Single-Layer Air-Core Inductor Design
by Martin E. Meserve
Place your mouse pointer over any of the categories above,
for an active menu of their contents.
Calculations propogate from one section to the next so that you can fine tune your requirements,
and your design, as you go. Initially, start with the
and then use the navigation menu for getting around.
A diagram of the coil and the basic equation used for the calculations are shown in the diagram.
It may not be absolutely clear in the diagram that the coil diameter includes the form diameter plus
one wire diameter. This then gives you the diameter from wire center to wire center.
In the Initial Design you are expected
to enter your basic coil requirements. Just enter the ones that you know for sure and leave the
others at their default. You can always go back to this data at a later date and make adjustments.
The output data is what is considered optimum for the input data provided.
The, in the section on Alternate Form Size,
you can make fine
adjustments to the coil's diameter. A list of common materials used for coil forms is listed
and an entry box for defining it. The initial number in the entry box is simply the calculation
brought down from the previous section. Note that drastically changing the coil's diameter will affect
the Length-To-Diameter Ratio and the Coil Efficiency.
Finally, in the section on Even Turns, you can avoid a fractional
turns specification and, again, re-calculates the coil. The final output is based on the input
information from the previous two sections and the even number of turns specified.
The View Design selection,
in the navigation bar, pops up a separate window with a synopsys of your coil design.
This is handy for printing purposes but it is also handy for developing your coil. The
synopsys window can be left up while the data is being adjusted in the main window.
When you want to update the synopsys window just click on the
View Design link from anywhere in the main window.
The old data will automatically be overwritten with current data.
The View Coil Taps selection, in the navigation bar,
pops up a separate window with the design data your your coil that has been optimized for specific form
diameter and number of turns. Following this data is a table that lists the Inductance at each of the
even numbered turns. This is handy if you want to design one coil for use on multiple frequency bands,
like a multi-band amplifier or antenna tuner. If the inductance ou need is not listed, just interpolate
between two close values. This should get you within a single turn of the correct tap point.
For Bare Wire, the spacing between turns is calculated as twice the wire
diameter. When winding, a length of the same size wire is used as a spacer, and then removed.
For Enamled Wire, the spacing between turns is calculated as the wire diameter + 0.005"
(.127 mm), which is the approximate thickness of the enamel coating.
For Insulated Wire, you need to determine the number of Turns-Per-Inch (TPI) when
the wire is close wound. To determine this, wrap some of the wire you will be using around a ruler
and count the number of turns in 1 inch (25.4mm). Then enter that data in the Turns-per-Inch
In every case spread the windings evenly over the calculated winding length before securing in place.
The Length-to-Diameter Ratio of a coil can affect the Q of a
single-layer close-wound coil. A high Q insures improved circuit efficiency, a narrower bandwidth
and less wide-band noise in oscillator circuits. In designing a high Q coil the following parameters
should be considered:
The wire size should be as large as practicable.
The turn spacing should be as close as practicable.
The coil form should have a low dielectric constant. Air is best.
The Length-to-Diameter Ratio should not exceed 4:1. Ratios of between 1:1 and 2:1 are preferred for most circuits.
The Turns per Inch area, above, is used as a Output when Bare or Enameled
wire is selected, however, it is used as a Input when Insulated wire is selected. See the information
in the section on Design Considerations for details on how to handle
different turns spacing.
Some Suggested Sources Of Coil Form Material
The calculated coil diameter is x.
In the space provided below, enter a new Coil Form Diameter.
Choose any diameter near the Calculated Coil Form OD.
The coil calculated, in the previous section, is optimized for the L/D Ratio and
Type of Wire you have specified. However, the calculated Form Diameter may not be practical.
The next series of calculations are based on a user specified coil form of a more practical size.
Some practical diameter suggestions are included in the tables. Also you may wish to use some pre-wound coil
stock available from Barker & Williamson. When you enter the
data, a new coil will be calculated based on the new Coil Form Diameter, and the previously entered/calculated
Adjusted design based on a user defined coil form diameter.
This part of the design is optional. For a variety of reasons, mostly mechanical, it would
be nice if the coil started and ended on the same side of the coil form. Making this coil is going to be difficult
enough without having to deal with the number of turns described to 3 decimal places. Round up or round down,
it's your choice.
The calculated number of turns, from the previous section, was
xxxxx. Enter a even number of turns, close to the calculated number, in this
red box ........