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.
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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.
Nomographs Available on RF Cafe: -
Decibel Nomograph
-
Voltage and Power Level Nomograph
- Voltage, Current, Resistance,
and Power Nomograph
- Resistor
Selection Nomogram
- Resistance
and Capacitance
- Capacitance
Nomograph
- Earth Curvature Nomograph
- Coil Design Nomograph
-
Coil Inductance Nomograph
- Antenna Gain Nomograph
-
Resistance and
Reactance Nomograph
Posted February 13, 2014
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