to bond a copper plate to (that's why it's always best to plan ahead!!). You can either remove turns from the bottom of the secondary to expose more coilform, or glue the secondary into the flange and bond the copper plate to the outside of the flange. Of course, then you won't be able to remove the secondary from the flange for easy disassembly/storage.
DO NOT wrap the bottom of the secondary with copper or aluminum tape! This will likely create a "shorted turn", which, being so close to the primary, will quickly overheat and melt a groove into your secondary. I watched someone do something similar with a stainless-steel worm-gear hose clamp, and the clamp became smoking hot very quickly.
When you do get your coil running well, there WILL be some risk of internal arcing down through the inside of the secondary. Again, installing a sealed baffle is well worth the effort if it prevents destroying a secondary.
Registered Member #2553
Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97
Herr Zapp wrote ...
lokey -
My recommendation is to permanently connect the bottom winding to some mechanical strain-relief attached to the bottom of the secondary coilform. Otherwise, eventually you'll break the wire. I use a copper plate epoxied to the coilform, with a machine nut or threaded boss soldered or brazed to the plate. The bottom turn of the winding is soldered directly to the copper plate, and your RF ground wire is secured to the nut with a short pan-head machine screw.
The terminations for both top and bottom turns, and baffle installation, have been covered previously in another thread:
Herr Zapp
how did you solder your nut onto the copper pad for your stress relief? i started by soldering the nut on, but solder filled in the threads so i couldnt get the screw in. then i soldered the nut with the screw in, but it soldered the screw into the nut so when i unscrewed it the head snapped off.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
lokey -
It's all in the art of soldering. First, use 400 grit sandpaper, steel wool, etc. to clean the plate and the nut to a nice, shiny, grease free dead-clean surface. Support the plate out at the edges so you can get the center of the plate hot enough to flow solder. Position the nut on the plate, and then gently heat the underside of the plate with a propane torch. Just keep sweeping the tip of the flame back and forth across the underside of the plate. Keep touching the top of the plate, right next to the nut, with the end of a piece of small-gage, flux-core solder. Apply just enough heat to cause the solder to flow and form a small fillet around the base of the nut. Heat everything too hot, and/or apply too much solder, and capillary action will draw it up into the nut threads.
Even if you're not careful and do get solder in the nut threads, you can run a bottoming tap into the nut and easily clean out the solder.
I now use high-temperature silver-brazing alloy to secure the nuts on my grounding plates, but the process is essentially identical to soldering with soft-solder; the brazing alloy is just much stronger than lead-alloy solder.
Remember, the whole point of the copper-plate thing is to provide a reliable strain-relief for the secondary wire. You want the copper plate to be as large as possible (within reason) so you've got lots of bonding area for the epoxy to adhere to. A little postage-stamp-sized plate won't provide much bonding area.
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best way to secure coil
