Sat Sep 1 21:21:26 EDT 2018 Mike www.w0btu.com wrote:
> It may very well be true that tank coils made from round copper have some
> slight advantages over edge-wound. But after all is said and done, does
> this really make a great deal of difference? :-)
> I have an edge-wound roller inductor in my homebrew amplifier.
> http://www.w0btu.com/833C_linear_amplifier.html
> It's reliable, doesn't arc, and doesn't get warm. Isn't that what's really
> important?
I recently ran across this discussion on another radio forum, claiming that
edge wound is inferior in terms of Q to round wire. It was said that on a flat
strip, the current tends to crowd to the two edges for the same reasons that
cause skin effect, thereby wasting much of the copper. Round wires would be
less prone to this "crowding effect" because they have no edges.
I checked out a few on-line sites, but they quickly get bogged down in long
series of long, tedious mathematical equations, and I was not able to come to
any clear conclusions. Some suggest that the subject is not well understood,
even suggesting Terman's textbook is wrong, or that the effect is negligible if
the radius of curvature at the edges of the strip is much greater than the skin
depth. The skin-effect proposition makes some sense, although I had never
thought of this. The closest I could come to a conclusion from the texts is in
regard to the "proximity effect" of closely spaced parallel round conductors,
in which the rf current density in each conductor crowds in the direction away
from the adjacent conductor. The rectangular conductor could be thought of as
approximating a large number of closely-spaced round conductors, so the current
density would tend to crowd towards the edges of the conductor. I would like
to see/hear some opinions on the subject.
I can understand that primary advantage of edge-wound would be for mechanical
reasons, for example for a rotary inductor where an effective sliding contact
is more difficult with round wire. Slip-on coil clips like the ones EF Johnson
used to make are designed to work with edge-wound coils. I would add that the
spacing between turns with edge-wound can be decrease compared to the same
amount of copper in round stock, thus increasing the number of turns per inch
with edge-wound, resulting in more rigidity for a given thickness of conductor.
Most broadcast transmitters, phasors and ATU units use edge-wound stock, so
there must be a good reason.
I threw together a prototype for the matching unit for my 160m vertical,
using some badly corroded round-wire coil stock from the junkbox, with
deteriorated plastic supporting strips. Once the final design of the coils
was determined by trial-and-error, I built a permanent version, using like-new
silver-plated edgewound coil stock, with the same coil diameter and turns
spacing. I had anticipated slightly better efficiency with the final version,
but as it turned out, the prototype and final version performed equally well,
neither one any better or any worse than the other. With the same DC input to
the final, the rf ammeter reading at the base of the vertical was exactly the
same with either set of matching coils.
Don k4kyv
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