Coil Winding Instructions

Coil Geometry:

The Rook coil is a type of basket weave (B/W) coil and as such is inherently higher in efficiency than a standard close wound coil using regular magnet wire. This results from the spaces between each winding which is similar to a standard space wound coil. The advantage of the B/W is that this is achieved without adding length to the coil and reducing its inductance. Higher efficiency also comes from the ability to completely remove the form once the coil is finished.

The primary advantage of the Rook’s unique geometry is that it has an outer surface that is truly cylindrical in shape and an inner diameter that is close to the outer diameter. This results in a larger effective diameter for the production of inductance. It may also be more pleasing to the eye. A standard basket weave coil is wound on a series of pegs and the resulting coil shell is significantly thicker and looks like a multi pointed star.

Other possible advantages unique to the Rook may include (but remain to be proven so…. here is where you come in):
- Improved Litz wire efficiency due to reduced bending radius.
- Use in variometer / variocoupler based on structural rigidity and both inner and outer surface symmetry.
- Others will be found through experimentation.


Dimensions

The outside diameter of the form is approximately 5 inches and with an inside diameter of close to 4 1/5 inches. The slot depth is sufficient to wind a 2 inch long coil. The slot width is 3/32 of an inch.

Wire Considerations:
This form impacts the type of wire used in three ways:
1. Slot Width limits wire to a diameter less than 3/32 of an inch.
2. Slot Depth limits wire diameter to coils with a total length of 2 inches.
3. Slot Dressing (smoothness of edges) limits wire to diameters and types that will not snag on the form while being removed.

This form should work fine, as is, on magnet wire in the range of 14 to 24 gauge. If finer wire is desired or the serving on Litz wire snags easily, you may wish to dress the slots down with a fine file, emery board or other abrasive tool. Likewise, if larger diameter wire is desired, then the slots could be widened by hand filing or some other method.

Weaving Techniques:
Standard B/W winding techniques can be employed with the Rook form. The simplest is a 1 Up and 1 Down (1U/1D) weave that alternates winding the wire over one semi-circular peg and under the next and then back up over the third etc. This results in a coil length equal to a standard close wound coil but with the improved efficiency of a space wound coil (when using magnet wire). Wire with thick insulation uses the insulation to produce this spacing but still may be used in a B/W to gain the advantage of a removable form.

Two other standard techniques result in a coil that is nearly 1/3 shorter than a 1U/1D coil. The 1U/2D is produced by winding the wire over the first peg and then under the next two and then back over the 4th etc. It is well spaced and structurally stable. The 2U/1D is produced (as the name implies) by winding the wire over the first and second peg and then under the third and back up over the 4th and 5th etc. It results in a smoother outer surface cylinder.

Counting Windings:
Take your time counting windings and check your count a second time. In a 1U/1D coil each wire on an individual semi-circular peg (except the very first winding) counts as two windings since one goes under the peg as well. On the 1U/2D and 12U/1D coils, each wire on an individual peg count as three since two go under each peg.

Winding Techniques:
This coil can comfortably be held in one hand while the other does the weaving. Even old hands may find this form easier to use than a standard B/W form especially those made from nails driven through board (those nails are sharp). As such this may be a better form than the standard B/W for school projects. Most coils can be wound in as little as 15 minutes and completely finished in less than a half hour.
Begin at the bottom of a slot and work your way outward. Keep each winding in place with just a bit of tension on the wire. Pause every few windings to compact the windings together.

The windings do not need to be stretched from peg to peg as in a standard B/W and a slightly loose winding will be easier to remove from the form. This should not be sloppy just not stretched tight.

Another suggestion would be to place the wire spool on a mandrel (could be as simple as a dowl) that is parallel to the axis that you will be winding your form in. This eliminates a twisting to the wire as it comes off the spool and results in ripples in the wire on the coil.
More experience is needed but the Rook does work well with bifilar (two winding) coils like that used in the Mystery crystal radio design. However, it does add a layer of complexity and you may choose to wind a standard weave for your very first coil.

Affixing the Coil Windings:
Many people use hot glue to good advantage with B/W coils and the Rook is no exception. A bead of glue on the inside of each wire intersection seam is usually all you need to make the coil rigid enough to be removed. A bit of glue looped around the last winding at this seam will hold it in place. This can be done with the first winding after it has been partially removed from the form and touched up after it is completely off the form. Some extra glue at the point where the winding begins and ends will hold the coil leads in place. Other glues will work but with hot glue you have an instant coil. In any case, make sure that the glue does not attach to the form and if it does (like at the very end where you loop some around the last winding) just pop it loose by prying up with the point of a pocketknife.

Another classic technique is to use string or dental floss to lace the coil together by passing it through the loops in the windings and then tying it off. Some people use wire in this way as a temporary way to get the coil off the form before it is made rigid using glue, lacings or special low loss pegs.

Some bifilar coils (like Mystery) can be less rigid and may require extra glue or string to keep them in shape. Experience will be your guide. Over time you will find that other coil types will require less binding material.

Coil Form Removal:
Once the coil has been made rigid, it can be teased off the form by prying the first winding at the slot intersection point and working your way around the circumference of the form. Take it slow as this is the time when your coil is most likely to be affected but any slight bends in a winding or loosened glue joints can be touched up later. If the coil is very difficult to remove you have probably wound it too tight.

Inductance:
Standard inductance formulas for air core solenoids are appropriate for these coils. Bear in mind that the effective diameter of basket weave coils is close to the average diameter of all the windings. In this case, 4 3/4 inches is a good estimate. One standard formula used is L=(rN)2/(9r+10l) where L is total inductance in micro Henrys, r is the effective radius in inches and l is the coil length in inches. If you would like a copy of an Excel Spreadsheet that does this calculation go here and select Professor Coyle in the top left corner.