[ARC5] Making Stable Inductors for 2 MHz

[Kenneth G. Gordon]

On 29 Jun 2011 at 8:17, J. Forster wrote:

> Hi Mike,
> 
> I've never played w/ a tube VFO, but I know the specs are well within
> the art in WW II. Every WW II receiver had what I need in it. I just
> have little experience with the design of such coils and was looking
> for advice.

The coils are fairly easily done. As I said, all it really requires 
is a form made of stable material, and then some careful winding. 
Although ceramic coil forms are usually best, those used in the VF-1 
are phenolic.

Toroid-cored coils are a bit more problematic since some ferrite 
material is not particularly temperature-stable. However, others have 
made them work quite well, especially when using solid-state active 
devices (FETs).

> 
> I really would rather stay away from a DDS solution, for a number of
> reasons, not the least of which is chips going unobtanium. Also, to
> shape the envelope correctly requires a bunch of other stuff.

The solid-state VFOs I have mentioned are NOT DDS types: they simply 
use an FET as the active device, and possibly another or a bipolar 
transistor, as a buffer. 

Otherwise, they are identical with tube-based VFOs.

The main idea behind and the result of using an FET is that the far 
lower power (voltage and current) required, and the lack of need for 
filament (heat) vastly reduces any thermal drift.

That one in Electric Radio magazine I mentioned is of such. Its drift 
characteristics are by no means the best achieveable, but are quite 
adequate for your needs.

Ken Gordon W7EKB