I only remember the stuff that melted when overworked. Or soldered too
close to. One can't expect to work a plastic supported coil at
incandescent wire temperatures and have the wires stay in place to not
have drifting tuning.
It wouldn't be hard to make such coils starting with strips of bare PC
board and bare copper wire plus a dollop of stiff epoxy. Wind the coil
close spaced on a suitable form, like a big dowel or a piece of water
pipe (plastic or metal). Slip the close wound coil off the basic form,
space it by running a rod through the helix, then mount the strips to
the wires with a bead of epoxy. Or using polystyrene rods from the hobby
shop, heat the wire and melt it into the polystyrene rods. Polystyrene
has very good RF characteristics but it melts easily. Or glue the wire
to the polystyrene rods with coil Q dope, that I think is polystyrene in
a solvent. Its not impossible to drill the strips of PC board and twirl
the helix through three or four strips, then needing no epoxy except
maybe for the end anchors. Takes little in materials other than the wire
and some scrap stripped PC board material, or polystyrene strips from
the hobby shop or plastic window glazing from the home center or to be
exotic, some Lexan from McMaster-Carr. Delrin and nylon may be a little
lossy for RF applications. And the custom wound tuner coil can easily
make room at the middle for a variable link.
Not to neglect the fact that such coils used to be factory products from
B&W and National as well as military surplus like the BC-610 plate coils
and might be found in some ham attics or sheds yet today. Some year I
need to catalog what I have and convert them to projects or cash.
Sometimes the quest for compactness conflicts with the need for adequate
conductor size to minimize conductor heating. Conservatively designed
equipment and parts that is expected to run a long time generally keeps
conductor temperatures under control by appropriate conductor sizes.
I don't know what the MFJ coils are like.
73, Jerry, K0CQ
On 1/14/2011 12:43 PM, Rick - NJ0IP / DJ0IP wrote:
With those coil stock you have to be careful.
B&W usually made two versions of coils, with the cheap one using plastic,
which melts when it is hot.
The more expensive ones used some kind of composite, which would take a lot
more heat before melting.
Maybe I'm being unfair, but I will assume the MFJ coils are the cheap
plastic versions.
73
Rick
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