Creating a high ratio transformer

Nickel-zinc ferrites go higher, and might get you to 30MHz.

Toroids or c-cores might be better for high frequency work. The parallel capacitance along each winding can be a problem at high frequencies, and inter-winding capacitance can be worse. <snip>

Pot cores are excellent for coupling, but for strays you need to apply some tape. Mylar tape between layers lowers capacitance (but adjacent winding-to-winding has such low relative voltage that capacitive energy there is minor). Cloth tape and varnish was the old-school approach. For the wire coming out of the center you want to preapply spaghetti tubing (to space it off from the outer layers).

I'd consider using foil for the one-turn, and put it in the center of the pot core (I assume you're using a one-section spool?) so that it is an easy-to-wind core for the many-turns layers. If you want to put a static shield over it, you want (of course) to make a second foil for that.

Current in a sheet is a better match to most transformer flux coupling models than that from a single round ring.

The one-fat-turn on inside minimizes resistance of t he low-Z winding by limiting length. The higher-Z outer winding is less power loss because less current there. Going with foil the inner winding takes very little of the throat.

In a previous job, we used 3M foam tape to add spacing between windings. Also used mylar tape.

I generally just cover possible shorting areas with the mylar tape, but I do have shrink tube that would work.

Yes, one section bobbin. Maybe I don't understand, it is easiest to wind the 10 turns first, add a spacing layer, then put the 1 turn sheet winding on, other wise it is difficult the get the leads out for a sheet winding on the inside. Or so it seems.

I have done that shield on a Flag Antenna (directional MF antenna) to prevent unwanted signal ingress, but, I put the shield between the to layers, What is the advantage of a shield in this situation? Also with battery operation, do I connect it to an external ground or low side of the LC connection? (Q meter, series tuned) One time build, so I'll put in the effort to add shields if effective.

I'll see if I can work out the lead exiting problem with the sheet winding going on first to reduce losses. Thanks for the input, Mikek

Mikek

OK, I have the inner foil turn figured our drawn glued to 0.004" brass shim stock, cut out and wrapped on the bobbin. Now, I need to know about the shield, Is it needed? Outside? Between layers? And if needed what is is it's purpose? Thanks, Mikek

I ordered it. I tried to get an inter-library loan, but the library is understaffed and they aren't doing inter-library loans at this time. Grumble! Mikek