Letters of a Radio-Engineer to His Son

by John Mills

For quite a range of wave-lengths we may use the same loading coil and tune the antenna circuit entirely by this series condenser. For some other range of wave-lengths we shall then need a different loading coil. In a well-designed set the wave-length ranges overlap. The calculation of the size of loading coil is quite easy but requires more arithmetic than I care to impose on you at present. I shall therefore merely give you illustrations based on the assumption that your antenna has a capacity of 0.0001 or of 0.0002 mf. and that the condensers which you have bought are 0.0005 and 0.001 for their maxima.

In Table I there is given, for each of several values of the inductance of the primary coil, the shortest and the longest wave-lengths which you can expect to receive. The table is in two parts, the first for an antenna of capacity 0.0001 mf. and the second for one of 0.0002 mf. Yours will be somewhere between these two limits. The shortest wave-length depends upon the antenna and not upon the condenser which you use in series with it for tuning. It also depends upon how much inductance there is in the coil which you have in the antenna circuit. The table gives values of inductance in the first column, and of minimum wave-length in the second. The third column shows what is the greatest wave-length you may expect if you use a tuning condenser of 0.0005 mf.; and the fourth column the slightly 226large wave-length which is possible with the larger condenser.

TABLE I
Part 1. (For antenna of 0.0001 mf.)
Inductance in mil-henries	Shortest wave-length in meters.	Longest wave-length in meters with 0.0005 mf.	Longest wave-length in meters with 0.001 mf.
0.10	103	169	179
0.20	146	238	253
0.40	207	337	358
0.85	300	490	515
1.80	400	700	760
2.00	420	750	800
4.00	600	1080	1130
5.00	660	1200	1260
10.00	900	1700	1790
30.00	1600	2900	3100
Part 2. (For antenna of 0.0002 mf.)
0.10	169	225	240
0.16	210	285	305
0.20	240	320	340
0.25	270	355	380
0.40	340	450	480
0.60	420	550	590
0.80	480	630	680
1.20	585	775	840
1.80	720	950	1020
3.00	930	1220	1320
5.00	1200	1600	1700
8.00	1500	2000	2150
12.00	1850	2400	2650
16.00	2150	2800	3050

From Table I you can find how much inductance you will need in the primary circuit. A certain amount you will need to couple the antenna and the secondary circuit. The coil which you wound at the beginning of your experiments will do well for that. Anything more in the way of inductance, which the antenna circuit requires to give a desired wave-length, you may consider as loading. In Table II are some data as to winding coils on straight 227cores to obtain various values of inductance. Your 26 s. s. c. wire will wind about 54 turns to the inch. I have assumed that you will have this number of turns per inch on your coils and calculated the inductance which you should get for various numbers of total turns. The first part of the table is for a core of 3.5 inches in diameter and the second part for one of 5 inches. The first column gives the inductance in mil-henries. The second gives number of turns. The third and fourth are merely for convenience and give the approximate length in inches of the coil and the approximate total length of wire which is required to wind it. I have allowed for bringing out taps. In other words 550 feet of the wire will wind a coil of 10.2 inches with an inductance of 8.00 mil-henries, and permit you to bring out taps at all the lower values of inductance which are given in the table.

TABLE II
Part 1. (For a core of 3.5 in. diam.)
Inductance in mil-henries	Number of turns	Length in inches	Feet of wire required.
0.10	25	0.46	25
0.16	34	0.63	36
0.20	39	0.72	42
0.25	44	0.81	49
0.40	58	1.07	63
0.60	75	1.38	80
0.80	92	1.70	100
0.85	96	1.78	104
1.00	108	2.00	118
1.20	123	2.28	133
1.80	164	3.03	176
2.00	180	3.33	190
3.00	242	4.48	250
4.00	304	5.62	310
5.00	366	6.77	370
8.00	550	10.20	550
Part 2. (For a core of 5.0 in. diam.)
2.00	120	2.22	160
3.00	158	2.93	215
4.00	194	3.58	265
5.00	228	4.22	310
8.00	324	6.00	450
10.00	384	7.10	530
12.00	450	8.30	625

228The coil which you wound at the beginning of your experiment had only 75 turns and was tapped so that you could, by manipulating the two switches of Fig. 112, get small variations in inductance. In Table III is given the values of the inductance which is controlled by the switches of that figure, the corresponding number of turns, and the wave-length to which the antenna should then be tuned. I am giving this for two values of antenna capacity, as I have done before. By the aid of these three tables you should have small difficulty in taking care of matters of tuning for all wave-lengths below about 3000 meters. If you want to get longer waves than that you had better buy a few banked-wound coils. These are coils in which the turns are wound over each other but in such a way as to avoid in large part the “capacity effects” which usually accompany such winding. You can try winding them for yourself but I doubt if the experience has much value until you have gone farther in the study of the mathematical theory of radio than this series of letters will carry you.

TABLE III
		Circuit of Fig. 112
Number	Inductance in	Wavelength with antenna of
of turns	mil-henries	0.0001 mf.	0.0002 mf.
14	0.04	120	170
20	0.07	160	220
28	0.12	210	290
36	0.18	250	360
44	0.25	300	420
56	0.38	370	520
75	0.60	460	650

In the secondary circuit there is only one capacity, that of the variable condenser. If it has a range of values from about 0.00005 mf. to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of wave-lengths from 270 to 860 m. Using half the coil the range is 150 to 480 m. With the larger condenser the ranges are respectively 270 to 1220 and 270 to 670. For longer wave-lengths load with inductance. Four times the inductance will tune to double these wave-lengths.

[11]

If you can afford to buy, or if you can borrow, ammeters and voltmeters of the proper range you should take the characteristic yourself.