This Chart and a Ruler Will Give You All You Need to Know
About Coil Design
March 1930 Radio News Article

March 1930 Radio News [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Here is what an app looked like in 1930. Even in today's world with computing devices everywhere sporting simulators and component calculating programs, there are still times when having a good old fashioned nomograph or chart handy can be very useful while in sitting at a bench selecting component values for tweaking or troubleshooting a design. The advantage of such visual aids is that they provide a big picture of what's happening as frequencies, lengths, widths, core materials, etc. change - being able to see both the trees and the forest, so to speak. When you are working in bands where the component physical size is a significant portion of the wavelength, things get more complicated and a combination of trial and error and calculations / simulations are needed. At the IC design level, of course, you have no choice but to rely solely on your computer, but if you regularly conduct part of your circuit design at a lab bench, I recommend gathering a collection of these kinds of charts and graphs either on your laptop, table, smartphone, or even in a paper folder. They can take a lot of the mystery out of what is happening as you work.

This Chart and a Ruler Will Give You All You Need to Know About Coil Design

By W. Bruce Ross

The September, 1929, issue of Radio News contained a calculation chart, showing how a coil of suitable inductance could be wound to cover a definite band of wavelengths when shunted by a variable condenser of known value. Or if the reader has a coil and condenser, it is possible, with this scale, to determine the tuning range of the two, when connected in a tuning circuit.

We have worked out a further application of the above-mentioned chart which makes unnecessary the use of paper and pencil calculations in estimating coil sizes and wavelength ranges. This involves the addition of another line (Number 8), which appears in the accompanying illustration, and which is exactly three-eighths of an inch from the original Index Line, No.5.

The graduations on this line represent the total number of turns on the coil. We shall call this line No.8. We already have scales marked "Number of turns per inch" (No. 6), and "Length of coil in inches" (No. 4), so that the new scale must fit in with these. Indeed, it is by means of No. 4 and No. 6 that we graduate No.8.

Imagine a coil 1 inch long, wound with wire of such a diameter that there are exactly 10 turns to the inch. There will naturally be 10 turns in the coil. If we lay our straight-edge between 1 on scale No. 4 and 10 on scale No. 6, then the point where we cross the new scale must be the "10" point. Make a short mark on line No. 8, and put 10 beside it lightly. Suppose now that the coil is still 1 inch long but that we wind it with wire having 15 turns to the inch. There will be 15 turns on the coil. Shifting the straight-edge so as to join 15 on No. 6 and 1 on No. 4, we make a mark where it crosses the new line and put a "15" beside it. Similarly an imaginary coil having 20 turns to the inch, and 1 inch long. gives us the 20-turn point on No. 8. and so on up to 50.

We can go back, and by joining 11, 12, 13, -, 19 on No. 6 with 1 on No. 4, find the points for 11, 12, 13, -, 19 on No. 8; then using points 22, 24, 26. -, 38 on No. 6. find corresponding points on No. 8. (You will easily see that the graduation marks on No. 8 will be closer than those on No. 6, so that, to avoid confusion on the former, we drop the points for 21, 23, -, 39, and all the forties but 45).

We might go on up to 150 on No. 8 by simply joining the points 60, 70, etc. on No. 6 with 1 on No. 4; but it will be found that for these higher numbers the straight-edge makes such an acute angle with the new line that it becomes increasingly difficult to judge precisely where they cross, and errors in graduation are likely to result. Fortunately, there is a simple dodge which will obviate this. Prolong our imaginary coil to 2 inches in length. If we wind it with wire having 30 turns per inch, we will have wound 60 turns in all. Placing the straight-edge so that it join 30 on No. 6 with 2 on No. 4, we see that it must cross No. 8 at the point "60" We therefore mark it as such. Again, if we wind the 2-inch long coil with 35-turns-per-inch wire, it will have a total of 70 turns. Joining 35 on No. 6 with 2 on No. 4, we locate "70" on No. 8. And similarly, joining points "40," "45." and "50" on No. 6 with "2" on No. 4 gives us points "80," "90," and "100" on No. 8. It will be found advisable to put in more than the 55. 65, 75, etc., points on the new scale, because otherwise the graduations would be too close for easy reading.

As a check, you will notice that the downward-sloping illustration line, indicating a coil 2 3/4 inches long wound with 35-turns-per-inch wire, cuts scale No. 8 at 96. The exact number of turns is 96.25, thus showing the scale to be quite accurate enough for any case in which the chart would be used.

There will be few calls, in all probability, for points corresponding to numbers greater than 100 on scale No. 8, but for completeness' sake and for possible future needs, the 110, 120, 130, - 200 points may be located by using point 2 on No. 4 and appropriate points on No. 6. For the 5, 6, 7, 8 and 9 positions on No. 8 we may joint 1/2 on No. 4 with 10, 12, 14, 16 and 18 on No. 6. It is little use going below 5 on No. 8, as the chart is not accurate for such short coils.

Caution - Do not rely on the point marked 1/4 on scale No. 4. It is out of place, its proper position being about 1/8 inch higher up on the line.

If you decide to ink your work in - and this is a very good plan - draw two faint lines to the right of No. 8 and parallel to it, at distances of 1/16 and 3/32 inches respectively. Prolong the marks representing the numbers 5, 6, 7, 8. 9, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150 and 200 to the 3/32 inch line, and the smaller divisions to the 1/16 inch line. Put numbers opposite the long lines only.

While you are doing this, it will be a good plan to make a further slight addition which will facilitate the use of the chart. Divide the Index Line (No.5) into equal sections (points 1/8 of an inch apart will do nicely) and number them consecutively - whether up or down is quite immaterial. If you do this, you will save marking up the chart with pencil every time you use it.

To illustrate by the broken lines printed on the chart: instead of actually drawing the line from 250 on scale No.3 to 2 on No. 7, simply read the intersection of the straight-edge with the Index Line. Suppose it is 21.9. Then swing the straight-edge to 35 on No. 6, check that it passes through 21.9 on the Index Line, and read off 96 turns on No. 8, and 2 3/4 inches on No. 4.

Posted February 11, 2022
(updated from original post on 2/13/2014)

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