[Amps] 160M PI network Toroidal Coil

[Manfred Mornhinweg]

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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