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Introduction
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As the title indicates, this page is for the design of a
single-layer, air-core, coil. 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 below.
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Program Description
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In the Initial Designyou 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.
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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
a 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
you need is not listed, just interpolate between two close values. This
should get you within a single turn of the correct tap point.
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Design Considerations
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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.
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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.
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Initial Design Input
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Initial Design Output Data
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Number of Turns =
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Wire Size =
| x
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Turns Spacing =
| x
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Coil Form OD =
| x
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Coil Length =
| x
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L/D Ratio =
| x
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Turns_per_Inch =
| x
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Alternate Form Size Selection
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Some Suggested Sources Of Coil Form Material
PVC Pipe
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Pipe Size
| Outside Diameter
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3/8"
| 0.675" (17.145 mm)
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1/2"
| 0.840" (21.336 mm)
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3/4"
| 1.050" (26.670 mm)
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1"
| 1.315" (33.401 mm)
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1-1/4"
| 1.660" (42.164 mm)
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1-1/2"
| 1.900" (48.260 mm)
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2"
| 2.375" (60.325 mm)
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2-1/2"
| 2.875" (73.025 mm)
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3"
| 3.50" (88.900 mm)
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3-1/2"
| 4.0" (101.600 mm)
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4"
| 4.50" (114.300 mm)
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Shotgun Shell Diameters
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.410 = .410" (10.414 mm)
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28-Ga. = .550" (13.970 mm)
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20-Ga. = .615" (15.621 mm)
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16-Ga. = .662" (16.814 mm)
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12-Ga. = .729" (18.516 mm)
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10-Ga. = .775" (19.685 mm)
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The calculated coil diameter is x.
In the space provided below, enter a new Coil Form Diameter.
Choose any diameter near calculated Coil Form OD.
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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
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Form Diameter =
| x
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Coil Diameter =
| x
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Coil Length =
| x
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Wire Type =
| x
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Wire Diameter =
| x
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Number of Turns =
| x
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Turns Per Inch =
| x
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Turns Spacing =
| x
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L/D Ratio =
| x
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Adjustment For An Even Number Of Turns
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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.
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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 ........ .
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Final design based on a user defined coil form
diameter and number of turns.
Inductance =
| x
| Form Diameter =
| x
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Coil Diameter =
| x
| Coil Length =
| x
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Wire Type =
| x
| Wire Diameter =
| x
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Number of Turns =
| x
| Turns Per Inch =
| x
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Turns Spacing =
| x
| L/D Ratio =
| x
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Wire Length =
| x
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