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Introduction

As the title indicates, this page is for the design of a Single-Layer, Air-Core, inductor. It is not equiped to handle multi-layer coils. The page is long and is split into several pieces. Much of it is explanation, but the intent is that, you start out with a required inductance and physical properties, then, through a series of calculations, finish with a practical coil design. These formulae provide close approximations of values for frequencies in the 1-30 MHz range that are sufficiently accurate for most Amateur Radio purposes. The calculations are useful in the VHF and UHF range but only as a basis for further calculation and experimentation.

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 Program Description 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. Note: 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.

Program Description

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.

Design Considerations

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 input area.

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.

Initial Design Input
Initial Design Input Data
Required
Inductance (uH)

Wire Size
(AWG, in, or mm)

Wire Type
(* See Below)

Turns per Inch
(* 25.4 mm)

Length/Diameter Ratio
(* 1.5:1 Recomended)

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.

Initial Design Output Data
Number of Turns = x
Wire Size = x
Turns Spacing = x
Coil Form OD = x
Coil Length = x
L/D Ratio = x
Turns_per_Inch = x
Alternate Form Size Selection

Some Suggested Sources
Of Coil Form Material

PVC Pipe
Pipe Size Outside Diameter
3/8" 0.675" (17.145 mm)
1/2" 0.840" (21.336 mm)
3/4" 1.050" (26.670 mm)
1" 1.315" (33.401 mm)
1-1/4" 1.660" (42.164 mm)
1-1/2" 1.900" (48.260 mm)
2" 2.375" (60.325 mm)
2-1/2" 2.875" (73.025 mm)
3" 3.50" (88.900 mm)
3-1/2" 4.0" (101.600 mm)
4" 4.50" (114.300 mm)
Shotgun Shell Diameters
.410 = .410" (10.414 mm)
28-Ga. = .550" (13.970 mm)
20-Ga. = .615" (15.621 mm)
16-Ga. = .662" (16.814 mm)
12-Ga. = .729" (18.516 mm)
10-Ga. = .775" (19.685 mm)

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 information.

Adjusted design based on a user
defined coil form diameter.

Inductance = x
Form Diameter = x
Coil Diameter = x
Coil Length = x
Wire Type = x
Wire Diameter = x
Number of Turns = x
Turns Per Inch = x
Turns Spacing = x
L/D Ratio = x
Adjustment For An Even Number Of Turns

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 ........  .

Final design based on a user defined coil form diameter and number of turns.
Inductance = x
Coil Diameter = x
Wire Type = x
Number of Turns = x
Turns Spacing = x
Form Diameter = x
Coil Length = x
Wire Diameter = x
Turns Per Inch = x
L/D Ratio = x
Wire Length = x