Bill,
Wind it on a piece of 3 inch diameter pipe under as much tension as you can
and you will have a strong, stiff coil with plenty of current carrying
capacity. It will have essentially zero loss and none of the insulation
problems of a toroid, nor the cost of the toroid.
You are wrong regarding the loss. The loss isn't "essentially zero",
unfortunately. Such a coil will likely end up having a Q between 300 and
350, perhaps lower if other metal objects are too close to it, or if it
uses some lossy support. Now if the loaded Q of the tank circuit is 12,
as is often done by design, then the coil will loose about 4% of the RF
power. At 1500W output that would be 60 watts, which is enough to make
that coil very hot, unless there is a strong stream of air cooling it.
Of course a toroid isn't a guaranteed solution for that problem. With a
toroid you need far less wire, thus there is lower loss in the wire, but
there is the loss in the iron powder. It really needs to be evaluated
which solution provides the better performance. But when space is at a
premium, in this frequency range a toroidal inductor usually ends up
significantly better than an air cored one. Instead when you have lots
of space, a good air core inductor is possible, and doesn't need special
material.
Regarding cost, considering today's copper prices, and that powdered
iron toroids aren't expensive, the difference isn't really large. With
the money you save by needing far less wire, you can about pay for the
toroid!
The only question is will it fit inside your amp?
That's the point. If there is plenty of room, I would use an air coil,
for simplicity. If instead the space is tight, I would use a toroidal coil.
Note also that magnetic cores may produce linearity problems, under some
conditions. Air instead is always linear!
Manfred
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