Part 18 (2/2)

You will have to keep changing the tuning of your detector circuit and of the antenna For each new setting very sloing the condenser plates in the oscillator circuit and see if you get a signal It will probably be easier to use the ”stand-by position,” which I have described, with switch _S_ open in the secondary circuit of Fig

118 In that case you have only to tune your antenna to 275 meters and then you will pick up a note when your local oscillator is in tune

After you have done so you can tune the secondary circuit which supplies the crystal

If you adopt thisbetween the antenna and the crystal circuit You will alant a very weak coupling between the oscillator circuit and the detector circuit You will also probably want a weaker coupling between tickler and tube input than you are at first inclined to believe will be enough Patience and some skill in manipulation is always required for this sort of experiment

When you have co a local oscillator, you are ready to try the regenerative circuit This has been illustrated in Fig 92 of Letter 18 and needs no further description You will have the advantage when you cos of the antenna circuit and the secondary tuned circuit You will need then only to adjust the coupling of the tickler and

After you have co of an adept at radio and are in a position to plan your final set For this set you will need to purchase certain parts complete from reputable dealers because many of the circuits which I have described are patented and should not be used except as rights to use are obtained by the purchase of licensed apparatus which e how radio receivers operate and why, you are now in a good condition to discuss with dealers the relativesets

[Illustration: Fig 119]

Before you actually buy a coe of frequency over which you are carrying out your experiths you will need to increase the inductance of your antenna so that it will be tuned to a lower frequency This is usually called ”loading” and can be done by inserting a coil in the antenna To obtain sths decrease the effective capacity of the antenna circuit by putting another condenser in series with the antenna Usually, therefore, one connects into his antenna circuit both a condenser and a loading coil By using a variable condenser the effective capacity of the antenna systeed The result is that this series condenserand the slide wire tuner is not needed Fig 119 shows the circuit

For quite a range of wave-lengths wecoil and tune the antenna circuit entirely by this series condenser For soths 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 iive you illustrations based on the assumption that your antenna has a capacity of 00001 or of 00002 ht are 00005 and 0001 for their iven, for each of several values of the inductance of the priths which you can expect to receive The table is in two parts, the first for an antenna of capacity 00001 mf and the second for one of 00002 mf

Yours will be soth 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 coluth in the second The third coluth youcondenser of 00005 th which is possible with the larger condenser

TABLE I

Part 1 (For antenna of 00001 est wave-length in meters mil-henries in meters with 00005 mf with 0001 mf

010 103 169 179 020 146 238 253 040 207 337 358 085 300 490 515 180 400 700 760 200 420 750 800 400 600 1080 1130 500 660 1200 1260 1000 900 1700 1790 3000 1600 2900 3100

Part 2 (For antenna of 00002 mf)

010 169 225 240 016 210 285 305 020 240 320 340 025 270 355 380 040 340 450 480 060 420 550 590 080 480 630 680 120 585 775 840 180 720 950 1020 300 930 1220 1320 500 1200 1600 1700 800 1500 2000 2150 1200 1850 2400 2650 1600 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 experi more in the way of inductance, which the antenna circuit requires to give a desired wave-length, youIn Table II are soht cores to obtain various values of inductance Your 26 s s c ind 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 35 inches in diameter and the second part for one of 5 inches The first coluives number of turns The third and fourth are th in inches of the coil and the approxith of hich is required to wind it I have allowed for bringing out taps In other words 550 feet of the ind a coil of 102 inches with an inductance of 800out taps at all the lower values of inductance which are given in the table

Table II

Part 1 (For a core of 35 in diath Feet of wire mil-henries of turns in inches required

010 25 046 25 016 34 063 36 020 39 072 42 025 44 081 49 040 58 107 63 060 75 138 80 080 92 170 100 085 96 178 104 100 108 200 118 120 123 228 133 180 164 303 176 200 180 333 190 300 242 448 250 400 304 562 310 500 366 677 370 800 550 1020 550

Part 2 (For core of 50 in diam)

200 120 222 160 300 158 293 215 400 194 358 265 500 228 422 310 800 324 600 450 1000 384 710 530 1200 450 830 625

The coil which you wound at the beginning of your experiment had only 75 turns and was tapped so that you could, by et siven the values of the inductance which is controlled by the switches of that figure, the corresponding nuth to which the antenna should then be tuned I a this for two values of antenna capacity, as I have done before By the aid of these three tables you should have s for all wave-lengths below about 3000 er waves than that you had better buy a few banked-wound coils