This Chart and a Ruler Will Give You All You Need to Know About Coil Design
March 1930 Radio News Article
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.
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine.
Here is a list of the Radio & Television News articles
I have already posted. As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
and a Ruler Will Give You All You Need to Know About Coil Design
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.
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.
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 13, 2014