When you say split you mean cut the coil in half electrically?
Chuck
Instead of two separate coils, use only one coil, with the same total inductance as the two separate coils in series.
Electrically and physically split it into two identical sections without widening the turns spacing between the sections at the split. Actually, slightly widening, or even doubling the spacing at the split point won't hurt if you have more than a just a few turns, but it is best to maintain uniform spacing between all the turns if possible.
Theoretically, you will need only 70.7% of the total number of turns (1.414 times the number on one of the separate coils), although practically, the number count will likely vary, due to such factors as turns spacing, coil diameter, length vs diameter, etc. See the coil formulae in the handbook, or use this handy calculator at
http://hamwaves.com/antennas/inductance.html . Over the years I have developed enough of a feel for the dimensions of a coil for a particular application that I can usually hit the number of turns by trial-and-error in one or two attempts, more quickly even than using the calculator, when building from existing coil stock on hand with some means of temporarily attaching taps to the turns to the coil and moving those taps around.
Once you have determined the number of turns and the dimensions, and constructed or tapped the coil, count turns from each end and determine the
exact mid-point of the coil. Think of each turn in terms of 360°, and roughly count degrees if the start and finish points of the coil are not exactly in line with the axis of the coil. (
If you plan to use an existing high quality commercially built edge-wound coil, I would suggest throwing together a mock-up as a prototype, using heavy-gauge scrap copper wire or small diameter tubing to make sure your estimate is close to correct before you cut and damage a good coil only to find it won't work.)
When the exact mid-point is determined, cut out a section of the conductor to form a gap in the winding at that exact point. Make the width of the gap wide enough to withstand the peak rf voltage on the line at the transmitter output, with ample safety margin. Connect the wire leads from the transmitter (or balun if you use one) to the cut ends of the coil stock at the gap, and mechanically secure the cut ends. With a homebrew coil, it is merely a matter of winding it in two identical, symmetrical sections. Make sure that the spiral of both coils continues in the same direction. The split in the coil serves as the low-Z input, and the outer ends serve as the hi-Z output where the variable capacitor goes and where the output line is attached.
