What is the advantage

[thedoc298]

I see many coils with 2 toroids, one larger and one small. What is the advantage of this. Thanks again.

[MRMILSTAR]

There are two reasons. Most of the time, someone is just trying to add more capacitance to the top load without going to the trouble of building a single larger toroid. Another possible reason for adding a smaller toroid below a larger one is to bring the electromagnetic  field closer to the top of the secondary coil to provide protection against streamers being emitted from the top of the secondary coil windings.

[thedoc298]

That sounds reasonable. Thanks again.

[Mads Barnkob]

A third reason is also to raise up the top toroid, to gain more distance between the toroid from where the streamers come out and the primary coil/strike rail/ground, a smaller diameter near the top of the secondary coil will also have a stronger electrical field that helps prevent corona discharge and streamers from the top turn of the secondary coil if there is a too large distance from that and topload.

[thedoc298]

A third reason is also to raise up the top toroid, to gain more distance between the toroid from where the streamers come out and the primary coil/strike rail/ground, a smaller diameter near the top of the secondary coil will also have a stronger electrical field that helps prevent corona discharge and streamers from the top turn of the secondary coil if there is a too large distance from that and topload.

Thanks for another good tip, takes long time to fully understand all the in's and out's. Would it be like if at the top of the secondary I had a 10 inch giraffe neck that it would be to my advantage to have a smaller turtle neck below the bigh toroid on top of the 10 inch gap for the reasons you stated. Thanks.

[MRMILSTAR]

Most people with SGTCs just use a single toroid mounted at the proper height to shield the top turns of the secondary from corona or sparks. My 8.6" pig-powered coil just has a single 30" x 9" aluminum toroid with no problem.

[MRMILSTAR]

JAVATC allows for multiple top loads in the input. You can try your different ideas and see what they do.