Chapter XXII: Letter 21: Your Receiving Set and How to Experiment

MY DEAR STUDENT:

In this letter I want to tell you how to experiment with radio apparatus. The first rule is this: Start with a simple circuit, never add anything to it until you know just why you are doing so, and do not box it up in a cabinet until you know how it is working and why.

Your antenna at the start had better be a single wire about 25 feet high and about 75 feet long. This antenna will have capacity of about 0.0001 m. f. If you want an antenna of two wires spaced about three feet apart I would make it about 75 feet long. Bring down a lead from each wire, twisting them into a pigtail to act like one wire except near the horizontal part of the antenna.

Your ground connection can go to a water pipe. To protect the house and your apparatus from lightning insert a fuse and a little carbon block lightning arrester such as are used by the telephone company in their installations of house phones. You can also use a so-called "vacuum lightning arrester." In either case the connections will be as shown in Fig. 111. If you use a loop antenna, of course, no arrester is needed.

At first I would plan to receive signals between 150 meters and 360 meters. This will include the amateurs who work between 160 and 200 m., the special amateurs who send C-W telegraph at 275 m., and the broadcasting stations which operate at 360 m. This range will give you plenty to listen to while you are experimenting. In addition you will get some ship signals at 300 m.

To tune the antenna to any of the wave lengths in this range you can use a coil of 75 turns wound on a cardboard tube of three and a half inches in diameter. You can wind this coil of bare wire if you are careful, winding a thread along with the wire so as to keep the successive turns separated. In that case you will need to construct a sliding contact for it. That is the simplest form of tuner.

On the other hand you can wind with single silk covered wire and bring out taps at the 0, 2, 4, 6, 8, 10, 14, 20, 28, 36, 44, 56, 66, and 75th turns. To make a tap drill a small hole through the tube, bend the wire into a loop about a foot long and pull this loop through the hole as shown in Fig. 110. Then give the wire a twist, as shown, so that it can't pull out, and proceed with your winding.

Use 26 s. s. c. wire. You will need about 80 feet and might buy 200 to have enough for the secondary coil. Make contacts to the taps by two rotary switches as shown in Fig. 112. You can buy switch arms and contacts studs or a complete switch mounted on a small panel of some insulating compound. Let switch _s_{1}_ make the contacts for taps between 14 and 75 turns, and let switch _s_{2}_ make the other contacts.

For the secondary coil use the same size of wire and of core. Wind 60 turns, bringing out a tap at the middle. To tune the secondary circuit you will need a variable condenser. You can buy one of the small ones with a maximum capacity of about 0.0003 mf., one of the larger ones with a maximum capacity of 0.0005 mf., or even the larger size which has a maximum capacity of 0.001 mf. I should prefer the one of 0.0005 mf.

You will need a crystal detector--I should try galena first--and a so-called "cat's whisker" with which to make contact with the galena. For these parts and for the switch mentioned above you can shop around to advantage. For telephone receivers I would buy a really good pair with a resistance of about 2500 ohms. Buy also a small mica condenser of 0.002 mf. for a blocking condenser. Your entire outfit will then look as in Fig. 112. The switch _S_ is a small knife switch.

To operate, leave the switch _S_ open, place the primary and secondary coils near together as in the figure and listen. The tuning is varied, while you listen, by moving the slider of the slide-wire tuner or by moving the switches if you have connected your coil for that method. Make large changes in the tuning by varying the switch _s_{1}_ and then turn slowly through all positions of _s_{2}_, listening at each position.

When a signal is heard adjust to the position of _s_{1}_ and _s_{2}_ which gives the loudest signal and then closing _S_ start to tune the secondary circuit. To do this, vary the capacity of the condenser in the secondary circuit. Don't change the primary tuning until you have tuned the secondary and can get the signal with good volume, that is loud. You will want to vary the position of the primary and secondary coils, that is, vary their coupling, for you will get sharper tuning as they are drawn farther apart. Sharper tuning means less interference from other stations which are sending on wave lengths near that which you wish to receive. Reduce the coupling, therefore, and then readjust the tuning. It will usually be necessary to make a slight change in both circuits, in one case with switch _s_{1}_ and in the other with the variable condenser.

As soon as you can identify any station which you hear sending make a note of the position of the switches _s_{1}_ and _s_{2}_, and of the setting of the condenser in the secondary circuit. In that way you will acquire information as to the proper adjustments to receive certain wave-lengths. This is calibrating your set by the known wave-lengths of distant stations.

After learning to receive with this simple set I should recommend buying a good audion tube. Ask the seller to supply you with a blue print of the characteristic[11] of the tube taken under the conditions of filament current and plate voltage which he recommends for its use. Buy a storage battery and a small slide-wire rheostat, that is variable resistance, to use in the filament circuit. Buy also a bank of dry batteries of the proper voltage for the plate circuit of the tube. At the same time you should buy the proper design of transformer to go between the plate circuit of your tube and the pair of receivers which you have. It will usually be advisable to ask the dealer to show you a characteristic curve for the transformer, which will indicate how well the transformer operates at the different frequencies in the audio range. It should operate very nearly the same for all frequencies between 200 and 2500 cycles.

The next step is to learn to use the tube as a detector. Connect it into your secondary circuit instead of the crystal detector. Use the proper value of C-battery as determined from your study of the characteristic of the tube. One or two small dry cells, which have binding-post terminals are convenient C-batteries. If you think you will need a voltage much different from that obtained with a whole number of batteries you can arrange to supply the grid as we did in Fig. 86 of Letter 18. In that case you can use a few feet of 30 German-silver wire and make connections to it with a suspender clip. Learn to receive with the tube and be particularly careful not to let the filament have too much current and burn out.

Now buy some more apparatus. You will need a grid condenser of about 0.0002 mf. The grid leaks to go with it you can make for yourself. I would use a piece of brown wrapping paper and two little metal eyelets. The eyelets can be punched into the paper. Between them coat the paper with carbon ink, or with lead pencil marks. A line about an inch long ought to serve nicely. You will probably wish to make several grid leaks to try. When you get satisfactory operation in receiving by the grid-condenser method the leak will probably be somewhere between a megohm and two megohms.

For this method you will not want a C-battery, but you will wish to operate the detector with about as high a voltage as the manufacturers will recommend for the plate circuit. In this way the incoming signal, which decreases the plate current, can produce the largest decrease. It is also possible to start with the grid slightly positive instead of being as negative as it is when connected to the negative terminal of the A-battery. There will then be possible a greater change in grid voltage. To do so connect the grid as in Fig. 115 to the positive terminal of the A-battery.

About this time I would shop around for two or three small double-pole double-throw switches. Those of the 5-ampere size will do. With these you can arrange to make comparisons between different methods of receiving. Suppose, for example, you connect the switches as shown in Fig. 113 so that by throwing them to the left you are using the audion and to the right the crystal as a detector. You can listen for a minute in one position and then switch and listen for a minute in the other position, and so on back and forth. That way you can tell whether or not you really are getting better results.

If you want a rough measure of how much better the audion is than the crystal you might see, while you are listening to the audion, how much you can rob the telephone receiver of its current and still hear as well as you do when you switch back to the crystal. The easiest way to do this is to put a variable resistance across the receiver as shown in Fig. 113. Adjust this resistance until the intensity of the signal when detected by the audion is the same as for the crystal. You adjust this variable resistance until it by-passes so much of the current, which formerly went through the receiver, that the "audibility" of the signal is reduced until it is the same as for the crystal detector. Carefully made resistances for such a purpose are sold under the name of "audibility meters." You can assemble a resistance which will do fairly well if you will buy a small rheostat which will give a resistance varying by steps of ten ohms from zero to one hundred ohms. At the same time you can buy four resistance spools of one hundred ohms each and perhaps one of 500 ohms. The spools need not be very expensive for you do not need carefully adjusted resistances. Assemble them so as to make a rheostat with a range of 0-1000 ohms by steps of 10 ohms. The cheapest way to mount is with Fahnestock clips as illustrated in Fig. 114. After a while, however, you will probably wish to mount them in a box with a rotary switch on top.

To study the effect of the grid condenser you can arrange switches so as to insert this condenser and its leak and at the same time to cut out the C-battery. Fig. 115 shows how. You can measure the gain in audibility at the same time.

After learning to use the audion as a detector, both by virtue of its curved characteristic and by the grid-condenser method, I would suggest studying the same tube as an amplifier. First I would learn to use it as an audio-frequency amplifier. Set up the crystal detector circuit. Use your audio-frequency transformer the other way around so as to step up to the grid. Put the telephone in the plate circuit. Choose your C-battery for amplification and _not detection_ and try to receive.

You will get better results if you can afford another iron-core transformer. If you can, buy one which will work between the plate circuit of one vacuum tube and the grid circuit of another similar tube. Then you will have the right equipment when you come to make a two-stage audio-frequency amplifier. If you buy such a transformer use the other transformer between plate and telephones as you did before and insert the new one as shown in Fig. 116. This circuit also shows how you can connect the switches so as to see how much the audion is amplifying.

The next step is to use the audion as an amplifier of the radio-signal before its detection. Use the proper C-battery for an amplifier, as determined from the blue print of the tube characteristic. Connect the tube as shown in Fig. 117. You will see that in this circuit we are using a choke coil to keep the radio-frequency current out of the battery part of the plate circuit and a small condenser, another one of 0.002 mf., to keep the battery current from the crystal detector. You can see from the same figure how you can arrange the switches so as to find whether or not you are getting any gain from the amplifier.

Now you are ready to receive those C-W senders at 275 meters. You will need to wind another coil like the secondary coil you already have. Here is where you buy another condenser. You will need it later. If before you bought the 0.0005 size, this time buy the 0.001 size or vice versa. Wind also a small coil for a tickler. About 20 turns of 26 wire on a core of 3-1/2 in. diameter will do. Connect the tickler in the plate circuit of the audion. Connect to the grid your new coil and condenser and set the audion circuit so that it will induce a current in the secondary circuit which supplies the crystal. Fig. 118 shows the hook-up.

You will see that you are now supplying the crystal with current from two sources, namely the distant source of the incoming signals and the local oscillator which you have formed. The crystal will detect the "beat note" between these two currents.

To receive the 275 meters signals you will need to make several adjustments at the same time. In the first place I would set the tuning of the antenna circuit and of the crystal circuit about where you think right because of your knowledge of the settings for other wave lengths. Then I would get the local oscillator going. You can tell whether or not it is going if you suddenly increase or decrease the coupling between the tickler coil and the input circuit of the audion. If this motion is accompanied by a click in the receivers the tube is oscillating.

Now you must change the frequency at which it is oscillating by slowly changing the capacity in the tuned input circuit of the tube. Unless the antenna circuit is properly tuned to the 275 meter signal you will get no results. If it is, you will hear an intermittent musical note for some tune of your local oscillator. This note will have the duration of dots and dashes.

You will have to keep changing the tuning of your detector circuit and of the antenna. For each new setting very slowly swing 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 this method you will want a close coupling between the antenna and the crystal circuit. You will always want 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 completed this experiment in heterodyne receiving, using 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 come to this of knowing very closely the proper settings of the antenna circuit and the secondary tuned circuit. You will need then only to adjust the coupling of the tickler and make finer adjustments in your tuning.

After you have completed this series of experiments you will be something 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 embodies the patented circuits. Knowing how radio receivers operate and why, you are now in a good condition to discuss with dealers the relative merits and costs of receiving sets.

Before you actually buy a completed set you may want to increase the range of frequency over which you are carrying out your experiments. To receive at longer wave-lengths 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 smaller wave-lengths 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 system may be easily changed. The result is that this series condenser method becomes the easiest method of tuning and the slide wire tuner is not needed. Fig. 119 shows the circuit.

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 large wave-length which is possible with the larger condenser.

TABLE I

Part 1. (For antenna of 0.0001 mf.)

Inductance in Shortest wave-length Longest wave-length in meters
mil-henries. in meters. with 0.0005 mf. 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