I calculate that you need about 10 turns of #12 wire on a 1 1/2 inch form, spaced out over about 2 3/4 inches.
You connect the coil top end to the base of the vertical radiator and coil bottom end to the radials' common point. I know it might sound weird to put a coil *across* the feedpoint, but trust me!
The shield of the coax cable connects to the radial common point/bottom of the coil. Then you take the center conductor on the coax and attach it a *couple turns up* from the bottom of the coil... I think 2 turns up should do it.
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x coil (10 turns #12, 1.5 inch form, 2.75 inch long)
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x -- center coax tapped 2 turns from bottom of coil
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O -- radials and coax shield meet here, coil bottom
It's a nice design because it keeps static from building up on the vertical radiator.
There are two ways you can adjust it.
Spreading and compressing the coil length will change the resonant frequency, and that will be the frequency where you get lowest SWR. If you find the SWR is lowest at the bottom of the band, spread the coil. If lowest at the high end of the band, compress it. You can also trim or lengthen the vertical radiator instead.
If you sweep across the channels and find the lowest SWR is in the middle of the band but it's not very low... like the lowest SWR is 2:1 but it happens on channel 20 or so, adjust the location of the coax center conductor tap on the coil.
You can change the coil a little bit but don't make huge changes... the diameter, number of turns and length should be about right.
Good luck with the project, you'll love the feeling of using your own homemade antenna and having it work well.
My first homemade antenna was a vertical dipole for CB inside some 1 1/2" PVC pipe. I clamped it to the side of the house and it worked really well.
