How I Make My Coils

The first step is to cut a piece of HDPE (high density polyethylene) to the size needed. You will need a protractor, ruler and a compass to lay out your form. (hint: If you expect to make a lot of forms, make one as a master template to trace others) The width of the form is determined by the diameter of your proposed coil. The height the same as the width plus 3/4 to one inch additional (18-24 mm) to accommodate the mounting area.

Cover the entire piece of HDPE with wide masking tape. Draw a vertical line that divides the width in half. Measure the width and divide by two. Then measure down from the top. This point will be the center of the coil, except for the mounting area. Use the compass and draw a circle to the full width of the form. You may have to darken the line by hand drawing. It depends on the hardness of your compass lead. Then make another circle for the hub size.

Now is a good time to make a line across the bottom of the form that is about 3/8 inch (9 mm) from the bottom. The mounting holes will be located along this line.

Next figure out how many degrees each slot will be positioned at. You may want to cut a circle of paper and do the layout there. Spider coils must have an odd number of slots to function. The number really depends on the mechanical aspects of your form. I like to have as many slots as I can get, being careful that the distance between slots at the hub are not too close. That would seriously weaken the form. With a 2 inch hub (50 mm), I like to use 11 slots.

Lay out the center positions of the 11 slots. Measure to the left and right for the width of the slits. I like 1/8 (3 mm) to 3/16 inch (4 mm) spacing when making a form for 660/46 litz. 1/8 inch is plenty for 165/46 litz and a little thinner than that for 40/44 size litz. If you are making a tracing template, make the slots a bit wider.

Once the slots are laid out, there are some at the bottom that have to be extended so that the wire will fit as you are winding your coil. You can go straight out or use an angle like I do.

Now find the positions of the 4 holes (or how ever you want to do it) and drill the holes. The hole size will depend on the screw size you will use.

Next cut the rounded top of the form with a scroll saw. Then carefully smooth the edges with a small belt sander.

Carefully cut the slits with your scroll saw. If you are familiar with your saw, I'm sure that the outcome will be pleasing. After the form is cut, the tape can be pealed off and you are ready to wind the coil.

The wire is then wound. I like to put the supply spool at a distance of about 10 feet (3 meters). That will give room for the wire to twist as it always does a little.

As you are winding, count the turns. When the coil is finished, count again before cutting. When I have a fixed amount of wire, that is close to being the right amount, I wind the first coil (if the two are different size, wind the bigger one first). Then I unwind all the wire from the supply spool, and wind the other form starting at the other end. I try to end up with a turn or two extra and when the windings on both coils are correct, then I make the cut. When the coils are installed and working, turns can be trimmed depending on your own antenna and variable capacitor situation.

If using a 365 to 400 pf capacitor, you want the bottom of the band to tune when the capacitor is nearly meshed. I make my sets tune to 520 khz at the bottom. The top usually ends being at the correct nearly open capacitor. The low distributed capacitance of a spider form will allow tuning the entire band without adding parallel capacitance for the bottom of the band.

Date: 2016-01-14; view: 954