Secondary coil is the coil that sits in the center of the assembly, its components consist of a "form" and fine ( thin) insulated wire. The form can be ( from worst to best) cardboard tubing, plastic pipe, fiberglass tubing, ( the nearly impossible..) self supporting. Materials that absorb moisture ( cardboard ) should be considered as a last choice for a form. The most commonly used material for the form is PVC drain pipe, it is thin walled and can be found up to 10" in diameter.
It has been found that there isnt too much of a performance increase to use the really exotic materials for a form. Besides, the costs of such materials are really ludicrous. For example... a 5' long section of Lexan or Acrylic 8" diameter tubing will set you back about 80 - 125$.
The wire used for the secondary is normally what is known as magnet wire, it is a small diameter wire insulated with a durable varnish. The standard for most coiling activities is the #22 magnet wire. Although there are instances of the # 30 wire being used on secondary coils. The thinner wire ( the higher the # ) has good and bad points to it. The thinner wire allows for more wraps to be made along the length of the form, thus increasing the inductance. But... it reduces the ability to put a fair amount of power into it.
To build a secondary coil, there is the hard way and the easy way. The hard way is to wind the coil by hand, rolling the form over a flat smooth surface as you feed the wire onto it. ( hard on the hands, wrists, and fingers, and it takes forever ). The easy way is to mount the form onto a lathe or make a jig that will hold the form. If you have a way to turn the form with a motor, this is even better.
There doesn't seem to be any problems with which way you wind the coil ( clock wise or CCW ) but it is normally started at the bottom of the coil form and wound along the length to the top. It is best to start the wire at least 2 inches from the bottom of the tube. This will give you room to add the RF ground plate to the secondary coil assembly. When winding the secondary, keeping tension on the wire is pretty important. It is best to have the wire pulled tight against the form to keep the wire from loosening up and slipping down the form once you stand it up. The wire should be wound to about 1/2 - 1" from the top of the form.
Once the form is wound with the wire, additional insulating is needed. The popular method for adding additional insulation is to apply several coats of polyeurathane varnish to the coil. This takes several days if you want a clean smooth finish.
Some tech tips on secondaries....
for smaller diameter secondaries, the height to width ratio should be 6:1
( 24" tall X 4" dia.)
medium sized coils have a reduced ratio 5:1
( 25" tall X 5" dia.)
larger coils
4.5:1 ( 27" tall X 7" dia.)
really big coils and up....
4:1 ( 32" tall X 8" dia)
(measurements are just for example)
These numbers are just a standard, there is nothing to stop you from winding a coil to 1.5 times the standard ratio. You could easily wind a 8" diameter form to 48" length without serious side effects. ( this is 6:1 for a large diameter coil )
The number of winds on a secondary is generally considered to be best at
900 - 1100 turns. Although there has been some coils with increased performance
at 1400 turns.